Zhongding Lei

A Comparative Study of QRD-M Detection and Sphere Decoding for MIMO-OFDM Systems

We present a comparative study of two tree search based detection algorithms, namely, the M-algorithm combined with QR decomposition (QRD-M) and the sphere decoding (SD) algorithms, for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. First, we show that nodes ordering before and during the tree search are important for both algorithms. With appropriate ordering, QRD-M can improve detection performance significantly and SD can reduce decoding complexity substantially. Then we compare the implementation complexity of the two algorithms, in terms of the number of nodes required to search or the required number of multiplications to achieve maximum likelihood detection performance. It is interesting to show that the average complexity of SD is lower than that of QRD-M, whereas the worst case complexity of SD is much higher than that of QRD-M

Pseudo-Inverse MMSE Based QRD-M Algorithm for MIMO OFDM

We propose a pseudo-inverse minimum mean squared error (MMSE) based QR decomposition (QRD) M-algorithm (QRD-M) for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. The proposed algorithm can effectively mitigate the detrimental effect of MIMO channel ill-conditions on the bit error rate (BER) performance. As a result, the number of retained branches (M value) can be reduced to half of the original QRD-M algorithm in Kyeong Jin Kim and R.A. Iltis (2002) in order to achieve the same performance for both the uncoded and coded systems. The detection complexity is therefore greatly reduced

A low complexity near ML V-BLAST algorithm

A low complexity MIMO detection scheme is proposed to achieve near optimal maximum likelihood (ML) detection performance. The scheme is especially efficient in high data rate packet transmission systems, such as wireless LAN based hotspots, fixed broadband wireless access systems, or 4G cellular based hotspots etc. The complexity of the optimal performance achieving scheme is significantly lower than that of the ML detection and the state of art technologies.

A list sphere decoder based turbo receiver for groupwise space time trellis coded (GSTTC) systems

A list sphere decoder (LSD) based turbo receiver is introduced for groupwise space time trellis coded (GSTTC) systems. It can approach the single-user bound by iteratively exchanging soft information between the LSD and the soft-input soft-output (SISO) maximum a posteriori (MAP) space time trellis (STT) decoder. The LSD approximates the SISO maximum likelihood (ML) channel detector using only a subset of hypotheses, and thus greatly reduces the computational complexity. Simulation results show that good FER and BER performances as well as fast convergence can be achieved with a very small list for simple STT codes with a small number of states. For more powerful codes with more states, a large list is required. Compared with the linear minimum mean square error (LMMSE) turbo receiver, the proposed receiver is more flexible and converges faster. Its performance can always be improved by increasing the list size, but at the cost of increased complexity. For higher order modulation, the performance gain is larger.

Iterative detection for Walsh-Hadamard transformed OFDM

In this paper, an iterative detection method is proposed to reduce the noise enhancement in Walsh-Hadamard transformed (WHT)-OFDM system. The iterative detection starts with conventional detection. The estimation of the received signal at the weakest subcarrier is calculated based on the detection of all data symbols after the hard decision or a combination of hard decision with soft decision, such as utilizing a clipping function. Then the actual received signal at the weakest subcarrier is replaced by the estimated signal and the conventional detection is applied repeatedly to replace the next weakest signal until all subcarriers are replaced. Through replacing the real subcarrier signal with the less noisy estimated signal one by one, iteratively, we can reduce the noise amplification significantly and smoothly. It is seen in the simulations, the bit error rate performance of the iterative detection is much better than that of the conventional detection for either WHT-OFDM system or OFDM system.

Iterative Receivers for Pre-transformed MIMO-OFDM Systems

In this paper, we present a pre-transformed (PT) multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) system, in short PT-MIMO-OFDM system. We propose two receiver structures that combine the iterative subcarrier reconstruction and iterative interference cancellation to fully exploit both the spatial and the frequency diversity of MIMO-OFDM systems. Significant performance gain can be achieved. Based on the performance and receiver complexity tradeoff, we also present a set of guidelines for choosing either one of the two receivers in practical systems

Near optimal list MIMO detection

A list MIMO detection scheme is proposed to achieve near optimal maximum likelihood detection performance. The scheme is especially efficient in high data rate packet transmission systems, such as wireless LAN based hotspots, fixed broadband wireless access systems, 4G cellular based hotspots etc. The complexity of the optimal performance achieving scheme is significantly lower than that of the ML detection and the state of art technologies

Performance analysis of iterative detection for pre-transformed OFDM

Recently, an iterative detection method is proposed for a pre-transformed OFDM system, where significant performance gain is shown to be achieved through linear processing (Z.Lei et al., Apr. 2003). An analytical treatment of the subject is given. The optimum transform design and reconstruction criteria are given to maximize the minimum SNR. In the case when no iteration is applied (i = 0), we compare the effect of the transform on the performance in a channel with and without diversity. Under the assumption that the previous detection is error free, it is shown analytically that the iterative method achieves a diversity advantage of i + 1 in the i/sup th/ iteration, thus providing an explanation on its superior performance. Simulations are conducted to corroborate the analysis. Due to the generality of the transform design, the analysis conducted is applicable for other common systems (such as MC-CDMA and SC-FDE systems) as well.