The major problem of Orthogonal FrequencyDivision Multiplexing (OFDM) is its High Peak-to-AveragePower Ratio (PAPR). The High PAPR increases the complexityof Analogue to Digital (A/D) and Digital to Analogue (D/A)converters and also reduces the efficiency of Radio FrequencyHigh Power Amplifier (RF HPA). In this paper, we presentDiscrete Fourier Transform (DFT), Discrete HartleyTransform (DHT) and Zadoff-Chu Transform (ZCT)precoders for both clipping and clipping and filtering to reducePAPR. The DFT precoder provides better PAPR comparedwith clipping, clipping and filtering OFDM. In addition toimprove the bit error rate (BER) this has also been taken as aperformance evaluation parameter. The DFT precoded systemis better than ZCT precoder by about 1.2 dB and better thanDHT precoder by about 1.5 dB at the same ComplementaryCumulative Distribution Function (CCDF) value. The DFTprecoder is better than clipping by 1dB and better thanclipping and filtering technique by about 6dB for OFDMsystem at the same parameters. The DFT Precoded system isbetter than other precoder as well as it improvement in BERperformance of the original OFDM by about 1 dB at the sameBER
Abstract-In this paper, Discrete Fourier Transform Precoded Orthogonal Frequency Division MultiplexingMultilayer Interleave Division Multiple Access (DFT P OFDM ML IDMA) communication system has been proposed. Chip by Chip (CBC) iterative receiver is used to recover the data in each layer. Also, different precoding techniques has been applied with and without clipping in the proposed P OFDM ML IDMA. Simulation results confirm that the DFT precoder provides better PAPR reduction compared with other precoded methods with and without clipping. BER and PAPR simulation results shows that the proposed DFT P OFDM ML IDMA brings about 7 dB PAPR reduction and 0.5 dB BER improvement compred with OFDM ML IDMA. The Simulations for the proposed DFT P OFDM ML IDMA with 2 -6 layers shows that the variation is 3.4 to 8.2 dB for the PAPR and 7.8 to 8.7 dB for the BER. So, the adaptive modulation can be simply realized by the proposed DFT P OFDM ML IDMA.Index Terms-PAPR, DFT, precoding, OFDM and IDMA. I. INTRODUCTIONInterleave-Division Multiple-Access (IDMA) was recently investigated. IDMA uses different chiplevel interleavers to distinguish different users, all adopting the same spreading sequence. The key principle of IDMA systems is that the chip-level interleavers should be different for different users. In addition, a low-cost CBC iterative detection can be utilized in the IDMA systems [1], [2]. Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission scheme that has become the technology choice for next generation wireless and wire line digital communication systems [3].IDMA can be combined with OFDM (OFDM-IDMA) integrates the advantages of both OFDM and IDMA in order to mitigate the intersymbol interference (ISI) and suppress the multiple access interference (MAI) simultaneously [4], [5]. OFDM-IDMA suffers PAPR problem. The transmitted signal of OFDM-IDMA has high PAPR, which may degrade the efficiency of power amplifier of the transmitters.A number of PAPR reduction schemes have been proposed for OFDM systems, such as clipping [6], partial transmit sequence method (PTS) [7], and selected mapping method (SLM) [8]. In [9],clipping is adopted in OFDM-IDMA systems for PAPR reduction but interference from clipping noise will reduce BER performance.To alleviate the above problem, precoding based techniques, however, show great promise as they are simple linear techniques to implement without the need of any side information. Precoding improves PAPR without increasing much complexity and without destroying the orthogonality between subcarriers. The precoding also improves the BER in comparison to normal OFDM system because of diversity gain obtained due to the spreading of data symbol among more than one subcarrier.In this paper, we present DFT precoder for clipped and Unclipped OFDM ML IDMA signals to reduce PAPR. Performance comparison is made with other transformbased precoders, namely Discrete Hartley Transform (DHT) and Zadoff-Chu Transform (ZCT) [7], [10].This paper is organized as follows: Section ...
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