Baseband OFDM System Block Diagram 

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... INTRODUCTION T he multiplexing principles (OFDM) of orthogonal modulation frequency have been division in existence for over forty years but the concept has only become popular in the last decade. The first proposal for OFDM was made in the early 1950’s and during the 1960s its theory was fully developed. Since then it has received tremendous attention from researchers and today, OFDM is a household name in the field of communication. The salient features of OFDM [1,2], have unsurprisingly made it the preferred modulation scheme for high data rate transmission schemes such as wireless LANs, WiMAX and LTE [3,4]. Its ability to mitigate multipath makes it suitable for outdoor mobile communications. OFDM has received great acclaim because of its advantageous features; these features however come along with some costs such as high Peak to Average Power Ratio (PAPR)[5]and inter carrier interference due to frequency offset issues [6, 7]. In OFDM systems instead of transmitting a single high-rate data stream, we transmit the data on several narrow-band sub- channels (with a different carrier corresponding to each sub- channel), so that each subcarrier experiences relatively flat fading. Thus, for a given overall data rate, increasing the number of subcarriers reduces the data rate that each individual carrier must convey, therefore lengthening the symbol duration on each subcarrier. To effectively eliminate ISI due to multipath fading, OFDM uses the concept of a cyclic prefix. This extends the symbol duration significantly so that it becomes far greater than the delay spread of the channel. Use of channel coding schemes significantly improves the performance of un-coded OFDM systems. The concatenated coding scheme originally proposed by Forney [8], is an error correcting coding scheme in which two different error correcting codes called the inner code and outer code are concatenated, achieving comparatively high coding gain by using codes that are relatively easy to decode. The main advantage of concatenated coding is that it can provide large coding gain with less implementation complexity. The concatenated coding scheme using the convolutional coding with Viterbi decoding for the inner code, and the Reed- Solomon (RS) code for the outer code is a special case of the general concatenated coding scheme proposed by Forney. The simulation results presented in this paper demonstrate the advantage of systems with error control coding over un- coded system performances in terms of the coding gain. Section II is an outline of the OFDM system model. The parameters assumed for various simulations are provided in section III and the bit error rate performance of various coded OFDM schemes over fading channels are provided in section IV. Finally Section V concludes the paper. II. OFDM SYSTEM MODEL OFDM represents a different system design approach. It can be thought of as a combination of modulation and multiple- access schemes that segments a communications channel in such a way that many users can share it. OFDM can be viewed as a form of frequency division multiplexing where each tone is orthogonal with every other tone. OFDM allows the spectrum of each tone to overlap, and because they are orthogonal, they do not interfere with each other. By allowing the tones to overlap, the overall amount of spectrum required is reduced. The orthogonality of the subcarriers also allows efficient modulator and demodulator implementation using the FFT algorithm on the receiver side, and IFFT on the transmitter side [9]. In order to totally get rid of ISI, OFDM employs the use of a cyclic prefix which increases the length of the symbol period so that it is much greater than the delay spread of the channel. Fig 1 below shows the block diagram of an OFDM system. A way to improve the system performance is to pass the input sequence through a forward error correction (FEC) encoder [10, 11]. FEC improves the small scale link performance by adding redundant bits in the transmitted message so that if an instantaneous fade occurs in the channel, the data may still be recovered at the receiver. The coded signal is then sent to the constellation mapper where the bits are mapped to a subcarrier amplitude and phase, which is represented by a complex in-phase and quadrature (IQ) format. The IFFT and the FFT are used for modulating and then demodulating the constellation mapped data onto the orthogonal sub carriers. The IFFT generally takes N symbols at a time where N represents the number of subcarriers in the system. Since the basis functions for an IFFT are N orthogonal sinusoids, each input symbol would act as a complex weight for the corresponding functions and the output will be a summation of all. When a convolutional block code is assumed as the channel encoder then at the receiver a Viterbi decoder is needed to do the decoding. For the concatenated coding schemes, a Reed Solomon encoder and Reed Solomon decoder pair are used as the outer coding and decoding schemes while the convolutional encoder and Viterbi decoder are used as the inner coding and decoding schemes. III. SIMULATED OFDM SPECIFICATION For obtaining the bit error rate performance over additive and multipath fading channels the parameters depicted in Table I have been assumed for modulation, channel coding and FFT size. For the COST 207 TU/BU channels 12-paths and for the Winner vehicular NLOS model 20-paths have been considered. Since in mobile wireless access the location of the end user changes and different users may be moving at different speeds, in this work performance was obtained for Doppler shifts of 100, 400 and 833 Hz (30km/hr, 120 km/hr, 250 km/hr). It was observed that the Doppler shifts due to relative motion degraded the performance and also caused an error floor in multipath fading channels. IV. SIMULATION RESULTS This section sets out to show the link-level BER performance of un-coded and coded QPSK-OFDM over AWGN and multipath Rayleigh fading channels. For channel coding, a rate R = 1⁄2 convolutional code and a concatenated code scheme of Reed Solomon/Convolutional code has been assumed. The link level BER performance for the un-coded QPSK- OFDM system over the AWGN channel is depicted in Fig. 2. The theoretical BER for QPSK is as in eq. (1) The observed SNR loss of approximately 0.5 dB in the simulated BER curve is as a result of the cyclic prefix introduced by OFDM. As stated in [1], the SNR loss can be calculated ...