Title of Invention	INTERLEAVED ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (IOFDM) SYSTEM
Abstract	Disclosed herein is an Interleaved Orthogonal Frequency Division Multiplexing (IOFDM) system, wherein the code rate is increased without increasing the number of sub carriers by adding only one ZP block of length L samples to P OFDM blocks and the code rate ratio, R1/R, being greater than one and that the redundancy due to ZP is less by a factor of P.

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FIELD OF TECHNOLOGY This invention relates to the field of Communication Technology. Further this invention relates to the area of digital communication. More particularly this invention pertains to Interleaved Orthogonal Frequency Division Multiplexing (lOFDM) system for digital broadcasting. DESCRIPTION OF PRIOR ART In recent years, Orthogonal Frequency Division Multiplexing (OFDM) (also similar to Discrete-Multitone (DMT) has been adopted as a standard for applications like Wireless systems, Cable Modems, DSL modems. Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB) etc. As an example. Wireless systems (LANS, Mobile wireless systems, and fixed wireless systems) need to transmit data over wireless channels with very high data rates. An OFDM commimication system uses the technique of transmitting bits based on the principle of using many sub-carriers in a given bandwidth. Typically, the channels in the above mentioned applications have a response, which are called as Finite-Impulse Response (ITR) channels. The FIR nature of the channel can lead to a channel, which is frequency selective. The conventional technique of combating such channels is to implement an equalizer, which is computationally very expensive for such high data rates. The OFDM technique is equivalent to converting the frequency selective chatmel into a set of flat fading channels, which makes the task of channel equalization trivial [1]. Figure 1 presents the OFDM system. The bit stream is mapped to an information symbol sequence s(n) using a particular modulation scheme. The sequence s{n) is divided into blocks of length K. The symbol block sin) is then mapped to a preceded block u(n), of length K, through a precoder matrix C. The role of C is to effectively convert a frequency selective fading channel into a number of flat fading channels. Let us assume that the upper bound on the channel length is known. The block x(n), of length N=K+L, is formed by using u(/i)and L number of zeros. The process of adding zeros at the end is known as zero padding (ZP). K and N are called OFDM block length and transmitted OFDM block length, respectively. The sequence x(n) is then serially transmitted through a transmitting antenna. Because of ZP, the channel induced inter block interference (IBI) between the blocks u(_n) is avoided and one can focus at each received block separately. IBI is introduced due to the FIR nature of the channels. The nature of the chamiel is such that the received data at a given instant of time is the true data corrupted by scaled and delayed versions of the past data. Thus, IBI is undesirable and is tackled by channel equalization. At the receiver, the received sequence y(n) is divided into blocks, of length N. After evaluating the z-transform of y(n)at K points in z-domain, simple equalization and maximal likelihood detection are performed to recover the information symbol sequence ^f/ij[l]. In order to avoid inter block interference (IBI) between OFDM blocks arising due to the frequency selective nature of the channel, either cyclic prefix (CP) or zero padding (ZP) is added to each OFDM block before transmission [3], [4]. The length of the CP/ZP should be longer than the channel delay profile to avoid IBL The redundancy introduced due to the addition of the CP/ZP results in an overhead. For example, it means that for every K samples of user data, one needs to transmit K+L samples. The code rate, denoted by R, is defined as the ratio of usefid samples to the total samples transmitted, i.e. R= K / (K+L). The objective of the invention is to propose a new scheme, which would increase the code rate value without increasing the number of sub-carriers in one OFDM block. This would impact the overall useful data rate of a communication system resulting in improved system performance. PROPOSED SOLUTION AND BRIEF DESCRIPTION OF INVENTION The present invention proposes to change the transmission scheme by appending only one cyclic prefix block for every P OFDM blocks at a time, thereby increasing the code rate R, without increasing the peak-to-average ratio of conventional OFDM systems. In order to reduce the complexity of the solution, an interleaving scheme is proposed which reduces computational complexity. OBJECTS OF THE INVENTION It is the primary object of mvention to invent an Interleaved Orthogonal Frequency Division Multiplexing (lOFDM), which is unique. It is another object of the invention to invent a novel Interleaved Orthogonal Frequency Division Multiplexing (lOFDM), which will result in reduction in redundancy in the system. It is another object of invention to invent a novel Interleaved Orthogonal Frequency Division Multiplexing (lOFDM) with a new scheme, which would increase the code rate value without increasing the number of sub-carriers in one OFDM block. It is another object of the invention to invent a novel Interleaved Orthogonal Frequency Division Multiplexing (lOFDM) wherein, inter-block interference is avoided. Further it is another object of the invention to invent a novel Interleaved Orthogonal Frequency Division Multiplexing (lOFDM) wherein, there is no degradation in system"s performance compared to prior art. Further objects of die invention will be clear from the following description. BRIEF DESCRIPTION OF THE DRAWINGS Figiiie. 1 Block diagram of the conventional OFDM system Figure. 2 Code rate improvement factor F = Ri / R as a function of P for two examples of Wireless systems Figure. 3 Base band model of the transmitter section of lOIDM system. Figure. 4 Receiver section of lOFDM system. Figure. 5 Performance of lOFDM for a case correspondence to the indoor wireless system as per the HIPERLAN channel model. Figure. 6 Performance of lOFDM for the case correspondence to the GSM HT channel model. DETAILED DESCRIPTION This invention thus provides an Interleaved Orthogonal Frequency Division Multiplexing (lOIDM) system, wherein the code rate is increased without increasing the number of sub carriers by adding only one ZP block of length L samples to P OFDM blocks and the code rate ratio, R/ /R, being greater than one and that the redundancy due to ZP is less by a factor of P. The same will be described in detail with reference to drawings. The key idea is to process P OFDM blocks at a time and add only one ZP block of length L. If we denote the code rate of the lOFDM system as Ri. then Ri= PK / (PK+L). It is easy to show Uiat the code rate improvement factor F"^ R] /R > J. All interleaving schemes (for P>i> which use this principle will have the code rate greater than the code rate of the conventional OFDM system. Fig. 1. describes the OFDM based communication system, which form prior art. Here, s(n) denotes the signal in complex domain which represents a modulated signal at time instant n. The next block is the serial to parallel block, which takes K complex numbers in serial mode and converts them into a parallel vector of length K, denoted by s{n) (the underline denoting that it is a vector). The next block denoted by C takes the vector and multiplies by a matrix C of ^ rows and K colimms and yields a vector of K rows and one column, denoted by «^(n). The next block functions as a zero padding (ZP) block, which appends L zeros to the input vector and yields x_(n), which is of length N=K+L. The next block denoted by P/S is a parallel to serial converter which takes the vector of length N and converts into a sequence of N niraibers denoted by x(^n), which is then fed to the antenna Tx. The transmitted signal x(n) is then passed through a channel denoted by h(n). The receiver antenna Rx receives the signal y(n). The sequence is then passed through a block denoted by S/P, which takes the sequence and converts into a vector of length N denoted by y(n). This vector is then passed through a box, which multiplies this vector by a matrix V yielding ^(n) of length K. The next block multiples by a constant (1/a) and again by another matrix R"" which does the job of channel equalization removing the inter block interference and yielding a vector of length K denoted by ?(n). This vector is then passed through a detector to obtain the estimates of s(n), denoted by S(n). The vector passing through P/S converter results in the estimated complex data sequence. Figure.2 shows an example of the increase in the code rate as a function of P for two sets of values of K and L. It is clear from the said figure that the increase in code rate is significant. As an illustration, for the GSM example (P=4, K=256, L=IOO), die improvement is more than 30% Similarly, for the example of an indoor wireless system, (P=8, K=64. L-16}, die unprovement is more dian 20%. This lOFDM scheme will, therefore, enhance tiie data rates of Wireless systems significantly. The block diagram in Figure.3 describes the base band model of an lOFDM transmitter section. Here, s(n) denotes the signal in complex domain which represents a modulated signal at lime instant n. The next block is the serial to parallel block which takes K complex numbers in serial mode and converts them into a parallel vector of length K, denoted by s(n) (the underline denoting that it is a vector). This vector is then passed through P blocks in parallel. In other words, the same vector s(n) is passed through each of the P blocks denoted by D° , D^,... D^"". The matrix D is a diagonal matrix with its kth element equal to expij 27i(k-l)/P). The next block denoted by C takes the vector and multiplies by a matrix C of K rows and K columns and yields a vector of K rows and one column, denoted by u(n). The block x(n), of length M=PK+L, is formed by interleaving the elements of u(nP),...,u(nP+P-l} and padding L zeros at the end of x(n). Because of interleaving, we call it as lOFDM system. The lOFDM block length is PK and the transmitted lOFDM block length is M. The block diagram in Figure. 4 describes the base band model of an lOFDM receiver section. We assume that the channel impulse response is constant over the transmission of lOFDM block. At the receiver, the received sequence y Figures 5 and 6 present the performance of the lOFDM system for the wireless indoor channel case and the Global System for Mobile Communication (GSM) channel case respectively, for different values of P. P=l corresponds to the conventional OFDM case. It is easily seen that even by increasing the value of P, the performance does not degrade but the code rate increases. I CLAIM: 1. An Interleaved Orthogonal Frequency Ltivision Multiplexing (lOFDM) system, wherein the code rate is increased without increasing the number of sub carriers by adding only one ZP block of length L samples to P OFDM blocks and the code rate ratio, Ri /R, being greater than one and that the redundancy due to ZP is less by a factor of P. 2. The Interleaved orthogonal frequency division multiplexing system, wherein the code rate (R/) is also increased by varying the values of K (OFDM block length) and L (no of zeros at the end of the block) and P (blocks of OFDM). 3. The Interleaved orthogonal frequency division multiplexing system as claimed in preceding claims, wherein P consecutive symbol blocks are considered as s(nP),...,s(nP+P-l) and the same are mapped to another set of P consecutive blocks s"(nP),...,s"(nP+P-l). 4. The Interleaved orthogonal frequency division multiplexing system as claimed in preceding claims, wherein s"(n) is the effective symbol block, which is mapped to a preceded block u{n) through the precoder matrix C and the block x(n), of length M=PK+L, is formed by interleaving the elements of u(nP),...,u(nP+P-l) and padding L zeros at the end of xfn). 5. The Interleaved orthogonal frequency division multiplexing system as claimed in preceding claims, wberem the channel impulse response kept constant over the transmission of lOFDM block and at the receiver, the received sequence y(n) is divided into blocks, of length M and recovering the information symbol sequence s(n) after evaluating the z-transform of yfn) at PK points in z-domain, by performing simple equalization, IDFT operation and maximal likelihood detectiou. 6. A wireless indoor channel including an interleaved orthogonal frequency division multiplexing system as claimed in claims 1 to 5. 7. A GSM channel including an interleaved orthogonal frequency division multiplexing system as claimed in claims 1 to 6. 8. An interleaved orthogonal frequency division multiplexing system is herein before described and illustrated with the accompanying drawings.

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