Xun Shao

Error performance analysis of linear zero forcing and MMSE precoders for MIMO broadcast channels

Linear precoding (LP) techniques for a multiuser multiple-input-multiple-output broadcast channel is investigated and analytical results of the achievable sum capacity and error performance for zero forcing (ZF) linear precoders is presented. It is shown that the detection signal-to-noise ratio of ZF-LP can be accurately approximated by a shrinking Chi-square distribution. The symbol error rate and its achievable diversity gain of ZF-LP are given. Then, an improved linear precoder based on the minimum mean-square error (MMSE) criterion is derived. Its error performance and sum capacity are analysed and compared with that of the ZF-LP. It is shown that the MMSE-LP can achieve much better error performance and a high sum capacity than the existing ZF-LP.

Antenna selection for MIMO-OFDM spatial multiplexing system

Transmit antenna selection is a promising closed-loop approach to introduce transmit-diversity for MIMO system. It offers performance improvement compared to the open loop schemes with equivalent complexity by only using low rate feedback link. Two categories of selection criteria were reported, which are capacity-oriented criteria (R. Blum, et al., 2002)and performance-oriented criteria (R.W. Heath, et al., 2001). In this paper, we focus on the later case. We present an analytical bound of transmit-diversity gain provided by antenna selection in flat fading channels. An antenna selection criterion is proposed for MIMO-OFDM in frequency selective fading channels.

A new differential space-time block coding scheme

Most existing space-time coding schemes rely heavily on accurate channel estimation at the receiver., which is a difficult task in rapidly fading environment. The previous Tarokh and Jafarkhani (see IEEE Journal on Selected Areas in Comm., vol.18, no.7, p.1169-1174, 2000) differential space-time block code (DSTBC) suffers from the signal constellation expansion for QPSK and 8PSK modulation. We present a new differential space-time block coding scheme without constellation expansion. This is achieved by choosing differential coefficients appropriately in the signal mapping. The new scheme can be decoded by a Viterbi decoder. More importantly, it provides a differential coding gain of 1 dB-1.5 dB due to the redundancy introduced in the differential coding and it is only 2 dB away from the coherent space-time block codes.

Precoder design for MIMO broadcast channels

This paper considers precoder designs in downlink MIMO broadcast channels (BC). A preceding scheme that is analog to the MMSE decision feedback equalizer in the uplink MIMO multiple access channels is first introduced. The proposed scheme can asymptotically achieve the sum-capacity of MIMO-BC at high SNRs. At medium SNRs, its achievable sum capacity is very close to that of MIMO-BC and larger than that of ZF-DR. The new scheme can also offer a significant gain over linear pre-equalization techniques in terms of average error probability. Furthermore, by linking the proposed precoder with the sphere encoder, a modified sphere encoder is devised. The modified sphere encoder is shown to be able to achieve the same diversity order as the maximum likelihood (ML) receiver in dual uplink. Its performance is also approaching the corresponding ML receiver.

Robust space-time trellis codes for OFDM systems over quasi-static frequency selective fading channels

This paper proposes the code design criteria for robust space-time trellis codes (STTCs) in orthogonal frequency division multiplexing (OFDM) systems over quasi-static frequency selective fading channels. The code design criteria consider the channels with multiple uncorrelated taps and various channel delay distributions such that the robust code performance over various channel conditions can be guaranteed. Based on the code design criteria, new 4, 8, and 16-state 4-PSK STTCs are constructed. Simulation shows that the new codes achieve the robust code performance over various quasi-static frequency selective fading channels.

New differential space-time block code for two, three and four transmit antennas

We present a new differential space-time block coding (DSTBC) scheme. The new scheme can be represented by a trellis and decoded using the Viterbi algorithm. It provides a differential coding gain of 1 dB due to the redundancy introduced in the differential coding and it is only 2 dB away from the coherent space-time block codes (STBC). Furthermore, we extend this design of DSTBC to three and four transmit antennas.

Performance analysis of space-time trellis coded OFDM over quasi-static frequency selective fading channels

This paper presents the performance analysis of space-time trellis codes in orthogonal frequency division multiplexing system (STTC-OFDM) over quasi-static frequency selective fading channels by considering the effect of channel delay distributions on the code performance. The analysis shows that various channel delay distributions do affect the code performance. It also shows that a better code performance can be achieved over the channel, which has a larger sum of the squared time delay differences between any two different channel taps. Simulation illustrates that the code performance of STTC-OFDM under various channel conditions agrees with the theoretical performance analysis.

Multiuser diversity for MIMO broadcast and multiple access channel with linear precoder and receiver

We study the multiuser diversity in both multiple-input multiple output (MIMO) multiple-access channel (MAC) and MIMO broadcast channel (BC). We consider multiuser scheduling with linear receivers for uplink and linear precoders for downlink. In each time slot, a group of users are selected in either transmission or reception. Based on the performance analysis, a criterion of selecting a number of active users to optimize error performance is presented. We show that the linear processing for downlink and uplink achieves the same SNR. This makes it convenient to use a single scheduling algorithm in both downlink and uplink. Furthermore, we present an analysis of the performance of the proposed scheduling algorithm. It is shown that the analysis agrees with the simulation.

Adaptive blind sequence detection for time varying channel

In rapidly time varying channels, the channel impulse response estimation might not be successful by the conventional decision directed scheme following the supervised training. The blind sequence detection would be the only solution in these scenarios. In this paper we proposed a new blind sequence detection algorithm, which has a significant robustness against bit ambiguity and whose performance with uncoded sequence is better than that of some existing blind algorithms with coded systems. Bit error performances have been investigated by computer simulation and compared with coded system. A significant gain has been achieved in SNR as much as 6 dB or more.

Precoder Design for MIMO Broadcast Channels

This paper considers precoder designs in downlink MIMO broadcast channels. We present a MMSE feedback precoding scheme, which is analogue to the MMSE generalized decision feedback equalizer (GDFE) in the uplink MIMO multiple access channel. The proposed scheme obtains the same level of total mean-square error as the MMSE-GDFE in the dual uplink with perfect decision, and it achieves a sum capacity that is very close to the sum capacity of MIMO BC. It can also offer a significant gain over the linear precoding techniques in term of average error probability