A link adaptation scheme is devised for vector-perturbation (VP) zero-forcing beamforming (ZFBF) MIMO precoding and a link-adaptable VP-ZFBF precoder is applied to multi-point three-dimensional (3D) beamformers to be used in mmWave-band wireless access systems. Channel coding schemes used in current systems, e.g., turbo codes, possess systematic code structures. The perturbation gain can thus be predicted by searching for perturbation vectors for the symbol vectors mapped from information bits. On the basis of this principle, we constructed an efficient iterative modulation-and-coding-set (MCS) selection procedure for VP-ZFBF precoding. Simulation results demonstrate that our proposed scheme suitably passed on the perturbation gain to the selection of higher MCS and thus achieved high throughputs by incorporating with multi-point 3D-beamformers.

The paper presents a cell search scheme embedded with carrier frequency synchronization for intercell asynchronous orthogonal frequency-division multiplexing code-division multiplexing (OFDM-CDM) systems. Several subcarriers are dedicated to a differentially encoded synchronization channel (SCH). In the other subcarriers, data symbols and pilot symbols are two-dimensionally spread in the time-frequency domain. The cell search scheme consists of a three-stage cell search and a two-stage carrier-frequency synchronization, that is, coarse carrier-frequency acquisition, fast Fourier transform window-timing detection, SCH frame-timing detection, fine carrier-frequency synchronization, and cell-specific scrambling code (CSSC) identification. Simulation demonstrated that this scheme can identify the CSSC with high detection probability while precisely synchronizing the carrier frequency in severe frequency-selective fading channels.

This paper proposes using a quasi-orthogonal (QO) full-rate space-time block code (STBC) and a fixed multibeam transmit array with low sidelobes for orthogonal frequency-division multiplexing code-division multiple access (OFDM-CDMA) downlink transmission. The proposed beam-time transmit diversity orthogonalizes signals between adjoining beams with a QO-STBC and suppresses unorthogonalized signals between separated beams with beam forming. Simulation results demonstrate that the proposed scheme retains orthogonality in narrow angular spread environments and provides two-beam diversity combining when a user is located between beams. In addition, the proposed scheme can alleviate multiuser interference (MUI) by allocating several users into different symbols in a QO-STBC matrix while assigning them all to the same spreading code.

We implemented a space-time equalizer using two sets of single-constrained sample matrix inversion array processors and a maximum-likelihood sequence estimator by using digital signal processors and field-programmable gate arrays. One of the array processors constrains the direct path, sends the one-symbol-delayed path component to the array output, and suppresses the paths with longer delays. The other array processor constrains the one-symbol-delayed path, sends the direct path component to the array output, and also suppresses the paths with longer delays. The desired paths, thus, extracted, whose signal-to-interference-plus-noise-ratios are improved in both path-diversity branches, are then combined by using a branch-metric-combining Viterbi equalizer. We implemented a receiver equipped with this equalizer and evaluated its bit-error rate performance by using a channel emulator. Experimental results indicate that the space-time equalizer provides both space diversity gains and path diversity gains while suppressing signals on paths with long delays.

This paper proposes employing a fixed multibeam transmit array and a space-time block code (STBC) in orthogonal frequency-division multiplexing code-division multiple access (OFDM-CDMA) downlink transmission. We use an STBC that has the same number of column vectors as the number of beams to orthogonalize the signals transmitted from all of the beams. Orthogonality is thus maintained among signals transmitted from a number of beams that have large sidelobes or are subject to wide angular spread. The results of simulation show that the proposed scheme provides beam diversity gain and that it can alleviate multiuser interference (MUI) by allocating several users to different symbols in an STBC matrix while assigning them all to the same spreading code.

We developed a space-time equalizer consisting of constraint SMI arrays and MLSE using multi-DSPs and FPGAs. The proposed scheme combines space diversity branches to maximize the SINR of the direct path and the one-symbol delayed path by using multiple space-time processing. Then path diversity branches are combined using a branch-metric-combining Viterbi equalizer. We evaluated the BER performance of this developed receiver by using an adaptive array test system. The experimental results indicate that the space-time equalizer provides both space diversity gains and path diversity gains while suppressing longer delayed paths.

A space and path diversity reception using a four-antenna array processing maximum likelihood sequence estimation (AP-MLSE) is implemented. The scheme combines space diversity branches to maximize the signal-to-interference-plus-noise-ratio (SINR) of the direct and one-symbol delayed paths using a pair of single constraint sample matrix inversion (SMI) array processors and a branch-metric-combining MLSE. A laboratory experiment is conducted to measure the bit error rate (BER) performance for frequency-selective fading channels in a 4.096 Mb/s time-division multiple-access (TDMA) transmission system. For comparison, the BER for a four-antenna SMI array processor (space-only diversity) is also implemented and measured simultaneously under identical conditions. The measured results show that the space and diversity reception using AP-MLSE provides a significantly improved BER compared with that for the SMI array.

We propose a path diversity reception technique that uses steering vector arrays and Viterbi algorithm-based sequence estimation techniques to cope with severe intersymbol interference (ISI). The proposed scheme suppresses some delayed paths with spatial and temporal processing based on the single constraint sample matrix inversion algorithm to reduce excess ISI for sequence estimation. The path diversity branches are weighted in proportion to the quality of the array output signals and then combined via the branch-metric-combining (BMC) Viterbi algorithm to estimate the most likely transmitted sequence. Maximum likelihood sequence estimation, limited-state sequence estimation, and decision-feedback sequence estimation are examined for their suitability as estimation techniques. Consequently, the proposed scheme is sufficient for managing various kinds of channel models. Thus, the proposed scheme is suitable for high-speed time-division multiple access (TDMA) mobile radio systems, because, in this kind of transmission, fading can be presumed to be nearly quasi-static during the burst, and the delayed paths are spread over several symbols.

In this paper, the joint processing of an adaptive array and a maximum likelihood sequence estimator (MLSE) is proposed to cope with frequency-selective fading. The proposed scheme generates replicas of multiple desired paths with different delays received at all antennas. The errors between the array outputs and the replicas are branch metric combined in the Viterbi algorithm to estimate the most likely transmitted sequence. The replicas are also used as reference signals for controlling array weights to suppress long delay paths. Simulation results show that the proposed scheme can provide a better bit error rate performance than a conventional adaptive array or a conventional MLSE in frequency-selective fading channels.