Fixed Sphere Decoder Research Articles

Fixed-Complexity Tree Search Schemes for Detecting Generalized Spatially Modulated Signals: Algorithms and Hardware Architectures

In the generalized spatial modulation (GenSM) multiple input multiple output (MIMO) system, each block of data bits is mapped to a set of spatially multiplexed (SMX) symbols and an index of transmit antenna combination (TAC) of active antennas. The difficulty for the GenSM MIMO receiver is to detect the SMX symbols and TAC index simultaneously. Recently, we applied the conventional sphere decoding algorithm (SDA) successively to achieve the exact maximum likelihood detection (MLD) of GenSM MIMO signals. The SDA scheme suffers from variable computational complexity and leads to hardware detectors with variable throughput rate. Instead, fixed-complexity tree search algorithms, e.g., reduced fixed sphere decoding (rFSD), with nearly MLD performance are employed to facilitate hardware implementation. Here, we propose to apply the rFSD successively and design a hardware architecture for detecting GenSM MIMO signals under the scenario of 5 transmit antennas, 2 transmit radio frequency chains, 4 receive antennas, and 64-QAM SMX symbols. The VLSI implementation results under the TSMC 90nm CMOS technology reveal that our architecture requires 276.7K gates and provides detection throughput 1.613 Gbps, while operating at 322.6 MHz. Compared with other related architectures, our architecture provides higher detection throughput rate and is of better hardware efficiency.

Beyond 100 Gb/s SEFDM Signal IM/DD Transmission Utilizing TDE With 20% Bandwidth Compression

We experimentally demonstrate a beyond 100 Gb/s intensity modulation/direct detection (IM/DD) 16-QAM optical spectrally efficient frequency division multiplexing (SEFDM) transmission over 2-km standard single mode fiber (SSMF) with 20% signal bandwidth compression. Time-domain equalization (TDE), which outperforms frequency-domain equalization (FDE) in our system, is adopted for SEFDM system due to the inter-carrier interference (ICI) induced by the spectrum compression. A hybrid cascaded binary-phase-shift-keying iterative detection (CBID) together with fixed sphere decoder (FSD) is successfully utilized to eliminate ICI in such a high-speed interconnection system. Experiment results show that SEFDM outperforms conventional discrete multi-tone (DMT), occupying less frequency spectrum resource and achieving superior resistance to bandwidth limitation and dispersion-induced power fading in fiber links. We also show that CBID is a more effective scheme with 10% bandwidth compression, considering the receiver complexity, while CBID-FSD is proposed for further bandwidth compression.

Downlink Design for Spectrum Efficient IoT Network

The Internet of Things (IoT) is a new network that connects massive devices which have the communication ability. With the requirement of various services and the wireless spectrums becoming increasingly scarce, we consider a nonorthogonal multicarrier transmission scheme that is spectrum efficient frequency division multiplexing (SEFDM) as the downlink transmission scheme for IoT network. SEFDM has the merit of improved bandwidth usage efficiency, but the serious inter carrier interference (ICI) will be introduced by multiplexing overlapped carriers. Thus, the signal detection is challenged for recovering the signal which is suffered from ICI due to the loss of the orthogonality. In this paper, a low complexity detector based on quasi-orthogonality compensation (QOC) is proposed in the downlink receiver. And the complexity of the QOC detector is also analyzed. Moreover, a novel detector which joint QOC and fixed sphere decoding (FSD) algorithm is proposed. The bit error rate performances of QOC and QOC-FSD detectors are evaluated by numerical simulations. Numerical results show that the QOC and QOC-FSD detectors can achieve better performance than conventional iterative detection (ID) and ID-FSD, respectively. Furthermore, QOC-FSD detector performs a lower complexity than ID-FSD detector.

Experimental Demonstration of 16-QAM DD-SEFDM With Cascaded BPSK Iterative Detection

To simplify the complexity of a spectrally efficient frequency-division multiplexing (SEFDM) system, cascaded binary-phase-shift-keying iterative detection (CBID) is proposed for square $M$ -ary quadrature-amplitude-modulation (M-QAM) SEFDM as the first decoding stage, in conjunction with a fixed sphere decoder (FSD). For 16-QAM SEFDM, the decoding complexity of FSD can be reduced to 1/4 with CBID-FSD replacing the iterative detection FSD algorithm to keep the same bit error rate. A 16∼20 Gb/s intensity-modulated single-sideband 16-QAM optical SEFDM is demonstrated by using the CBID-FSD algorithm.

A Low Complexity Fixed Sphere Decoder with Statistical Threshold for MIMO Systems

A Low Complexity Fixed Sphere Decoder with Statistical Threshold for MIMO Systems

Vlsi Architecture of Mimo Detector Using Fixed Complexity Sphere Decoding

Fixed Sphere Decoding is a near optimum tree search detection technique for the spatial multiplexing scheme. The algorithm performs a fixed number of operations to detect the signal independent of the noise level and channel conditions. In this paper, a Soft Input Soft Output Fixed Complexity Sphere Decoding algorithm is proposed for the MIMO receiver using 16 QAM modulation scheme. As the system performance was far from the channel capacity limit, MIMO channel could not support higher spectral efficiencies. Therefore, to obtain power efficiency very close to the Shannon limit, Turbo codes are implemented in MIMO system and provide higher spectral efficiency. The proposed FSD detector is capable of providing a throughput of 1.18 Gbits/s with a critical path delay of 9.603 ns.

An Improved Fixed Sphere Decoder Employing Soft Decision for the Detection of Non-orthogonal Signals

This letter proposes a hybrid soft iterative method together with Fixed Sphere Decoding (FSD) concurrently optimize performance and complexity. We show that for bandwidth compression factors of up to 25 percent, we can achieve the same performance as Orthogonal Frequency Division Multiplexing (OFDM). For systems with bandwidth compression higher than 25 percent, the complexity/performance trade-offs of the hybrid method are better than those of Truncated Singular Value Decomposition-FSD (TSVD-FSD).

Reduced Complexity FSD Algorithm for MIMO Detection

Detection is a major challenge for the utilization of multiple-input multiple-output (MIMO) system. Even the fixed sphere decoding (FSD), which is known for its simplicity in calculation, requests too much computation in high order modulation and large number antenna system especially for mobile battery-operated devices. In this paper, a reduced FSD algorithm is proposed to simplify the calculation complexity of the FSD while maintaining the performance at the same time. Simulation results in a 4×4, 16-QAM system show that up to 89% calculation can be saved while the performance drop is less than 0.1dB when SNR=24.

Reduced complexity FSD algorithm based on noise variance

multiple-input multiple-output (MIMO) system has very high spectrum efficiency. However, detection is a major challenge for the utilization of MIMO system. But even the fixed sphere decoding (FSD), which is known for its simplicity in calculation, requests too much computation in high order modulation and large number antenna system, especially for mobile battery-operated devices. In this paper, a reduced FSD algorithm is proposed to simplify the calculation complexity of the FSD while maintaining the performance at the same time. Simulation results show the effect of the proposed algorithm. Especially the results in a 4×4 64QAM system show that up to 81.2% calculation can be saved while the performance drop is less than 0.1dB when SNR=30.