Design of COFDM Transceiver Using VHDL

International Journal of Computer Applications (0975 – 8887) Volume 5– No.7, August 2010 Design of COFDM Transceiver Using VHDL Hemant Kumar Sharma Sanjay P. Sood Balwinder Singh Research Scholar HOD, ACS, HI & Electronics Division Design Engineer C-DAC, Mohali C-DAC, Mohali C-DAC, Mohali ABSTRACT results are described in section 5. Section 6 includes conclusion of OFDM is combined with channel coding scheme i.e. FEC this work and in section 7 contains the references. (Forward Error Correction) called CODED OFDM or COFDM is especially suitable for high speed environment because it provides 2. COFDM SYSTEM MODEL data transfer at higher speed with reliable transmission. Main OFDM is a wireless technology patented in 1970 that turns on focus of this paper is to design COFDM transceiver which principle of transmitting data by dividing the data stream into operates with FEC, scrambling, block interleaver with multiple parallel bit stream. OFDM transmits the data by using a conventional OFDM system to overcome the problem in wireless large number of narrow bandwidth carriers. These carriers are link i.e. ISI, ICI, fading etc. In this paper, COFDM transceiver regularly spaced in frequency. The carriers are sent in such a way designing details and simulation results are discussed after that that the carriers are orthogonal, meaning that they do not cause COFDM modulator is compared with previous work which interference to each other. Currently there are two principle 4G ensures present work is better than previous work. COFDM development technologies contending for attention: CDMA (Code Transceiver is designed and synthesized using Xilinx Project Division Multiplexing) and OFDM. CDMA is well known Navigator XILINX ISE 9.1i and simulated using ModelSim 5.8c standard and has been used for several years, but OFDM is simulator. relatively new technology which is being used by Nokia, Cisco, Lucent and Philips Semiconductor etc. General Terms Digital System Design, Wireless Communication, Data and OFDM with particular channel coding scheme or with error information system control coding called COFDM provides the higher data rate environment with removing the effect of frequency fading and reliable data rate transmission. Thus COFDM is adopted by many Keywords wireless standards like DAB, DSL, WLAN, IEEE 802.11 a, IEEE OFDM, COFDM, FEC, Convolution Encoder, Viterbi Decoder 802.11 g, IEEE 802.11 n etc [12]. With increasing the data rate in wireless communication system time of each transmission 1. INTRODUCTION becomes shorter. But delay time due to the multi-path fading In the field of wireless communication OFDM (Orthogonal remains constant, because of this ISI, ICI becomes main Frequency Division Multiplexing) is the suitable candidate to problems. COFDM avoids these problems by splitting the channel overcome the problems like multi-path fading, ISI (Inter Symbol into sub-channels and transmitting the data using sub-channels in Interference), ICI (Inter Carrier Interference), low bit rate such a way that data are transmitted orthogonally so that 50% capacity, need higher power to transfer the data etc [3] [4]. With bandwidth is saved than conventional system as shown in Figure increasing data transfer rate, it is very important that data must be 1. Figure 1(a) shows the simple conventional method in which the transmitted with carefully and reliable transmission should be take bandwidth used to transmit the signal is more than COFDM as place in wireless link. So OFDM is combined with suitable shown in Figure 1(b). channel coding scheme to perform reliable transmission, when COFDM technique is another form of multi-carrier modulation OFDM is combined with channel coding scheme then it is called which basic principles is transmission of data with reliable and at CODED OFDM or COFDM [3]. Coding scheme must be chosen higher data rate by converting data bit into several parallel bit for any system which satisfies requirements of high data rate as streams and each of these bit are modulated by each sub-carrier. well as good error capability and according to complexity, COFDM signal can be presented as shown in Figure 2. suitable delay and desired coding gain for system. The aim of this work to show the compatibility of designing and implementing a COFDM transceiver on FPGA. In this work COFDM transmitter and receiver is designed using VHDL after that results are verified then COFDM modulator is compared with previous work which shows the present work provides better performance than previous work and present work utilize less number of devices than previous work. This work can be organized as: In section 2 COFDM system is explained, section 3 describes the significance of FEC with Interleaving, design details or implementation details is covered in section 4, then simulation Figure 1 Transmission Techniques 11  International Journal of Computer Applications (0975 – 8887) Volume 5– No.7, August 2010 Data Input Scra FEC Puunct Interle Constell IFFT mbler Encoder urer aver ation Mapper (a) (b) Desc FEC Depun De- Constella FFT Figure 2 Spectra of (a) COFDM sub-channel and Decoder ramb cturer Interlea tion ler ver Demappe (b) COFDM signal r COFDM signal generation is presented in Figure 3. To generate Data Output Figure 4 Basic block of COFDM Transceiver COFDM successfully the relationship between all the carriers must be carefully controlled to maintain the orthogonality of the carriers. For this reason, COFDM is generated by firstly choosing 3. FEC WITH INTERLEAVING the spectrum required, based on the input data, and modulation Interleaving is a powerful technique that is used to perform burst scheme used. The spectrum which is desired is then converted error correction in which the bit errors occur sequentially in time back to its time domain signal using an IFFT. The FFT converts a and as groups [5]. Interleaving is used in digital communications cyclic time domain signal into its equivalent frequency spectrum. systems to enhance the random error correcting capabilities of After performing OFDM function using IFFT & FFT, signal is error correcting codes to the point that they can be effective in a send to COFDM receiver where the data transmitted through burst noise environment [11]. The interleaver subsystem COFDM transmitter is received. rearranges the encoded symbols over multiple code blocks as shown in Figure 5. FEC process with interleaving enhances the performance of system with reasonable cost of price and hardware. Figure 5 shows that data is first encoded through FEC encoder then data is sent to interleaver after mapped using particular modulating scheme then transmitted, at receiving side data is first de-mapped using same modulating scheme that employed at transmitter side then sent to de-interleaver which performs reverse operation of interleaver and rearranges the data then data is decoded using FEC decoder [10]. Interleaving techniques are traditionally used to enhance the Figure 3 COFDM Generation quality of digital transmission over the bursty radio channel. This Basic block of COFDM transmitter and receiver is shown in is usually accomplished by scrambling successive symbols of the Figure 4 which consists of scrambler, convolution encoder as FEC transmitted sequence into different time slots. A channel is encoder, puncturer, block interleaver, QPSK scheme as considered fully interleaved when consecutive symbols of the constellation mapper and IFFT (Inverse Fast Fourier Transform) received sequence appear to be independent not affect by the same at transmission side. While COFDM receiver includes FFT (Fast burst error. Fourier Transform), Constellation de-mapper, de-interleaver, depuncturer, FEC decoder and descrambler. FEC encoder and FEC decoder performs FEC process at transmitter and receiver Encoder Interleaver Mapper side. An IFFT and FFT block performs OFDM process to map the QPSK value into OFDM symbol so that bandwidth forInput transmission of signals is saved. The COFDM transmitter block is divided into several parts with each block functioning differently and this is to ensure that the Output system works effectively. The Scrambler serializes input data and Decoder De-interleaver De-mapper then performs an “exclusive-OR” operation on the data [6]. Data is then encoded by the convolution encoder which is used as FEC encoder to perform FEC scheme or to encode the data and passes Figure 5 FEC with Interleaving through puncturer. Then interleaver rearranges the bit sequence and passed through mapper after it QPSK value bit is mapped into FEC coding gives the best performance when it is used with OFDM symbols using IFFT. IFFT is performed to represent the interleaving with respect to the BER (Bit Error Rate), frequency signal or data into time domain because when data is transmitted diversity, fading environment, multi-path environment etc [2]. in analog form through transmitter and represented in time Due to the bursty nature of many radio interference sources and domain. the characteristics of the demodulator, it is more likely that 12  International Journal of Computer Applications (0975 – 8887) Volume 5– No.7, August 2010 erroneous bits will clump together. To combat this problem Then comes to the constellation mapper which is used to mapped interleaving of the coded data is performed after encoding in the the interleaved bits into QPSK value. The mapper converts the transmitter and de-interleaving before decoding in the receiver. input data into complexed valued constellation points, according The main objective of interleaving is to ensure that adjacent to a given constellation. Some typical constellations for wireless symbols in the coded sequence are spaced out in the transmitted applications are BPSK, QPSK and QAM [14], in this work QPSK sequence, so that any clumps of bit errors in the received mapper is used at transmitter side. At receiver side QPSK sequence are spread out more uniformly by the de-interleaver, demapper is used as QPSK mapper is used at transmitter side. The letting the decoder work under optimum conditions [11]. constellation graph of QPSK can be shown in Figure 7 and carrier phase shift corresponding to various input bits can be shown in Table 1. 4. IMPLEMENTATION DETAILS The basic design methodology to implement the COFDM transmitter and receiver is the divide and conquer scheme in which first COFDM transceiver is divided into two main parts: COFDM Transmitter and COFDM Receiver. After it COFDM transmitter block is further divided into sub-blocks and same in case of COFDM receiver is performed. Then sub-blocks of COFDM transmitter and receiver is designed and results are verified. After verification of each block’s result blocks are merged then COFDM transmitter and COFDM receiver are assembled which called COFDM Transceiver and then result of COFDM Transceiver is verified. Whole design is designed using Figure 7 QPSK constellation graph [14] Xilinx Project Navigator XILINX ISE 9.1i and result is verified using ModelSim XE 5.8c simulator. The device selected for this Table 1. Carrier Phase Shifts corresponding to various input work is Virtex-5 XC5vlx30-3ff324. Basic functioning and main bits designing issues of each block is discussed in next. Information bits Phase change First block of COFDM transmitter is scrambler which serializes the input data bits and used to generate the random number of 11 ∏/4 sequence and work as pseudo noise sequence (PN) generator. The 01 3∏/4 PN generator has applications in communications in CDMA, 00 -3∏/4 system testing etc [6]. In this work M-length sequence generator 10 -∏/4 is used for PAPR reduction and easiest way to generate bits. De- scrambler is used to perform just reverse operation of scrambler at To design FFT/IFFT DIT (Decimation in Time) radix-2 approach receiver side. Then data is encoded using convolution encoder is used. FFT transforms the signals in time domain to frequency which is used as FEC encoder to perform FEC process as shown domain. In 1973 it was discovered that FFT could be used in in Figure 6. In this work convolution encoder of code rate ½ and multi carrier systems like OFDM. The equation can be written as constraint length L=3 is used to encode the data stream generated follows by scrambler. Viterbi decoder is used as FEC decoder at receiver side to decode bit streams. This decoder is based on “viterbi algorithm” in which the possible received bit sequence form a “tree” structure and the viterbi tracks likely paths through the tree structure [7] [8]. (1) After this data is punctured and rearranged using interleaver, in The quantity WNnk is defined as below this work block interleaver is used to remove burst type errors and bit type errors. An Interleaver spreads the scrambled data in some (2) deterministic manner. Interleaving is a process of rearranging the data symbols so that burst error can be overcome and corrected by This factor also called twiddle factor is calculated and put in a means of FEC which is most effective technique for it. At table in order to make the computation easier and can run receiving side reverse operation is performed using depuncturer simultaneously. The twiddle factor table is depending on the and de-interleaver to recover the data. number of points used, here 8-point DIT radix-2 approach is used. During the computation of IFFT, the factor does not need to be recalculated since it can refer to the twiddle factor table thus it Data U1save time since calculation is done concurrently. IFFT is defined input as the equation given below 1 2 3 (3) U2 Figure 6 Convolution Encoder [7] 13  International Journal of Computer Applications (0975 – 8887) Volume 5– No.7, August 2010 A lot of devices are utilized by IFFT/FFT block than any other block. So design the FFT/IFFT block with minimum device utilization summary is main challenge. 5. SIMULATION RESULTS COFDM Transceiver system is designed using VHDL and synthesized using Xilinx Projector navigator XILINX ISE 9.1i. Results are verified using ModelSim XE 5.8c simulator. The design is mapped on Virtex-5 XC5vlx30-3ff324. Internal RTL view of COFDM Transmitter, COFDM Receiver and COFDM Transceiver can be shown in Figure 8, Figure 9 and Figure 10 respectively. Figure 10 Internal RTL View of COFDM Transceiver Figure 8 and Figure 9 shows the internal RTL view of COFDM transmitter and receiver which shows the combination of all blocks which are transmitter or receiver side respectively. In Figure 10 internal view of COFDM transceiver is shown which shows the combination of COFDM transmitter, COFDM receiver and OFDM blocks. Simulation results of COFDM transceiver is shown in Figure 11. Figure 8 Internal RTL View of COFDM Transmitter Figure 11 Simulation result of COFDM Transceiver Simulation results verified that the transceiver would perform with suitably low bit error for the full range of coding and modulation schemes and for variation of all the channel impairments. VHDL functional verification confirmed that the HDL design exhibited good performance. Figure 9 Internal RTL View of COFDM Receiver 14  International Journal of Computer Applications (0975 – 8887) Volume 5– No.7, August 2010 6. CONCLUSION devices to perform better than previous work. The timing analysis The main focus of this work is to shows the capability of of COFDM Transceiver is presented as below designing and simulating COFDM system which includes OFDM integrated with FEC technique. This work’s main emphasis was Minimum period: 2.451ns (Maximum Frequency: 407.914MHz) on designing and simulation of synthesizable VHDL code of the COFDM transceiver using Xilinx’s ISE 9.1i and simulated using Minimum input arrival time before clock: 2.056ns ModelSim XE 5.8c simulator. Device Utilization summary of Maximum output required time after clock: 2.505ns COFDM Transceiver is given in Table 2. Table 3 shows the total memory used by COFDM transceiver system. Resource Maximum combinational path delay: No path found Utilization of COFDM modulator is compared with previous work in Table 4 and results shows the present design utilizes the 7. REFERENCES less number of devices. [1] S. B Weinstein and P.M. Ebert, “Data Transmission by Table 2. 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