In the reverse link of orthogonal frequency division multiplexing (OFDM)/ space division multiple access (SDMA) systems, each receive antenna of a base station receives a multiplexed version of signals transmitted from users, where the transmitted signals have individual amounts of frequency offset. Therefore, a frequency offset compensation scheme which is different from those used in general OFDM systems is required. For this requirement, frequency offset compensation schemes using the feedback transmission from the base station to user terminals have been proposed for multiuser OFDM systems. These schemes work with good precision when the feedback information is correct and is transmitted without errors. However, when the offset information is incorrectly received at user terminals, the frequency offset is not accurately compensated for. In OFDM/SDMA systems, one user is enough for causing inter-carrier interference to all users. Therefore, a frequency offset compensation scheme without feedback transmission is sometimes preferable. In this paper, we propose a frequency offset compensation scheme without feedback transmission. To compensate for frequency offset in every transmitted signal, the multiplexed received signals must be separated into each user's component before the offset compensation. Thus, we adopt the principle of the parallel interference cancellation (PIC). By employing PIC, the received signals can be separated before the offset compensation. Thus, the frequency offset of every user's signal can be compensated for. Simulation results show the bit error rate performance of the proposed scheme attains almost the same as that of the conventional scheme using the feedback transmission without errors.

This paper proposes a bandwidth division type parallel combinatory (PC) spread spectrum (SS) modulation scheme. In the proposed system, a given system bandwidth for the conventional single-carrier PC-SS system is divided into H subbands, and H PC-SS signals are transmitted in parallel. We evaluate the frame error rate (FER) of the proposed system under the asynchronous CDMA environment. We show that the proposed scheme provides a smaller FER than the single-carrier PC-SS system for a given information bit rate. We also show that the proposed scheme attains a higher information bit rate than the single-carrier PC-SS system for a given FER.

In space division multiplexing with orthogonal frequency division multiplexing (SDM-OFDM) systems, since co-channel interference (CCI) degrades the demodulation performance, CCI suppression is essential. For CCI suppression, the turbo detector using the soft-input soft-output maximum likelihood detector (SISO-MLD) is proposed. Although SISO-MLD can deal with soft information, SISO-MLD updates only a priori information of code bits at each iteration. This prevents the performance to be improved. Meanwhile, a turbo detector with soft cancellation (SC) followed by minimum mean square error (MMSE) filter, in which the variance of the residual interference components as well as a priori information can be updated at each iteration, is well known. However, the performance of SC/MMSE detector is also limited because it is proposed to reduce the computational complexity at the cost of performance degradation. In this paper, we propose an SC-type turbo detector which uses SISO-MLD for SDM-OFDM systems. In our proposed detector, the soft cancellation unit of the SC-type turbo detector is modified to cope with the SISO-MLD. From the simulation results, the proposed SC/MLD provides a better BER performance than the turbo detector using SISO-MLD. Furthermore, the proposed SC/MLD can attain the bit error rate (BER) equivalent to that of the SC/MMSE detector with a smaller computational complexity.

Space-time turbo codes have both advantages of space-time codes and turbo codes, and the space-time turbo code proposed by Su and Geraniotis is known to achieve full coding rate and full antenna diversity. This paper presents some improvements of their space-time turbo code in a two-antenna configuration. We first propose a new condition for full antenna diversity which imposes less constraints on the interleaver. Next, by applying a method used to improve turbo trellis-coded modulation to the space-time turbo code, we propose a new decoding algorithm which utilizes more precise estimates on extrinsic information. Simulation results show that the proposed condition assures full antenna diversity and the new decoding algorithm provides a better performance than that of Su and Geraniotis'.

In cellular systems, autonomous reuse partitioning (ARP) is one of the channel assignment strategy which attains the high spectral efficiency. In the strategy, the movement of mobile stations (MSs) causes the disturbance of reuse partition. Furthermore the smaller cell size causes the spectral efficiency worse. In this paper, we propose a new ARP strategy with reuse partitioning reconstructing, named RP-reconstructing ARP strategy, for microcellular systems. We evaluate the performance of the proposed strategy with blocking rate and forced call termination rate by the computer simulation. The results show that the system with the proposed strategy accommodates 1.5 times as many users as the system with ARP does.

Non-orthogonal multiple access (NOMA) enables multiple mobile devices to share the same frequency band. In a conventional NOMA scheme, the receiver of a far user detects its desired signal without canceling the signal for a near user. However, the signal for the near user acts as interference and degrades the accuracy of likelihood values for the far user. In this paper, a joint maximum likelihood detection scheme for the far user of the NOMA downlink is proposed. The proposed scheme takes the interference signal into account in calculating the likelihood values. Numerical results obtained through computer simulation show that the proposed scheme improves the performance by from 0.2dB to 3.1dB for power allocation coefficients of 0.2 to 0.4 at a bit error rate (BER) of 10-2 relative to the conventional scheme.

In multi-input multi-output orthogonal frequency division multiplexing (MIMO-OFDM) systems, phase tracking schemes suffer from co-channel interference (CCI) and inter-carrier interference (ICI) caused by residual frequency offset. In this paper, we propose a residual frequency offset compensation scheme using feedback phase tracking to eliminate the effect of both ICI and CCI for MIMO-OFDM systems. The proposed phase tracking scheme estimates the amount of residual frequency offset in the frequency domain, and compensates for it in the time domain, periodically. Thus, the effect of ICI can be reduced. Furthermore, we consider two methods of channel estimation that enable the system to estimate the channel response several times within a packet to eliminate the effect of CCI. This is because the channel is generally estimated at the beginning of a packet, and this estimation is affected by residual frequency offset. First is the method that employs midambles. Second is the one that reuses the preamble. When the channel is estimated several times within a packet, the effect of CCI can be reduced. Simulation results show the proposed scheme can compensate for residual frequency offset and CCI more accurately than the conventional scheme, and improve the packet error rate (PER) performance.