Title of Invention	" METHOD AND APPARATUS FOR CONTROLLING DOWNLINK TRANSMISSION BETWWEN SUBSCRIBER STATION AND BASE STATION IN AN ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING NETWORK"
Abstract	The invention relates to a method of controlling downlink transmissions to a subscriber station capable of communicating with a base station of an orthogonal frequency division multiplexing (OFDM) network. The method comprises the steps of: receiving a first pilot signal from a first base station antenna; receiving a second pilot signal from a second base station antenna; and estimating the channel between the base station and subscriber station based on the received first and second pilot signals. The method also comprises determining a set of OFDM symbol processing parameters based on the step of estimating the channel and transmitting the OFDM symbol processing parameters to the base station. The base station uses the OFDM symbol processing parameters to control the relative gains and the relative delays of OFDM symbols transmitted from the first and second antennas.

Title of Invention

" METHOD AND APPARATUS FOR CONTROLLING DOWNLINK TRANSMISSION BETWWEN SUBSCRIBER STATION AND BASE STATION IN AN ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING NETWORK"

Abstract

The invention relates to a method of controlling downlink transmissions to a subscriber station capable of communicating with a base station of an orthogonal frequency division multiplexing (OFDM) network. The method comprises the steps of: receiving a first pilot signal from a first base station antenna; receiving a second pilot signal from a second base station antenna; and estimating the channel between the base station and subscriber station based on the received first and second pilot signals. The method also comprises determining a set of OFDM symbol processing parameters based on the step of estimating the channel and transmitting the OFDM symbol processing parameters to the base station. The base station uses the OFDM symbol processing parameters to control the relative gains and the relative delays of OFDM symbols transmitted from the first and second antennas.

Full Text	SYSTEM AND METHOD FOR CHANNEL ESTIMATION IN A DELAY DIVERSITY WIRELESS COMMUNICATION SYSTEM TECHNICAL FIELD OF THE INVENTION The present disclosure relates generally to wireless communications and, more specifically, to an apparatus and method for performing channel estimation in an orthogonal frequency division multiplexing (OFDM) network or an orthogonal frequency division multiple access (OFDMA) network. BACKGROUND OF THE INVENTION Conventional orthogonal frequency division multiplexing (OFDM) networks and orthogonal frequency division multiple access (OFDMA) network are able to improve the reliability of the channel by spreading and/or coding data traffic and control signals over multiple subcarriers (i.e., tones). However, if the channel is flat, frequency diversity cannot be achieved. In order to overcome this, it is possible to introduce artificial frequency diversity into the transmitted signal. A technique for artificially introducing frequency diversity into an OFDM environment was disclosed in U.S Patent Application Serial No. 11/327,799, filed on January 6, 2006 and incorporated by reference above. In the device disclosed in Serial No. 11/327,799, multiple copies of the same OFDM symbol are delayed by different delay values, then amplified by the same or different gain values, and then transmitted from different antennas. This artificially introduces frequency-selective fading in the OFDM channel, thereby allowing frequency selectivity to be exploited using frequency-domain scheduling for low-to-medium speed mobile devices or frequency diversity for higher speed mobile devices. However, when selecting the symbol processing parameters (i.e., delay values and the gain values) applied to the OFDM symbols, it is important to take into consideration the user channel type and the mobile speed. To accomplish this, channel estimation is performed and the symbol processing parameters are determined based on the channel estimates and mobile speed. Therefore, there is a need for improved apparatuses and methods for performing channel estimation in an OFDM wireless network that artificially introduces frequency diversity by delaying and amplifying multiple copies of the same OFDM symbol and then transmitting the delayed and amplified OFDM symbols from different transmit antennas. SUMMARY OF THE INVENTION A method of controlling downlink transmissions to a subscriber station is provided for use in a subscriber station capable of communicating with a base station of an orthogonal frequency division multiplexing (OFDM) network. The method comprises the steps of: receiving a first pilot signal from a first antenna of the base station; receiving a second pilot signal from a second antenna of the base station; estimating the channel between the base station and subscriber station based on the received first and second pilot signals; determining a set of OFDM symbol processing parameters based on the step of estimating the channel, wherein the OFDM symbol processing parameters are usable by the base station to control the relative gains and the relative delays of OFDM symbols transmitted from the first and second antennas; and transmitting the OFDM symbol processing parameter set to the base station. According to another embodiment of the present disclosure, a subscriber station capable of communicating with a base station of an orthogonal frequency division multiplexing (OFDM) network is provided. The subscriber station comprises: receive path circuitry capable of receiving a first pilot signal from a first antenna of the base station and receiving a second pilot signal from a second antenna of the base station; and channel estimating circuitry capable of estimating the channel between the base station and subscriber station based on the received first and second pilot signals and capable of determining a set of OFDM symbol processing parameters based on a channel quality estimate. The OFDM symbol processing parameters are usable by the base station to control the relative gains and the relative delays of OFDM symbols transmitted from the first and second antennas and wherein the subscriber station is capable of transmitting the OFDM symbol processing parameters to the base station. According to yet another embodiment of the present disclosure, a base station is provided for use in an orthogonal frequency division multiplexing (OFDM) network. The base station comprises: 1) receive path circuitry capable of receiving an uplink signal from a subscriber station, estimating the channel between the base station and subscriber station based on the received uplink signal, and determining a set of OFDM symbol processing parameters based on a channel quality estimate; and 2) transmit path circuitry capable of using the OFDM symbol processing parameters to control the relative gains and the relative delays of processed OFDM symbols transmitted from a first antenna and a second antenna of the base station. The base station is capable of transmitting the OFDM symbol processing parameters to the subscriber station. The OFDM symbol processing parameters are based on the multipath characteristics and the frequency selectivity characteristics of the channel. Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms "include" and "comprise," as well as derivatives thereof, mean inclusion without limitation; the term "or," is inclusive, meaning and/or; the term "each" means every one of at least a subset of the identified items; the phrases "associated with" and "associated therewith," as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term "controller" means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: FIGURE 1 illustrates an exemplary orthogonal frequency division multiplexing (OFDM) wireless network that is capable of performing channel estimation according to the principles of the present disclosure; FIGURE 2A is a high-level diagram of the orthogonal frequency division multiplexing (OFDM) transmit path in a base station according to one embodiment of the disclosure; FIGURE 2B is a high-level diagram of the orthogonal frequency division multiplexing (OFDM) receive path in a subscriber station according to one embodiment of the disclosure; FIGURE 3 illustrates the OFDM symbol processing block in the base station in greater detail according to an exemplary embodiment of the present disclosure; FIGURE 4A illustrates data traffic transmitted in the downlink from a base station to a subscriber station according to an exemplary embodiment of the present disclosure; FIGURE 4B is a flow diagram illustrating the determination of the user channel type based on the measurements on the preamble according to an exemplary embodiment of the disclosure; FIGURE 5 is a message flow diagram illustrating the transmission of OFDM symbols from a base station to a subscriber station according to the principles of the disclosure; FIGURE 6 is a flow diagram illustrating the processing of pilot signals and OFDM data symbols according to an exemplary embodiment of the present disclosure; FIGURE 7 is a message flow diagram illustrating the transmission of OFDM symbols from a base station to a subscriber station according to another embodiment of the disclosure; and FIGURE 8 is a message flow diagram illustrating the transmission of OFDM symbols from a base station to a subscriber station according to another embodiment of the disclosure. DETAILED DESCRIPTION OF THE INVENTION FIGURES 1 through 8, discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless network. The present disclosure is directed to apparatuses and algorithms for channel estimation and channel quality estimation for demodulation and data rate selection in an orthogonal frequency division multiplexing (OFDM) wireless network that uses delayed diversity. Such a delayed diversity wireless network was disclosed previously U.S Patent Application Serial No. 11/327,799, incorporated by reference above. The present disclosure uses a number of factors, including user channel type and mobile speed, to select OFDM symbol processing parameters (i.e., delays D1, D2,..., DP and gains g0, g1, ..., gP) for OFDM symbols transmitted from up to P antennas (i.e., ANT1 to ANTP). Therefore, different OFDM symbol processing parameters may be used to transmit to different mobile devices that are scheduled simultaneously, depending upon their channel types. It is noted that the scope of the present disclosure is not limited to orthogonal frequency division multiplexing (OFDM) wireless networks. The present disclosure is also applicable to orthogonal frequency division multiple access (OFDMA) wireless networks. However, for simplicity and brevity, the embodiments described below are directed to OFDM wireless networks, except where otherwise noted or where the context indicates otherwise. For relatively low-speed mobile devices, it is usually possible to track changes in the channel, thereby allowing channel sensitive scheduling to improve performance. Thus, the OFDM symbol processing parameters may be selected in such a way that relatively large coherence bandwidth results. That is, a relatively larger number of subcarriers experience similar fading. This goal may be achieved by keeping the delays for OFDM symbols from different antennas relatively small. A mobile device may then be scheduled on a subband consisting of contiguous subcarriers. For relatively high-speed mobile devices, channel quality variations cannot be tracked accurately, so that frequency-diversity may be helpful. Thus, the OFDM symbol processing parameters are selected in such a way that relatively small coherence bandwidth results. That is, potentially independent fading may occur from subcarrier to subcarrier. This goal may be achieved by having relatively large delays for OFDM symbols transmitted from different antennas. The symbol processing parameters may also be selected based on the degree of frequency-selectivity already present in the channel. For example, if a channel already has a lot of multipath effects and is, therefore, frequency selective, there may be little or no need for additional frequency selectivity. The OFDM symbol processing parameters may be selected on a user-by-user basis because different mobile devices experience different channel types. FIGURE 1 illustrates exemplary orthogonal frequency division multiplexing (OFDM) wireless network 100, which is capable of performing channel estimation according to the principles of the present disclosure. In the illustrated embodiment, wireless network 100 includes base station (BS) 101, base station (BS) 102, base station (BS) 103, and other similar base stations (not shown). Base station 101 is in communication with base station 102 and base station 103. Base station 101 is also in communication with Internet 130 or a similar IP-based network (not shown). Base station 102 provides wireless broadband access (via base station 101) to Internet 130 to a first plurality of subscriber stations within coverage area 120 of base station 102. The first plurality of subscriber stations includes subscriber station 111, which may be located in a small business (SB), subscriber station 112, which may be located in an enterprise (E), subscriber station 113, which may be located in a WiFi hotspot (HS), subscriber station 114, which may be located in a first residence (R), subscriber station 115, which may be located in a second residence (R), and subscriber station 116, which may be a mobile device (M), such as a cell phone, a wireless laptop, a wireless PDA, or the like. Base station 103 provides wireless broadband access (via base station 101) to Internet 130 to a second plurality of subscriber stations within coverage area 125 of base station 103. The second plurality of subscriber stations includes subscriber station 115 and subscriber station 116. In an exemplary embodiment, base stations 101-103 may communicate with each other and with subscriber stations 111-116 using OFDM or OFDMA techniques. Base station 101 may be in communication with either a greater number or a lesser number of base stations. Furthermore, while only six subscriber stations are depicted in FIGURE 1, it is understood that wireless network 100 may provide wireless broadband access to additional subscriber stations. It is noted that subscriber station 115 and subscriber station 116 are located on the edges of both coverage area 120 and coverage area 125. Subscriber station 115 and subscriber station 116 each communicate with both base station 102 and base station 103 and may be said to be operating in handoff mode, as known to those of skill in the art. Subscriber stations 111-116 may access voice, data, video, video conferencing, and/or other broadband services via Internet 130. In an exemplary embodiment, one or more of subscriber stations 111-116 may be associated with an access point (AP) of a WiFi WLAN. Subscriber station 116 may be any of a number of mobile devices, including a wireless-enabled laptop computer, personal data assistant, notebook, handheld device, or other wireless-enabled device. Subscriber stations 114 and 115 may be, for example, a wireless-enabled personal computer (PC), a laptop computer, a gateway, or another device. FIGURE 2A is a high-level diagram of the transmit path in orthogonal frequency division multiplexing (OFDM) transmitter 200 according to an exemplary embodiment of the disclosure. FIGURE 2B is a high-level diagram of the receive path in orthogonal frequency division multiplexing (OFDM) receiver 250 according to an exemplary embodiment of the disclosure. OFDM transmitter 200 comprises quadrature amplitude modulation (QAM) modulator 205, serial-to-parallel (S-to-P) block 210, Inverse Fast Fourier Transform (IFFT) block 215, parallel-to-serial (P-to-S) block 220, add cyclic prefix block 225, and OFDM symbol processing block 230. OFDM receiver 250 comprises remove cyclic prefix block 260, serial-to-parallel (S-to-P) block 265, Fast Fourier Transform (FFT) block 270, parallel-to-serial (P-to-S) block 275, quadrature amplitude modulation (QAM) demodulator 280, and channel estimation block 285. At least some of the components in FIGURES 2A and 2B may be implemented in software while other components may be implemented by configurable hardware or a mixture of software and configurable hardware. In particular, it is noted that the FFT blocks and the IFFT blocks described in FIGURES 2A and 2B may be implemented as configurable software algorithms, where the values of FFT and IFFT sizes may be modified according to the implementation. QAM modulator 205 receives a stream of input data and modulates the input bits (or symbols) to produce a sequence of frequency-domain modulation symbols. Serial-to-parallel block 210 converts (i.e., de-multiplexes) the serial QAM symbols to parallel data to produce M parallel symbol streams where M is the IFFT/FFT size used in OFDM transmitter 200 and OFDM receiver 250. IFFT block 215 then performs an IFFT operation on the M parallel symbol streams to produce time-domain output signals. Parallel-to-serial block 220 converts (i.e., multiplexes) the parallel time-domain output symbols from IFFT block 215 to produce a serial time-domain signal. Add cyclic prefix block 225 then inserts a cyclic prefix to each OFDM symbol in the time-domain signal. As is well known, the cyclic prefix is generated by copying the last G samples of an N sample OFDM symbol and appending the copied G samples to the front of the OFDM symbol. Finally, OFDM symbol processing block 230 processes the incoming OFDM symbols as described in FIGURE 3 and as described in U.S Patent Application Serial No. 11/327,799. The process OFDM samples at the output of OFDM symbol processing block 230 are then sent to up-conversion circuitry (not shown) prior to being transmitted from multiples transmit antennas. The transmitted RF signal arrives at OFDM receiver 250 after passing through the wireless channel and reverse operations to those in OFDM transmitter 200 are performed. Remove cyclic prefix block 260 removes the cyclic prefix to produce the serial time-domain baseband signal. Serial-to-parallel block 265 converts the time-domain baseband signal to parallel time domain signals. FFT block 270 then performs an FFT algorithm to produce M parallel frequency-domain signals. Parallel-to-serial block 275 converts the parallel frequency-domain signals to a sequence of QAM data symbols. QAM demodulator 280 then demodulates the QAM symbols to recover the original input data stream. Channel estimation block 285 also receives the QAM data symbols from parallel-to-serial block 275 and performs channels estimation. As will be described below in greater detail, the channel estimation values are used to determine a parameter set of gain values and delay values that are used in OFDM symbol processing block 230 in OFDM transmitter 200 and are used by QAM demodulator 280 to demodulate the QAM data symbols. The exemplary transmit path of OFDM transmitter 200 may be representative of the transmit paths of any one of base stations 101-103 or any one of subscriber stations 111-116. Similarly, the exemplary receive path of OFDM receiver 250 may be representative of the transmit paths of any one of base stations 101-103 or any one of subscriber stations 111-116. However, since multiple antenna configurations are more common in base stations than in subscriber stations or other mobile devices, for the sake of simplicity and clarity, the descriptions that follow will be directed toward transactions between a base station (e.g., BS 102) that implements a transmit path similar to OFDM transmitter 200 and a subscriber station (e.g., SS 116) that implements a receive path similar to OFDM receiver 250. However, such an exemplary embodiment should not be construed to limit the scope of the present disclosure. It will be appreciated by those skilled in the art that in cases where multiple antennas are implemented in a subscriber station, the transmit path and the receiver path of both the base station and the subscriber station may be implemented as in shown in FIGURES 2A and 2B. FIGURE 3 illustrates OFDM symbol processing block 230 in greater detail according to an exemplary embodiment of the present disclosure. OFDM symbol processing block 230 comprises P delay elements, including exemplary delay elements 311 and 312, P+1 amplifiers, including exemplary amplifiers 321, 322 and 323, and P+l transmit antennas, including exemplary antennas 331, 332 and 333. Delay elements 311 and 312 are arbitrarily labeled "D1" and "DP", respectively. OFDM symbol processing block 230 receives incoming ODFM symbols and forwards P+l copies of each OFDM symbol to the P+l transmit antennas. Each OFDM symbol comprises N+G samples, where N is the number of samples in the original data symbol and G is the number of samples in the cyclic prefix appended to the original symbol. A first copy of each OFDM symbol is applied directly to the input of amplifier 321, amplified by a gain value, gO, and sent to antenna 331. A second copy of each OFDM symbol is delayed by delay element 311, applied to the input of amplifier 322, amplified by a gain value, gl, and sent to antenna 332. Other copies of each OFDM symbol are similarly delayed and amplified according to the number of antennas. By way of example, the P+1 copy of each OFDM symbol is delayed by delay element 312, applied to the input of amplifier 323, amplified by a gain value, gP, and sent to antenna 333. The gain values and the delay values are determined by the values in an OFDM symbol processing parameter set, as described hereafter and in U.S. Patent Application Serial No. 11/327,799. The result is that multiple copies of each OFDM are transmitted, wherein each copy of an OFDM symbol is amplified by a selected amount and delayed by a selected amount relative to other OFDM symbol copies. U.S. Patent Application Serial No. 11/327,799, incorporated by reference above, describes a number of architectures for OFDM symbol processing block 230 that achieve such a result. In an advantageous embodiment, the delays introduced by OFDM symbol processing block 230 are cyclic delays, as disclosed in U.S. Patent Application Serial No. 11/327,799. FIGURE 4A illustrates data traffic transmitted in the downlink from base station 102 to subscriber station 116 according to an exemplary embodiment of the present disclosure. An exemplary frame of OFDM data is 10 milliseconds in length and comprises fifteen (15) transmit time intervals (TTIs), namely TTI 1 through TTI 15, where each one of TTI 1 through TTI 15 is 0.667 milliseconds in duration. Within each of TTI 2 through TTI 15, there are four OFDM data symbols, where each OFDM data symbol is 0.1667 milliseconds in duration. In the first TTI, namely TTI 1, there are three OFDM data symbols that are preceded by a pilot preamble symbol. The pilot preamble symbol is used by SS 116 to perform synchronization channel estimation and to determine the OFDM symbol processing parameter set. FIGURE 4B is a flow diagram illustrating the determination of the user channel type based on the measurements on the preamble according to an exemplary embodiment of the disclosure. In an OFDM system, a known pilot sequence is transmitted for one or more OFDM symbol durations. Channel estimation block 285 in the receiver (i.e., SS 116) detects the known pilot signal, which is then use to perform synchronization (process step 410). Channel estimation block 285 uses the detected preamble symbols to determine the degree of multipath effects in the channel and, therefore, the frequency selectivity in the channel between BS 102 and SS 116 (process step 420). Based on the profile of the channel, channel estimation block 285 (or another processing element or controller in SS 116) determines (i.e., calculates) a set of OFDM symbol processing parameters (i.e., gain values and delay values) that may be used in BS 102 to improve reception of OFDM symbols in SS 116 (process step 430). SS 116 then feeds back the OFDM symbol processing parameter set to BS 102 in the uplink (process step 440). Other factors, such as mobile speed, can also be used in determining (or calculating) the OFDM symbol processing parameters. The channel type may also be determined by using other mechanisms, such as reference in time-frequency. In this manner, BS 102 receives an OFDM symbol processing parameter set from each subscriber station. Thereafter, as BS 102 schedules each subscriber station to receive data, BS 102 uses the OFDM symbol processing parameter set for that subscriber station to modify the OFDM symbols transmitted from each antenna for BS 102. For example, BS 102 may use OFDM Symbol Processing Parameter Set A to transmit OFDM symbols from two or more antennas to SS 116 and may use OFDM Symbol Processing Parameter Set B to simultaneously transmit OFDM symbols from two or more antennas to SS 115. FIGURE 5 is a message flow diagram illustrating the transmission of OFDM symbols from base station 102 to subscriber station 116 according to one embodiment of the disclosure. In this example, base station 102 uses two transmit antennas (first antenna ANT1 and second antenna ANT 2) to transmit to SS 116. SSI 16 receives a first pilot signal (Pilot 1) from antenna ANT1 and receives a second pilot signal (Pilot2) from antenna ANT 2. SS A then determines OFDM Symbol Processing Parameter Set A as described above in FIGURES 4A and 4B. Next, SS 116 transmits OFDM Symbol Processing Parameter Set A to BS 102 in signal 505. Thereafter, BS 102 uses OFDM Symbol Processing Parameter Set A to transmit OFDM data symbols in the downlink back to SS 116. As noted above, the OFDM symbol processing parameters in Parameter Set A consist of symbol delays and gains from the two antennas. By way of example, in signal 520, BS 102 transmits from ANT1 processed OFDM symbols that were processed using Parameter Set A. In signal 525, BS 102 simultaneously transmits from ANT2 processed OFDM symbols that were processed using Parameter Set A. BS 102 also simultaneously transmits pilot signal 510 (Pilotl) and pilot signal 515 (Pilot2) from the two transmit antennas, ANT 1 and ANT 2. In the embodiment in FIGURE 5, Pilotl and Pilot2 are not processed using the parameters in OFDM Symbol Processing Parameter Set A. This is due to the fact that another transmission may be scheduled at the same time for another subscriber station on other OFDM subcarriers using a different set of OFDM symbol processing parameters. The pilot signals must be correctly understood by all the subscriber stations scheduled in the cell, so the pilot signals are not modified using OFDM Symbol Processing Parameter Set A. FIGURE 6 is a flow diagram illustrating the processing of pilot signals and OFDM data symbols according to an exemplary embodiment of the present disclosure. Because the OFDM symbols in signals 520 and 525 are processed using the values in OFDM Symbol Processing Parameter Set A, signals 520 and 525 are combined during transmission over the radio link in such a way that single OFDM symbols are received in SS 116 from BS 102 (process step 650). Since pilot signals 510 and 515 (Pilotl and Pilot2) are transmitted on orthogonal subcarriers from antenna ANT1 and antenna ANT2, pilot signals 510 and 515 are received separately at SS 116 (process steps 605 and 610). In order to get correct channel estimation for demodulation, SS 116 compensates pilot signals 510 and 515 (Pilotl and Pilot2) from antennas ATN1 and ANT2 using the Parameter Set A received from channel estimation block 285 (process steps 615 and 620). Compensated pilot signals 510 and 515 are then combined (process step 630) and the overall channel estimate is obtained (process step 640). This overall channel estimate is then used by demodulator 280 to demodulate the processed data symbols carried in the OFDM subcarriers (process step 660). FIGURE 7 is a message flow diagram illustrating the transmission of OFDM symbols from base station 102 to subscriber station 116 according to another embodiment of the disclosure. In FIGURE 7, the OFDM symbol processing parameters are determined in base station (BS) 102, rather than in subscriber station (SS) 116. BS 102 may determine (or estimate) the OFDM symbol processing parameters in Parameter Set A from a number of different uplink signals 705 transmitted by SS 116, including pilot signals 705, preamble signals 705 and/or data signals 705 from SS 116. In this example, since Pilotl signal 710 and Pilot2 signal 715 are not processed using Parameter Set A, BS 102 transmits OFDM Symbol Processing Parameter Set A to SS 116 in control message 720. SS 116 then uses the OFDM symbol processing parameters as described in FIGURES 2-6. BS 102 transmits processed OFDM symbols 725 from ANT1 and processed symbols 730 from ANT2 using the gain and delay values in Parameter Set A. SS 116 uses the same gain and delay parameters in control message 720 to compensate the pilots and to perform the overall channel estimation for data demodulation. FIGURE 8 is a message flow diagram illustrating the transmission of OFDM symbols from base station 102 to subscriber station 116 according to another embodiment of the disclosure. Similar to FIGURE 7, the OFDM symbol processing parameters in FIGURE 8 are again determined in base station (BS) 102 for the case of two transmit antennas, rather than in subscriber station (SS) 116. BS 102 may determine (or estimate) the OFDM symbol processing parameters in Parameter Set A from a number of different uplink signals 805 transmitted by SS 116, including pilot signals 805, preamble signals 805 and/or data signals 805 from SS 116. However, unlike FIGURE 7, Pilotl signal 810 from ANT1 and Pilot2 signal 815 from ANT2 are processed using Parameter Set A. In this case, the Pilotl signal and the Pilot2 signal both use the same OFDM subcarriers. In other words, the two pilots are not transmitted on orthogonal subcarriers. Therefore, the Pilotl signal and the Pilot2 signal are received in SS 116 as a single signal that can be directly used for overall channel estimation. The channel estimates are then used for data demodulation. BS 102 also transmits processed OFDM symbols 825 from ANT1 and processed symbols 830 from ANT2 using the gain and delay values in Parameter Set A. In a scenario where the Pilotl signal and the Pilot2 signal are not compensated, the channel quality estimate is based on the pilot signal strengths SS 116 receives from the two transmit antennas, ANT1 and ANT2. SS 116 compensates the Pilotl signal and the Pilot2 signal using the OFDM symbol processing parameters. This gives an estimate of the expected channel quality when BS 102 transmits OFDM symbols using the OFDM symbol processing parameters for SS 116. SS 116 then transmits a channel quality estimate (CQE) message back to BS 102. BS 102 determines an optimum data rate based on the channel quality estimate (CQE) message from SS 116 and then transmits processed OFDM symbols at that data rate. In SS 116, processed OFDM symbols containing data are processed using gain gO from ANT1, gain gl from ANT2 and delay Dl from ANT 2. These operations reverse the operations in OFDM symbol processing block 230 in FIGURE 3, assuming only transmit antenna 331 (i.e., ANT1) and transmits antenna 332 (i.e., ANT2) are used. In SS 116, an FFT operation is performed on the received OFDM symbols in order to retrieve the information in the frequency domain. The data and pilot symbols carried on orthogonal subcarriers are separated in the frequency domain. The pilot signals are converted back to the time domain by performing an IFFT operation. In this process, the subcarriers carrying data are set to 0. Also, when an ANT1 OFDM symbol is generated, the subcarriers carrying ANT2 OFDM symbols are set to 0. Similarly, when an ANT2 OFDM symbol is generated, the subcarriers carrying the ANT1 OFDM symbols are set to 0. SS 116 multiplies the pilot OFDM symbols from ANT1 with gain gO and the pilot OFDM symbols from ANT2 with gain gl. The receiver also delays the pilots from ANT2 with delay D1. Again, these operations reverse the operations in OFDM symbol processing block 230 in FIGURE 3, assuming only transmit antenna 331 (i.e., ANT1) and transmits antenna 332 (i.e., ANT2) are used. The two resulting pilots are then combined to get the overall pilot. An FFT operation is performed on the overall pilot to get the overall channel response in the frequency domain. The channel estimate in the frequency domain is then used for data demodulation in the frequency domain. This additional compensation on the pilot signals allows for estimation of the additional processing done in BS 102 on the OFDM symbols containing data. The effect of the actual radio channel is also reflected in the overall channel estimate because the received pilot signals travel via the radio channel. The compensation needs to be done on the pilot symbols only, and not the data symbols, because the data symbols are already processed in BS 102. In an OFDM system, the pilot and data symbols are carried on OFDM subcarriers. Therefore, the compensation can either be done on the time domain OFDM symbol or directly in the frequency domain. In order to do compensation in the frequency domain, the affect of OFDM symbol delay in the time-domain must be accounted for in the frequency domain. In general, a time delay in the time domain translates into a phase rotation in the frequency domain. Therefore, the OFDM subcarriers carrying the pilot symbols may be appropriately phase rotated in the frequency domain to account for time delays. While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods may be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The exemplary embodiments disclosed are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. It is intended that the disclosure encompass all alternate forms within the scope of the appended claims along with their full scope of equivalents. We Claim: 1. A method of controlling downlink transmissions to a base station in a subscriber station capable of communicating with a base station of an orthogonal frequency division multiplexing (OFDM) network, comprising the steps of: receiving a first pilot signal from a first antenna of the base station; receiving a second pilot signal from a second antenna of the base station; estimating the channel between the base station and subscriber station based on the received first and second pilot signals; determining a set of OFDM symbol processing parameters based on the step of estimating the channel, wherein the OFDM symbol processing parameters are used by the base station to control the relative gains and the relative delays of OFDM symbols transmitted from the first and second antennas; transmitting the OFDM symbol processing parameter set to the base station; receiving in the subscriber station first processed OFDM symbols transmitted from the first antenna; and receiving in the subscriber station second processed OFDM symbols transmitted from the second antenna. 2. The method as claimed in Claim 1, wherein the step of determining is based on the multipath characteristics and the frequency selectivity characteristics of the channel. 3. The method as claimed in Claim 2, wherein the first processed OFDM symbols have been processed by the base station using the OFDM symbol processing parameters received from the subscriber station, and wherein the second processed OFDM symbols have been processed by the base station using the OFDM symbol processing parameters received from the subscriber station. 4. The method as claimed in Claim 3, comprising the step of demodulating the first and second processed OFDM symbols. 5. The method as claimed in Claim 3, comprising the steps of: receiving a first pilot signal from the first antenna; receiving a second pilot signal from the second antenna; compensating the first pilot signal according to the OFDM symbol processing parameters to generate a first compensated pilot signal; compensating the second pilot signal according to the OFDM symbol processing parameters to generate a second compensated pilot signal; and combining the first and second compensated pilot signals to generate a combined pilot signal. 6.The method as claimed in Claim 5, comprising the step of estimating the channel based on the combined pilot signal. 7. The method as claimed in Claim 6, comprising the step of demodulating the first and second processed OFDM symbols using the channel estimate from the step of further estimating the channel based on the combined pilot signal. 8. A subscriber station capable of communicating with a base station of an orthogonal frequency division multiplexing (OFDM) network, wherein the subscriber station comprises: receive path circuitry capable of receiving a first pilot signal from a first antenna of the base station, receiving a second pilot signal from a second antenna of the base station, receiving first processed OFDM symbols transmitted from the first antenna, and receiving second processed OFDM symbols transmitted from the second antenna; and channel estimating circuitry capable of estimating the channel between the base station and subscriber station based on the received first and second pilot signals and capable of determining a set of OFDM symbol processing parameters based on a channel quality estimate, wherein the OFDM symbol processing parameters are used by the base station to control the relative gains and the relative delays of OFDM symbols transmitted from the first and second antennas and wherein the subscriber station is capable of transmitting the OFDM symbol processing parameters to the base station. 9. The subscriber station as claimed in Claim 8, wherein the channel estimating circuitry determines the set of OFDM symbol processing parameters based on the multipath characteristics and the frequency selectivity characteristics of the channel. 10. The subscriber station as claimed in Claim 9, wherein the first and second OFDM symbols have been processed by the base station using the OFDM symbol processing parameters transmitted by the subscriber station. 11. The subscriber station as claimed in Claim 10, comprising demodulation circuitry capable of demodulating the first and second processed OFDM symbols. 12. The subscriber station as claimed in Claim 10, wherein the receive path circuitry is capable of receiving a first pilot signal from the first antenna and receiving a second pilot signal from the second antenna and wherein the channel estimating circuitry is capable of: compensating the first pilot signal according to the OFDM symbol processing parameters to generate a first compensated pilot signal; compensating the second pilot signal according to the OFDM symbol processing parameters to generate a second compensated pilot signal; and combining the first and second compensated pilot signals to generate a combined pilot signal. 13. The subscriber station as claimed in Claim 12, wherein the channel estimating circuitry is capable of estimating the channel based on the combined pilot signal. 14. The subscriber station as claimed in Claim 13, wherein the demodulation circuitry is capable of demodulating the first and second processed OFDM symbols using a channel estimate based on the combined pilot signal. 15. A base station in an orthogonal frequency division multiplexing (OFDM) network, comprising: receive path circuitry capable of receiving an uplink signal from a subscriber station, estimating the channel between the base station and subscriber station based on the received uplink signal, and determining a set of OFDM symbol processing parameters based on a channel quality estimate; and transmit path circuitry capable of using the OFDM symbol processing parameters to control the relative gains and the relative delays of processed OFDM symbols transmitted from a first antenna and a second antenna of the base station, wherein the base station is capable of transmitting the OFDM symbol processing parameters to the subscriber station. 16. The base station as claimed in Claim 15, wherein the OFDM symbol processing parameters are transmitted to the subscriber station by using control message. 17. The base station as claimed in Claim 15, wherein the set of OFDM symbol processing parameters are determined based on the multipath characteristics and the frequency selectivity characteristics of the channel. 18. The base station as claimed in Claim 15, wherein the uplink signal is one of: 1) a pilot signal transmitted by the subscriber station, 2) a preamble signal transmitted by the subscriber station, and 3) a data signal transmitted by the subscriber station. 19. A method of controlling downlink transmissions to the subscriber station in a base station of an orthogonal frequency division multiplexing (OFDM) network capable of communicating with a subscriber station, comprising the steps of: receiving an uplink signal from the subscriber station; estimating the channel between the base station and subscriber station based on the received uplink signal; determining a set of OFDM symbol processing parameters based on a channel quality estimate; using the OFDM symbol processing parameters to control the relative gains and the relative delays of processed OFDM symbols transmitted from a first antenna and a second antenna of the base station; and transmitting the OFDM symbol processing parameters to the subscriber station. 20. The method as claimed in Claim 19, wherein the OFDM symbol processing parameters are transmitted to the subscriber station by using control message. 21. The method as claimed in Claim 19, wherein the set of OFDM symbol processing parameters are based on the multipath characteristics and the frequency selectivity characteristics of the channel. 22.The method as claimed in Claim 19, wherein the uplink signal is one of: 1) a pilot signal transmitted by the subscriber station, 2) a preamble signal transmitted by the subscriber station, and 3) a data signal transmitted by the subscriber station. 23. The method as claimed in Claim 19, comprising the steps of: receiving in the subscriber station first processed OFDM symbols transmitted from the first antenna, wherein the first processed OFDM symbols have been processed by the base station using the OFDM symbol processing parameters; and receiving in the subscriber station second processed OFDM symbols transmitted from the second antenna, wherein the second processed OFDM symbols have been processed by the base station using the OFDM symbol processing parameters. 24. The method as claimed in Claim 23, comprising the step of demodulating the first and second processed OFDM symbols. ABSTRACT TITLE: "METHOD AND APPARATUS FOR CONTROLLING DOWNLINK TRANSMISSIONS BETWEEN SUBSCRIBER STATION AND BASE STATION IN AN ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING NETWORK" The invention relates to a method of controlling downlink transmissions to a subscriber station capable of communicating with a base station of an orthogonal frequency division multiplexing (OFDM) network. The method comprises the steps of: receiving a first pilot signal from a first base station antenna; receiving a second pilot signal from a second base station antenna; and estimating the channel between the base station and subscriber station based on the received first and second pilot signals. The method also comprises determining a set of OFDM symbol processing parameters based on the step of estimating the channel and transmitting the OFDM symbol processing parameters to the base station. The base station uses the OFDM symbol processing parameters to control the relative gains and the relative delays of OFDM symbols transmitted from the first and second antennas.

Full Text

SYSTEM AND METHOD FOR CHANNEL ESTIMATION IN A
DELAY DIVERSITY WIRELESS COMMUNICATION SYSTEM
TECHNICAL FIELD OF THE INVENTION
The present disclosure relates generally to wireless communications and, more
specifically, to an apparatus and method for performing channel estimation in an
orthogonal frequency division multiplexing (OFDM) network or an orthogonal
frequency division multiple access (OFDMA) network.
BACKGROUND OF THE INVENTION
Conventional orthogonal frequency division multiplexing (OFDM) networks
and orthogonal frequency division multiple access (OFDMA) network are able to
improve the reliability of the channel by spreading and/or coding data traffic and
control signals over multiple subcarriers (i.e., tones). However, if the channel is flat,
frequency diversity cannot be achieved. In order to overcome this, it is possible to
introduce artificial frequency diversity into the transmitted signal. A technique for
artificially introducing frequency diversity into an OFDM environment was disclosed
in U.S Patent Application Serial No. 11/327,799, filed on January 6, 2006 and
incorporated by reference above. In the device disclosed in Serial No. 11/327,799,
multiple copies of the same OFDM symbol are delayed by different delay values, then
amplified by the same or different gain values, and then transmitted from different
antennas. This artificially introduces frequency-selective fading in the OFDM
channel, thereby allowing frequency selectivity to be exploited using
frequency-domain scheduling for low-to-medium speed mobile devices or frequency
diversity for higher speed mobile devices.
However, when selecting the symbol processing parameters (i.e., delay values
and the gain values) applied to the OFDM symbols, it is important to take into
consideration the user channel type and the mobile speed. To accomplish this, channel
estimation is performed and the symbol processing parameters are determined based
on the channel estimates and mobile speed. Therefore, there is a need for improved
apparatuses and methods for performing channel estimation in an OFDM wireless
network that artificially introduces frequency diversity by delaying and amplifying
multiple copies of the same OFDM symbol and then transmitting the delayed and

amplified OFDM symbols from different transmit antennas.
SUMMARY OF THE INVENTION
A method of controlling downlink transmissions to a subscriber station is
provided for use in a subscriber station capable of communicating with a base station
of an orthogonal frequency division multiplexing (OFDM) network. The method
comprises the steps of: receiving a first pilot signal from a first antenna of the base
station; receiving a second pilot signal from a second antenna of the base station;
estimating the channel between the base station and subscriber station based on the
received first and second pilot signals; determining a set of OFDM symbol processing
parameters based on the step of estimating the channel, wherein the OFDM symbol
processing parameters are usable by the base station to control the relative gains and
the relative delays of OFDM symbols transmitted from the first and second antennas;
and transmitting the OFDM symbol processing parameter set to the base station.
According to another embodiment of the present disclosure, a subscriber station
capable of communicating with a base station of an orthogonal frequency division
multiplexing (OFDM) network is provided. The subscriber station comprises: receive
path circuitry capable of receiving a first pilot signal from a first antenna of the base
station and receiving a second pilot signal from a second antenna of the base station;
and channel estimating circuitry capable of estimating the channel between the base
station and subscriber station based on the received first and second pilot signals and
capable of determining a set of OFDM symbol processing parameters based on a
channel quality estimate. The OFDM symbol processing parameters are usable by the
base station to control the relative gains and the relative delays of OFDM symbols
transmitted from the first and second antennas and wherein the subscriber station is
capable of transmitting the OFDM symbol processing parameters to the base station.
According to yet another embodiment of the present disclosure, a base station is
provided for use in an orthogonal frequency division multiplexing (OFDM) network.
The base station comprises: 1) receive path circuitry capable of receiving an uplink
signal from a subscriber station, estimating the channel between the base station and
subscriber station based on the received uplink signal, and determining a set of OFDM
symbol processing parameters based on a channel quality estimate; and 2) transmit
path circuitry capable of using the OFDM symbol processing parameters to control the
relative gains and the relative delays of processed OFDM symbols transmitted from a

first antenna and a second antenna of the base station. The base station is capable of
transmitting the OFDM symbol processing parameters to the subscriber station. The
OFDM symbol processing parameters are based on the multipath characteristics and
the frequency selectivity characteristics of the channel.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION
below, it may be advantageous to set forth definitions of certain words and phrases
used throughout this patent document: the terms "include" and "comprise," as well as
derivatives thereof, mean inclusion without limitation; the term "or," is inclusive,
meaning and/or; the term "each" means every one of at least a subset of the identified
items; the phrases "associated with" and "associated therewith," as well as derivatives
thereof, may mean to include, be included within, interconnect with, contain, be
contained within, connect to or with, couple to or with, be communicable with,
cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have
a property of, or the like; and the term "controller" means any device, system or part
thereof that controls at least one operation, such a device may be implemented in
hardware, firmware or software, or some combination of at least two of the same. It
should be noted that the functionality associated with any particular controller may be
centralized or distributed, whether locally or remotely. Definitions for certain words
and phrases are provided throughout this patent document, those of ordinary skill in
the art should understand that in many, if not most instances, such definitions apply to
prior, as well as future uses of such defined words and phrases.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present disclosure and its advantages,
reference is now made to the following description taken in conjunction with the
accompanying drawings, in which like reference numerals represent like parts:
FIGURE 1 illustrates an exemplary orthogonal frequency division multiplexing
(OFDM) wireless network that is capable of performing channel estimation according
to the principles of the present disclosure;
FIGURE 2A is a high-level diagram of the orthogonal frequency division
multiplexing (OFDM) transmit path in a base station according to one embodiment of
the disclosure;
FIGURE 2B is a high-level diagram of the orthogonal frequency division
multiplexing (OFDM) receive path in a subscriber station according to one

embodiment of the disclosure;
FIGURE 3 illustrates the OFDM symbol processing block in the base station in
greater detail according to an exemplary embodiment of the present disclosure;
FIGURE 4A illustrates data traffic transmitted in the downlink from a base
station to a subscriber station according to an exemplary embodiment of the present
disclosure;
FIGURE 4B is a flow diagram illustrating the determination of the user channel
type based on the measurements on the preamble according to an exemplary
embodiment of the disclosure;
FIGURE 5 is a message flow diagram illustrating the transmission of OFDM
symbols from a base station to a subscriber station according to the principles of the
disclosure;
FIGURE 6 is a flow diagram illustrating the processing of pilot signals and
OFDM data symbols according to an exemplary embodiment of the present
disclosure;
FIGURE 7 is a message flow diagram illustrating the transmission of OFDM
symbols from a base station to a subscriber station according to another embodiment
of the disclosure; and
FIGURE 8 is a message flow diagram illustrating the transmission of OFDM
symbols from a base station to a subscriber station according to another embodiment
of the disclosure.
DETAILED DESCRIPTION OF THE INVENTION
FIGURES 1 through 8, discussed below, and the various embodiments used to
describe the principles of the present disclosure in this patent document are by way of
illustration only and should not be construed in any way to limit the scope of the
disclosure. Those skilled in the art will understand that the principles of the present
disclosure may be implemented in any suitably arranged wireless network.
The present disclosure is directed to apparatuses and algorithms for channel
estimation and channel quality estimation for demodulation and data rate selection in
an orthogonal frequency division multiplexing (OFDM) wireless network that uses
delayed diversity. Such a delayed diversity wireless network was disclosed previously
U.S Patent Application Serial No. 11/327,799, incorporated by reference above. The
present disclosure uses a number of factors, including user channel type and mobile

speed, to select OFDM symbol processing parameters (i.e., delays D1, D2,..., DP and
gains g0, g1, ..., gP) for OFDM symbols transmitted from up to P antennas (i.e., ANT1
to ANTP). Therefore, different OFDM symbol processing parameters may be used to
transmit to different mobile devices that are scheduled simultaneously, depending
upon their channel types.
It is noted that the scope of the present disclosure is not limited to orthogonal
frequency division multiplexing (OFDM) wireless networks. The present disclosure
is also applicable to orthogonal frequency division multiple access (OFDMA) wireless
networks. However, for simplicity and brevity, the embodiments described below are
directed to OFDM wireless networks, except where otherwise noted or where the
context indicates otherwise.
For relatively low-speed mobile devices, it is usually possible to track changes in
the channel, thereby allowing channel sensitive scheduling to improve performance.
Thus, the OFDM symbol processing parameters may be selected in such a way that
relatively large coherence bandwidth results. That is, a relatively larger number of
subcarriers experience similar fading. This goal may be achieved by keeping the
delays for OFDM symbols from different antennas relatively small. A mobile device
may then be scheduled on a subband consisting of contiguous subcarriers.
For relatively high-speed mobile devices, channel quality variations cannot be
tracked accurately, so that frequency-diversity may be helpful. Thus, the OFDM
symbol processing parameters are selected in such a way that relatively small
coherence bandwidth results. That is, potentially independent fading may occur from
subcarrier to subcarrier. This goal may be achieved by having relatively large delays
for OFDM symbols transmitted from different antennas.
The symbol processing parameters may also be selected based on the degree of
frequency-selectivity already present in the channel. For example, if a channel already
has a lot of multipath effects and is, therefore, frequency selective, there may be little
or no need for additional frequency selectivity. The OFDM symbol processing
parameters may be selected on a user-by-user basis because different mobile devices
experience different channel types.
FIGURE 1 illustrates exemplary orthogonal frequency division multiplexing
(OFDM) wireless network 100, which is capable of performing channel estimation
according to the principles of the present disclosure. In the illustrated embodiment,
wireless network 100 includes base station (BS) 101, base station (BS) 102, base
station (BS) 103, and other similar base stations (not shown). Base station 101 is in

communication with base station 102 and base station 103. Base station 101 is also in
communication with Internet 130 or a similar IP-based network (not shown).
Base station 102 provides wireless broadband access (via base station 101) to
Internet 130 to a first plurality of subscriber stations within coverage area 120 of base
station 102. The first plurality of subscriber stations includes subscriber station 111,
which may be located in a small business (SB), subscriber station 112, which may be
located in an enterprise (E), subscriber station 113, which may be located in a WiFi
hotspot (HS), subscriber station 114, which may be located in a first residence (R),
subscriber station 115, which may be located in a second residence (R), and subscriber
station 116, which may be a mobile device (M), such as a cell phone, a wireless laptop,
a wireless PDA, or the like.
Base station 103 provides wireless broadband access (via base station 101) to
Internet 130 to a second plurality of subscriber stations within coverage area 125 of
base station 103. The second plurality of subscriber stations includes subscriber
station 115 and subscriber station 116. In an exemplary embodiment, base stations
101-103 may communicate with each other and with subscriber stations 111-116 using
OFDM or OFDMA techniques.
Base station 101 may be in communication with either a greater number or a
lesser number of base stations. Furthermore, while only six subscriber stations are
depicted in FIGURE 1, it is understood that wireless network 100 may provide
wireless broadband access to additional subscriber stations. It is noted that subscriber
station 115 and subscriber station 116 are located on the edges of both coverage area
120 and coverage area 125. Subscriber station 115 and subscriber station 116 each
communicate with both base station 102 and base station 103 and may be said to be
operating in handoff mode, as known to those of skill in the art.
Subscriber stations 111-116 may access voice, data, video, video conferencing,
and/or other broadband services via Internet 130. In an exemplary embodiment, one
or more of subscriber stations 111-116 may be associated with an access point (AP) of
a WiFi WLAN. Subscriber station 116 may be any of a number of mobile devices,
including a wireless-enabled laptop computer, personal data assistant, notebook,
handheld device, or other wireless-enabled device. Subscriber stations 114 and 115
may be, for example, a wireless-enabled personal computer (PC), a laptop computer,
a gateway, or another device.
FIGURE 2A is a high-level diagram of the transmit path in orthogonal frequency
division multiplexing (OFDM) transmitter 200 according to an exemplary

embodiment of the disclosure. FIGURE 2B is a high-level diagram of the receive path
in orthogonal frequency division multiplexing (OFDM) receiver 250 according to an
exemplary embodiment of the disclosure. OFDM transmitter 200 comprises
quadrature amplitude modulation (QAM) modulator 205, serial-to-parallel (S-to-P)
block 210, Inverse Fast Fourier Transform (IFFT) block 215, parallel-to-serial
(P-to-S) block 220, add cyclic prefix block 225, and OFDM symbol processing block
230. OFDM receiver 250 comprises remove cyclic prefix block 260, serial-to-parallel
(S-to-P) block 265, Fast Fourier Transform (FFT) block 270, parallel-to-serial
(P-to-S) block 275, quadrature amplitude modulation (QAM) demodulator 280, and
channel estimation block 285.
At least some of the components in FIGURES 2A and 2B may be implemented
in software while other components may be implemented by configurable hardware or
a mixture of software and configurable hardware. In particular, it is noted that the FFT
blocks and the IFFT blocks described in FIGURES 2A and 2B may be implemented
as configurable software algorithms, where the values of FFT and IFFT sizes may be
modified according to the implementation.
QAM modulator 205 receives a stream of input data and modulates the input bits
(or symbols) to produce a sequence of frequency-domain modulation symbols.
Serial-to-parallel block 210 converts (i.e., de-multiplexes) the serial QAM symbols to
parallel data to produce M parallel symbol streams where M is the IFFT/FFT size used
in OFDM transmitter 200 and OFDM receiver 250. IFFT block 215 then performs an
IFFT operation on the M parallel symbol streams to produce time-domain output
signals. Parallel-to-serial block 220 converts (i.e., multiplexes) the parallel
time-domain output symbols from IFFT block 215 to produce a serial time-domain
signal.
Add cyclic prefix block 225 then inserts a cyclic prefix to each OFDM symbol in
the time-domain signal. As is well known, the cyclic prefix is generated by copying
the last G samples of an N sample OFDM symbol and appending the copied G samples
to the front of the OFDM symbol. Finally, OFDM symbol processing block 230
processes the incoming OFDM symbols as described in FIGURE 3 and as described in
U.S Patent Application Serial No. 11/327,799. The process OFDM samples at the
output of OFDM symbol processing block 230 are then sent to up-conversion circuitry
(not shown) prior to being transmitted from multiples transmit antennas.
The transmitted RF signal arrives at OFDM receiver 250 after passing through
the wireless channel and reverse operations to those in OFDM transmitter 200 are

performed. Remove cyclic prefix block 260 removes the cyclic prefix to produce the
serial time-domain baseband signal. Serial-to-parallel block 265 converts the
time-domain baseband signal to parallel time domain signals. FFT block 270 then
performs an FFT algorithm to produce M parallel frequency-domain signals.
Parallel-to-serial block 275 converts the parallel frequency-domain signals to a
sequence of QAM data symbols. QAM demodulator 280 then demodulates the QAM
symbols to recover the original input data stream. Channel estimation block 285 also
receives the QAM data symbols from parallel-to-serial block 275 and performs
channels estimation. As will be described below in greater detail, the channel
estimation values are used to determine a parameter set of gain values and delay values
that are used in OFDM symbol processing block 230 in OFDM transmitter 200 and are
used by QAM demodulator 280 to demodulate the QAM data symbols.
The exemplary transmit path of OFDM transmitter 200 may be representative
of the transmit paths of any one of base stations 101-103 or any one of subscriber
stations 111-116. Similarly, the exemplary receive path of OFDM receiver 250 may
be representative of the transmit paths of any one of base stations 101-103 or any one
of subscriber stations 111-116. However, since multiple antenna configurations are
more common in base stations than in subscriber stations or other mobile devices, for
the sake of simplicity and clarity, the descriptions that follow will be directed toward
transactions between a base station (e.g., BS 102) that implements a transmit path
similar to OFDM transmitter 200 and a subscriber station (e.g., SS 116) that
implements a receive path similar to OFDM receiver 250. However, such an
exemplary embodiment should not be construed to limit the scope of the present
disclosure. It will be appreciated by those skilled in the art that in cases where
multiple antennas are implemented in a subscriber station, the transmit path and the
receiver path of both the base station and the subscriber station may be implemented
as in shown in FIGURES 2A and 2B.
FIGURE 3 illustrates OFDM symbol processing block 230 in greater detail
according to an exemplary embodiment of the present disclosure. OFDM symbol
processing block 230 comprises P delay elements, including exemplary delay
elements 311 and 312, P+1 amplifiers, including exemplary amplifiers 321, 322 and
323, and P+l transmit antennas, including exemplary antennas 331, 332 and 333.
Delay elements 311 and 312 are arbitrarily labeled "D1" and "DP", respectively.
OFDM symbol processing block 230 receives incoming ODFM symbols and forwards
P+l copies of each OFDM symbol to the P+l transmit antennas. Each OFDM symbol

comprises N+G samples, where N is the number of samples in the original data
symbol and G is the number of samples in the cyclic prefix appended to the original
symbol.
A first copy of each OFDM symbol is applied directly to the input of amplifier
321, amplified by a gain value, gO, and sent to antenna 331. A second copy of each
OFDM symbol is delayed by delay element 311, applied to the input of amplifier 322,
amplified by a gain value, gl, and sent to antenna 332. Other copies of each OFDM
symbol are similarly delayed and amplified according to the number of antennas. By
way of example, the P+1 copy of each OFDM symbol is delayed by delay element
312, applied to the input of amplifier 323, amplified by a gain value, gP, and sent to
antenna 333. The gain values and the delay values are determined by the values in an
OFDM symbol processing parameter set, as described hereafter and in U.S. Patent
Application Serial No. 11/327,799. The result is that multiple copies of each OFDM
are transmitted, wherein each copy of an OFDM symbol is amplified by a selected
amount and delayed by a selected amount relative to other OFDM symbol copies.
U.S. Patent Application Serial No. 11/327,799, incorporated by reference above,
describes a number of architectures for OFDM symbol processing block 230 that
achieve such a result. In an advantageous embodiment, the delays introduced by
OFDM symbol processing block 230 are cyclic delays, as disclosed in U.S. Patent
Application Serial No. 11/327,799.
FIGURE 4A illustrates data traffic transmitted in the downlink from base
station 102 to subscriber station 116 according to an exemplary embodiment of the
present disclosure. An exemplary frame of OFDM data is 10 milliseconds in length
and comprises fifteen (15) transmit time intervals (TTIs), namely TTI 1 through TTI
15, where each one of TTI 1 through TTI 15 is 0.667 milliseconds in duration. Within
each of TTI 2 through TTI 15, there are four OFDM data symbols, where each OFDM
data symbol is 0.1667 milliseconds in duration. In the first TTI, namely TTI 1, there
are three OFDM data symbols that are preceded by a pilot preamble symbol. The pilot
preamble symbol is used by SS 116 to perform synchronization channel estimation
and to determine the OFDM symbol processing parameter set.
FIGURE 4B is a flow diagram illustrating the determination of the user
channel type based on the measurements on the preamble according to an exemplary
embodiment of the disclosure. In an OFDM system, a known pilot sequence is
transmitted for one or more OFDM symbol durations. Channel estimation block 285
in the receiver (i.e., SS 116) detects the known pilot signal, which is then use to

perform synchronization (process step 410). Channel estimation block 285 uses the
detected preamble symbols to determine the degree of multipath effects in the channel
and, therefore, the frequency selectivity in the channel between BS 102 and SS 116
(process step 420).
Based on the profile of the channel, channel estimation block 285 (or another
processing element or controller in SS 116) determines (i.e., calculates) a set of
OFDM symbol processing parameters (i.e., gain values and delay values) that may be
used in BS 102 to improve reception of OFDM symbols in SS 116 (process step 430).
SS 116 then feeds back the OFDM symbol processing parameter set to BS 102 in the
uplink (process step 440). Other factors, such as mobile speed, can also be used in
determining (or calculating) the OFDM symbol processing parameters. The channel
type may also be determined by using other mechanisms, such as reference in
time-frequency.
In this manner, BS 102 receives an OFDM symbol processing parameter set
from each subscriber station. Thereafter, as BS 102 schedules each subscriber station
to receive data, BS 102 uses the OFDM symbol processing parameter set for that
subscriber station to modify the OFDM symbols transmitted from each antenna for BS
102. For example, BS 102 may use OFDM Symbol Processing Parameter Set A to
transmit OFDM symbols from two or more antennas to SS 116 and may use OFDM
Symbol Processing Parameter Set B to simultaneously transmit OFDM symbols from
two or more antennas to SS 115.
FIGURE 5 is a message flow diagram illustrating the transmission of OFDM
symbols from base station 102 to subscriber station 116 according to one embodiment
of the disclosure. In this example, base station 102 uses two transmit antennas (first
antenna ANT1 and second antenna ANT 2) to transmit to SS 116. SSI 16 receives a
first pilot signal (Pilot 1) from antenna ANT1 and receives a second pilot signal
(Pilot2) from antenna ANT 2. SS A then determines OFDM Symbol Processing
Parameter Set A as described above in FIGURES 4A and 4B.
Next, SS 116 transmits OFDM Symbol Processing Parameter Set A to BS 102
in signal 505. Thereafter, BS 102 uses OFDM Symbol Processing Parameter Set A to
transmit OFDM data symbols in the downlink back to SS 116. As noted above, the
OFDM symbol processing parameters in Parameter Set A consist of symbol delays
and gains from the two antennas. By way of example, in signal 520, BS 102 transmits
from ANT1 processed OFDM symbols that were processed using Parameter Set A. In
signal 525, BS 102 simultaneously transmits from ANT2 processed OFDM symbols

that were processed using Parameter Set A.
BS 102 also simultaneously transmits pilot signal 510 (Pilotl) and pilot signal
515 (Pilot2) from the two transmit antennas, ANT 1 and ANT 2. In the embodiment
in FIGURE 5, Pilotl and Pilot2 are not processed using the parameters in OFDM
Symbol Processing Parameter Set A. This is due to the fact that another transmission
may be scheduled at the same time for another subscriber station on other OFDM
subcarriers using a different set of OFDM symbol processing parameters. The pilot
signals must be correctly understood by all the subscriber stations scheduled in the
cell, so the pilot signals are not modified using OFDM Symbol Processing Parameter
Set A.
FIGURE 6 is a flow diagram illustrating the processing of pilot signals and
OFDM data symbols according to an exemplary embodiment of the present
disclosure. Because the OFDM symbols in signals 520 and 525 are processed using
the values in OFDM Symbol Processing Parameter Set A, signals 520 and 525 are
combined during transmission over the radio link in such a way that single OFDM
symbols are received in SS 116 from BS 102 (process step 650). Since pilot signals
510 and 515 (Pilotl and Pilot2) are transmitted on orthogonal subcarriers from
antenna ANT1 and antenna ANT2, pilot signals 510 and 515 are received separately
at SS 116 (process steps 605 and 610).
In order to get correct channel estimation for demodulation, SS 116
compensates pilot signals 510 and 515 (Pilotl and Pilot2) from antennas ATN1 and
ANT2 using the Parameter Set A received from channel estimation block 285 (process
steps 615 and 620). Compensated pilot signals 510 and 515 are then combined
(process step 630) and the overall channel estimate is obtained (process step 640).
This overall channel estimate is then used by demodulator 280 to demodulate the
processed data symbols carried in the OFDM subcarriers (process step 660).
FIGURE 7 is a message flow diagram illustrating the transmission of OFDM
symbols from base station 102 to subscriber station 116 according to another
embodiment of the disclosure. In FIGURE 7, the OFDM symbol processing
parameters are determined in base station (BS) 102, rather than in subscriber station
(SS) 116. BS 102 may determine (or estimate) the OFDM symbol processing
parameters in Parameter Set A from a number of different uplink signals 705
transmitted by SS 116, including pilot signals 705, preamble signals 705 and/or data
signals 705 from SS 116.
In this example, since Pilotl signal 710 and Pilot2 signal 715 are not processed

using Parameter Set A, BS 102 transmits OFDM Symbol Processing Parameter Set A
to SS 116 in control message 720. SS 116 then uses the OFDM symbol processing
parameters as described in FIGURES 2-6. BS 102 transmits processed OFDM
symbols 725 from ANT1 and processed symbols 730 from ANT2 using the gain and
delay values in Parameter Set A. SS 116 uses the same gain and delay parameters in
control message 720 to compensate the pilots and to perform the overall channel
estimation for data demodulation.
FIGURE 8 is a message flow diagram illustrating the transmission of OFDM
symbols from base station 102 to subscriber station 116 according to another
embodiment of the disclosure. Similar to FIGURE 7, the OFDM symbol processing
parameters in FIGURE 8 are again determined in base station (BS) 102 for the case of
two transmit antennas, rather than in subscriber station (SS) 116. BS 102 may
determine (or estimate) the OFDM symbol processing parameters in Parameter Set A
from a number of different uplink signals 805 transmitted by SS 116, including pilot
signals 805, preamble signals 805 and/or data signals 805 from SS 116.
However, unlike FIGURE 7, Pilotl signal 810 from ANT1 and Pilot2 signal
815 from ANT2 are processed using Parameter Set A. In this case, the Pilotl signal
and the Pilot2 signal both use the same OFDM subcarriers. In other words, the two
pilots are not transmitted on orthogonal subcarriers. Therefore, the Pilotl signal and
the Pilot2 signal are received in SS 116 as a single signal that can be directly used for
overall channel estimation. The channel estimates are then used for data
demodulation. BS 102 also transmits processed OFDM symbols 825 from ANT1 and
processed symbols 830 from ANT2 using the gain and delay values in Parameter Set
A.
In a scenario where the Pilotl signal and the Pilot2 signal are not compensated,
the channel quality estimate is based on the pilot signal strengths SS 116 receives from
the two transmit antennas, ANT1 and ANT2. SS 116 compensates the Pilotl signal
and the Pilot2 signal using the OFDM symbol processing parameters. This gives an
estimate of the expected channel quality when BS 102 transmits OFDM symbols using
the OFDM symbol processing parameters for SS 116. SS 116 then transmits a channel
quality estimate (CQE) message back to BS 102. BS 102 determines an optimum data
rate based on the channel quality estimate (CQE) message from SS 116 and then
transmits processed OFDM symbols at that data rate.
In SS 116, processed OFDM symbols containing data are processed using gain
gO from ANT1, gain gl from ANT2 and delay Dl from ANT 2. These operations

reverse the operations in OFDM symbol processing block 230 in FIGURE 3, assuming
only transmit antenna 331 (i.e., ANT1) and transmits antenna 332 (i.e., ANT2) are
used. In SS 116, an FFT operation is performed on the received OFDM symbols in
order to retrieve the information in the frequency domain. The data and pilot symbols
carried on orthogonal subcarriers are separated in the frequency domain. The pilot
signals are converted back to the time domain by performing an IFFT operation. In
this process, the subcarriers carrying data are set to 0. Also, when an ANT1 OFDM
symbol is generated, the subcarriers carrying ANT2 OFDM symbols are set to 0.
Similarly, when an ANT2 OFDM symbol is generated, the subcarriers carrying the
ANT1 OFDM symbols are set to 0.
SS 116 multiplies the pilot OFDM symbols from ANT1 with gain gO and the
pilot OFDM symbols from ANT2 with gain gl. The receiver also delays the pilots
from ANT2 with delay D1. Again, these operations reverse the operations in OFDM
symbol processing block 230 in FIGURE 3, assuming only transmit antenna 331 (i.e.,
ANT1) and transmits antenna 332 (i.e., ANT2) are used. The two resulting pilots are
then combined to get the overall pilot. An FFT operation is performed on the overall
pilot to get the overall channel response in the frequency domain. The channel
estimate in the frequency domain is then used for data demodulation in the frequency
domain. This additional compensation on the pilot signals allows for estimation of the
additional processing done in BS 102 on the OFDM symbols containing data. The
effect of the actual radio channel is also reflected in the overall channel estimate
because the received pilot signals travel via the radio channel.
The compensation needs to be done on the pilot symbols only, and not the data
symbols, because the data symbols are already processed in BS 102. In an OFDM
system, the pilot and data symbols are carried on OFDM subcarriers. Therefore, the
compensation can either be done on the time domain OFDM symbol or directly in the
frequency domain. In order to do compensation in the frequency domain, the affect of
OFDM symbol delay in the time-domain must be accounted for in the frequency
domain. In general, a time delay in the time domain translates into a phase rotation in
the frequency domain. Therefore, the OFDM subcarriers carrying the pilot symbols
may be appropriately phase rotated in the frequency domain to account for time
delays.
While several embodiments have been provided in the present disclosure, it
should be understood that the disclosed systems and methods may be embodied in
many other specific forms without departing from the spirit or scope of the present

disclosure. The exemplary embodiments disclosed are to be considered as illustrative
and not restrictive, and the intention is not to be limited to the details given herein. It
is intended that the disclosure encompass all alternate forms within the scope of the
appended claims along with their full scope of equivalents.

We Claim:
1. A method of controlling downlink transmissions to a base station in a
subscriber station capable of communicating with a base station of an orthogonal
frequency division multiplexing (OFDM) network, comprising the steps of:
receiving a first pilot signal from a first antenna of the base station;
receiving a second pilot signal from a second antenna of the base station;
estimating the channel between the base station and subscriber station
based on the received first and second pilot signals;
determining a set of OFDM symbol processing parameters based on the
step of estimating the channel, wherein the OFDM symbol processing
parameters are used by the base station to control the relative gains and the
relative delays of OFDM symbols transmitted from the first and second
antennas;
transmitting the OFDM symbol processing parameter set to the base
station;
receiving in the subscriber station first processed OFDM symbols
transmitted from the first antenna; and
receiving in the subscriber station second processed OFDM symbols
transmitted from the second antenna.

2. The method as claimed in Claim 1, wherein the step of determining
is based on the multipath characteristics and the frequency selectivity
characteristics of the channel.
3. The method as claimed in Claim 2, wherein the first processed
OFDM symbols have been processed by the base station using the OFDM
symbol processing parameters received from the subscriber station, and
wherein the second processed OFDM symbols have been processed by the
base station using the OFDM symbol processing parameters received from
the subscriber station.
4. The method as claimed in Claim 3, comprising the step of
demodulating the first and second processed OFDM symbols.
5. The method as claimed in Claim 3, comprising the steps of:
receiving a first pilot signal from the first antenna;
receiving a second pilot signal from the second antenna;
compensating the first pilot signal according to the OFDM symbol processing
parameters to generate a first compensated pilot signal;

compensating the second pilot signal according to the OFDM symbol
processing parameters to generate a second compensated pilot signal; and
combining the first and second compensated pilot signals to generate a
combined pilot signal.
6.The method as claimed in Claim 5, comprising the step of estimating the
channel based on the combined pilot signal.
7. The method as claimed in Claim 6, comprising the step of
demodulating the first and second processed OFDM symbols using the
channel estimate from the step of further estimating the channel based on
the combined pilot signal.
8. A subscriber station capable of communicating with a base station of an
orthogonal frequency division multiplexing (OFDM) network, wherein the
subscriber station comprises:
receive path circuitry capable of receiving a first pilot signal from a first
antenna of the base station, receiving a second pilot signal from a second
antenna of the base station, receiving first processed OFDM symbols
transmitted from the first antenna, and receiving second processed OFDM
symbols transmitted from the second antenna; and

channel estimating circuitry capable of estimating the channel between the
base station and subscriber station based on the received first and second
pilot signals and capable of determining a set of OFDM symbol processing
parameters based on a channel quality estimate,
wherein the OFDM symbol processing parameters are used by the base
station to control the relative gains and the relative delays of OFDM symbols
transmitted from the first and second antennas and wherein the subscriber
station is capable of transmitting the OFDM symbol processing parameters to
the base station.
9. The subscriber station as claimed in Claim 8, wherein the channel
estimating circuitry determines the set of OFDM symbol processing
parameters based on the multipath characteristics and the frequency
selectivity characteristics of the channel.
10. The subscriber station as claimed in Claim 9, wherein the first and
second OFDM symbols have been processed by the base station using the
OFDM symbol processing parameters transmitted by the subscriber station.

11. The subscriber station as claimed in Claim 10, comprising
demodulation circuitry capable of demodulating the first and second
processed OFDM symbols.
12. The subscriber station as claimed in Claim 10, wherein the receive
path circuitry is capable of receiving a first pilot signal from the first antenna
and receiving a second pilot signal from the second antenna and wherein the
channel estimating circuitry is capable of:
compensating the first pilot signal according to the OFDM symbol processing
parameters to generate a first compensated pilot signal;
compensating the second pilot signal according to the OFDM symbol
processing parameters to generate a second compensated pilot signal; and
combining the first and second compensated pilot signals to generate a
combined pilot signal.
13. The subscriber station as claimed in Claim 12, wherein the channel
estimating circuitry is capable of estimating the channel based on the
combined pilot signal.

14. The subscriber station as claimed in Claim 13, wherein the
demodulation circuitry is capable of demodulating the first and second
processed OFDM symbols using a channel estimate based on the combined
pilot signal.
15. A base station in an orthogonal frequency division multiplexing
(OFDM) network, comprising:
receive path circuitry capable of receiving an uplink signal from a subscriber
station, estimating the channel between the base station and subscriber
station based on the received uplink signal, and determining a set of OFDM
symbol processing parameters based on a channel quality estimate; and
transmit path circuitry capable of using the OFDM symbol processing
parameters to control the relative gains and the relative delays of processed
OFDM symbols transmitted from a first antenna and a second antenna of the
base station,
wherein the base station is capable of transmitting the OFDM symbol
processing parameters to the subscriber station.
16. The base station as claimed in Claim 15, wherein the OFDM symbol
processing parameters are transmitted to the subscriber station by using
control message.

17. The base station as claimed in Claim 15, wherein the set of OFDM
symbol processing parameters are determined based on the multipath
characteristics and the frequency selectivity characteristics of the channel.
18. The base station as claimed in Claim 15, wherein the uplink signal
is one of: 1) a pilot signal transmitted by the subscriber station, 2) a
preamble signal transmitted by the subscriber station, and 3) a data signal
transmitted by the subscriber station.
19. A method of controlling downlink transmissions to the subscriber
station in a base station of an orthogonal frequency division multiplexing
(OFDM) network capable of communicating with a subscriber station,
comprising the steps of:
receiving an uplink signal from the subscriber station;
estimating the channel between the base station and subscriber station
based on the received uplink signal;
determining a set of OFDM symbol processing parameters based on a
channel quality estimate;

using the OFDM symbol processing parameters to control the relative gains
and the relative delays of processed OFDM symbols transmitted from a first
antenna and a second antenna of the base station; and
transmitting the OFDM symbol processing parameters to the subscriber
station.
20. The method as claimed in Claim 19, wherein the OFDM symbol
processing parameters are transmitted to the subscriber station by using
control message.
21. The method as claimed in Claim 19, wherein the set of OFDM
symbol processing parameters are based on the multipath characteristics and
the frequency selectivity characteristics of the channel.
22.The method as claimed in Claim 19, wherein the uplink signal is one of: 1)
a pilot signal transmitted by the subscriber station, 2) a preamble signal
transmitted by the subscriber station, and 3) a data signal transmitted by the
subscriber station.

23. The method as claimed in Claim 19, comprising the steps of:
receiving in the subscriber station first processed OFDM symbols
transmitted from the first antenna, wherein the first processed OFDM symbols
have been processed by the base station using the OFDM symbol processing
parameters; and
receiving in the subscriber station second processed OFDM symbols
transmitted from the second antenna, wherein the second processed OFDM
symbols have been processed by the base station using the OFDM symbol
processing parameters.
24. The method as claimed in Claim 23, comprising the step of
demodulating the first and second processed OFDM symbols.

ABSTRACT

TITLE: "METHOD AND APPARATUS FOR CONTROLLING DOWNLINK
TRANSMISSIONS BETWEEN SUBSCRIBER STATION AND BASE STATION IN AN
ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING NETWORK"
The invention relates to a method of controlling downlink transmissions to a
subscriber station capable of communicating with a base station of an orthogonal
frequency division multiplexing (OFDM) network. The method comprises the
steps of: receiving a first pilot signal from a first base station antenna; receiving
a second pilot signal from a second base station antenna; and estimating the
channel between the base station and subscriber station based on the received
first and second pilot signals. The method also comprises determining a set of
OFDM symbol processing parameters based on the step of estimating the
channel and transmitting the OFDM symbol processing parameters to the base
station. The base station uses the OFDM symbol processing parameters to
control the relative gains and the relative delays of OFDM symbols transmitted
from the first and second antennas.

" METHOD AND APPARATUS FOR CONTROLLING DOWNLINK TRANSMISSION BETWWEN SUBSCRIBER STATION AND BASE STATION IN AN ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING NETWORK"

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