Throughput is an important performance measure for data communications over a wireless channel. Orthogonal frequency division multiplexing (OFDM) with hybrid automatic repeat request (HARQ) achieves a good throughput performance over a frequency-selective fading channel. In OFDM with HARQ, however, during the first transmission uncoded information packet is transmitted. Consequently, the channel frequencyselectivity cannot be exploited since frequency-domain equalization (FDE) is not designed to take the channel advantages. In particular, the HARQ throughput performance of OFDM cannot be improved even for a high signal-to-noise power ratio (SNR). In this paper, to increase the HARQ throughput of conventional OFDM, we present the use of OFDM combined with time division multiplexing (OFDM/TDM) using minimum mean square error FDE (MMSE-FDE) designed to exploit the channel frequency-selectivity. It was shown, by computer simulation, that OFDM/TDM using MMSE-FDE with HARQ achieves a higher throughput than the conventional OFDM due to frequency diversity gain during the first transmission. It was also shown that OFDM/TDM using MMSE-FDE performs better in a stronger frequency-selective fading channel.

We present, in this paper, pilot-assisted channel estimation (CE) for orthogonal frequency division multiplexing combined with time division multiplexing (OFDM/TDM) using minimum mean square error frequency-domain equalization (MMSE-FDE) over a nonlinear and frequency-selective fading channel. Joint use of time-domain filtering to increase the signal-to-noise ratio (SNR) of pilot signal and frequency-domain interpolation for OFDM/TDM is presented. The simulation results show that OFDM/TDM with proposed pilot-assisted CE provides a better performance than OFDM since the peak-to-average power ratio (PAPR) problem can be reduced.

Orthogonal frequency division multiplexing (OFDM) combined with time division multiplexing (TDM), in this paper called OFDM/TDM, can overcome the high peak-to-average-power ratio (PAPR) problem of the conventional OFDM and improve the robustness against long time delays. In this paper, the bit error rate (BER) performance of OFDM/TDM in a frequency-selective Rayleigh fading channel is evaluated by computer simulation. It is shown that the use of frequency-domain equalization based on minimum mean square error criterion (MMSE-FDE) can significantly improve the BER performance, compared to the conventional OFDM, by exploiting the channel frequency-selectivity while reducing the PAPR or improving the robustness against long time delays. It is also shown that the performance of OFDM/TDM de- signed to reduce the PAPR can bridge the conventional OFDM and single-carrier (SC) transmission by changing the design parameter.

In this letter, we introduce frequency-domain space-time transmit diversity (STTD) encoding/decoding to orthogonal frequency division multiplexing combined with time division multiplexing (OFDM/TDM) on a frame-by-frame basis (i.e., over several concatenated OFDM signals in the frequency-domain) to achieve both spatial and frequency diversity gains and improve the bit error rate (BER) performance. The theoretical BER performance is evaluated by numerical computation using the derived conditional BER and confirmed by computer simulation.

The OFDM signals have a problem of high peak-to-average power ratio (PAPR). Hence, a large transmit-power backoff or amplitude clipping is required. The amplitude clipping causes signal degradation and the BER performance increases. A trade-off between the PAPR reduction and the BER performance is present; the PAPR reduces as the level of clipping reduces, but the BER degrades due to signal distortion. Recently, we proposed OFDM combined with time division multiplexing (OFDM/TDM) to alleviate the high PAPR problem, while achieving better BER performance than OFDM. In this paper, a theoretical bit error rate (BER) analysis of clipped OFDM/TDM system in a frequency-selective fading channel is developed. The average BER performance is evaluated by numerical computation using the derived conditional BER and by computer simulation. It is shown that OFDM/TDM can significantly reduce the amplitude clipping level and the required average signal energy per bit-to-AWGN power spectrum density ratio Eb/No for the given BER in comparison to conventional OFDM.

This paper presents a hybrid automatic repeat request (HARQ) with variable spreading factor (VSF) suitable for a multicarrier code division multiple access (MC-CDMA). We consider a HARQ based on incremental redundancy (IR) strategy. The throughput of MC-CDMA HARQ is a tradeoff among the frequency diversity gain, the coding gain and the inter-code interference (ICI). In order to effectively exploit the channel frequency-selectivity and the channel coding, the spreading factor is changed between the initial transmission and the succeeding retransmissions. The throughput performance of HARQ with VSF in a frequency-selective Rayleigh fading channel is evaluated by the computer simulation and compared with that with fixed spreading factor (FSF). It is shown that HARQ with VSF can provide better throughput performance than with FSF and is particularly useful when high-level modulation (e.g., 16QAM and 64QAM) is used. The impacts of fading correlation between packet retransmissions and the channel frequency-selectivity on achievable throughput performance are discussed

Orthogonal frequency division multiplexing (OFDM) combined with time division multiplexing (TDM), called OFDM/TDM, can bridge the conventional OFDM and single-carrier (SC) transmission by using frequency-domain equalization (FDE). Accurate channel estimation is required for FDE. A well-known channel estimation scheme is pilot-based channel estimation that uses periodically transmitted pilot signals. If a channel estimation error is present, intersymbol interference (ISI) is produced in OFDM/TDM. We apply a Gaussian approximation to the channel estimation error and theoretically investigate the impact of imperfect channel estimation on the average bit error rate (BER) performance of OFDM/TDM. It is shown by numerical evaluation that the channel estimation error degrades the BER performance more when OFDM/TDM approaches SC.

Orthogonal frequency division multiplexing (OFDM) combined with time division multiplexing (TDM), called OFDM/TDM, can overcome the high peak-to-average-power ratio (PAPR) problem of the conventional OFDM. Its bit error rate (BER) performance in a frequency-selective fading channel can be significantly improved by the use of frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion. However, MMSE-FDE requires accurate channel estimation. In this paper, a pilot-assisted channel estimation scheme suitable for OFDM/TDM with MMSE-FDE is presented and the achievable BER performance is evaluated by computer simulation. Keyword: OFDM/TDM, frequency-domain equalization, pilot- assisted channel estimation

For alleviating the high peak-to-average power ratio (PAPR) problem of orthogonal frequency division multiplexing (OFDM) while improving the bit error rate (BER) performance, the OFDM combined with time division multiplexing (TDM) using frequency-domain equalization (FDE) was recently proposed. In this paper, the theoretical BER analysis of the OFDM/TDM in a frequency-selective fading channel is presented. A conditional BER expression is derived, based on the Gaussian approximation of the residual inter-symbol interference (ISI) after FDE, for the given set of channel gains. Various FDE techniques, i.e., zero forcing (ZF), maximum ratio combining (MRC) and minimum mean square error (MMSE) criteria are considered. The average BER performance is evaluated by Monte-Carlo numerical computation method using the derived conditional BER expression.

Abstract: Orthogonal frequency division multiplexing (OFDM) combined with time division multiplexing (TDM) can bridge the conventional OFDM and single carrier (SC) transmission by using minimum mean square error (MMSE) frequencydomain equalization (FDE). The channel frequency-selectivity can be exploited by MMSE-FDE to improve the bit error rate (BER) performance. A much better BER performance is achieved in comparison to conventional OFDM while reducing the peak-to-average power ratio (PAPR). Further performance improvement can be achieved by transmit/receive antenna diversity technique. In this paper, the joint use of space-time transmit diversity (STTD) and receive antenna diversity is discussed.

One of the main problems of orthogonal frequency division multiplexing (OFDM) is high peak-to-average power ratio (PAPR). In this paper, a novel approach to alleviate the PAPR problem of OFDM is presented. A generalized OFDM (GOFDM) with frequency-domain equalization (FDE) is proposed. Various FDE techniques based on zero forcing (ZF), maximum ratio combining (MRC) and minimum mean square error (MMSE) are considered. The bit error rate (BER) performance of GOFDM in a frequency-selective fading channel is evaluated by computer simulation and compared with those of conventional OFDM and single-carrier (SC) transmission

あらまし OFDM は高速データ信号を多数の直交サブキャリアを用いて並列伝送する.遅延パスの影響を取り除く ため,ガードインターバル(GI)を付加する.しかし,GI を超える長遅延パスが存在すると,サブキャリア間干 渉(ICI)とシンボル間干渉(ISI)が発生し,OFDM 伝送特性が著しく劣化してしまう.そこで,本論文では,GI 付加前の OFDMシンボルを K個まとめて,K倍の長さの GIを付加する一般化 OFDM(GOFDM)を用いることを 提案している.GOFDM 受信では,従来の K 倍の長さの FFT 窓を用いて GOFDM 信号を周波数変換して周波数領 域等化した上で,OFDM 復調する.このような GOFDM では,従来の K 倍のパス遅延まで許容することができる. 本論文では,計算機シミュレーションにより,超遅延パスの存在下での GOFDM 伝送のビット誤り率特性を求め, 従来の OFDM伝送より優れた BER特性が得られることを明らかにしている. キーワード OFDM,周波数領域等化,周波数選択性フェージング.