Optimum data detection schemes for dual layer multi-user multiple-input multiple-output (MU-MIMO) systems are studied. A joint maximum likelihood (ML) modulation classification (MC) of the co-scheduled user and data detection receiver is developed. By expanding the max-log-maximum-a-posteriori MC approach to include distances of counter ML hypothesis symbols, the decision metric for MC is shown to be an accumulation over a set of tones of Euclidean distance computations also used by the ML detector for bit log-likelihood ratio soft decision generation. With a small complexity overhead, the proposed approach achieves near-optimal performance. An efficient hardware architecture is presented for the proposed approach.

In this paper, dual-layer multi-user multiple-input multiple-output systems are studied. Building on the low-complexity layered orthogonal lattice detector (LC-LORD), an efficient sub-optimal joint modulation classification (MC) of the co-scheduled user and data detection receiver is developed. By adjusting the Max-Log-Maximum-a-Posteriori MC approach to the limitations of LC-LORD, and expanding it to include distances of counter maximum likelihood hypothesis symbols, the decision metric for MC is shown to be an accumulation over a set of tones of Euclidean distance computations also used by the LC-LORD detector for bit log-likelihood ratio soft decision generation. Simulations demonstrate that with a small complexity overhead, the proposed approaches achieve near interference-aware performance. An efficient hardware implementation scheme is presented.

We present novel soft-input soft-output (SISO) multiple-input multiple-output (MIMO) detectors based on the Chase detection principle [1] in the context of iterative and decoding (IDD). The proposed detector complexity is linear in the signal modulation constellation size and the number of spatial streams. Two variants of the SISO detector are developed, referred to as SISO B-Chase and SISO L-Chase. An efficient method is presented that uses the decoder output to modulate the signal constellation decision boundaries inside the detector leading to the SISO detector architecture. The performance of these detectors significantly improves with just a few number of IDD iterations. The effect of transmit and receive antenna correlation is simulated. For the high-correlation case, the superiority of SISO B-Chase over the SISO L-Chase is demonstrated.

New expressions for the soft decision bit log-likelihood ratio (LLR) of a MIMO system using quadrature amplitude modulation (QAM) taking into account channel estimation error (CEE). The bit LLR for the maximum likelihood (ML) and the linear minimum mean-squared error (MMSE) receivers are derived, showing in both receivers explicit scaling of the LLR that is a function of the QAM symbol and the CEE variance. These new expressions for the LLRs are used to show that only modest improvements in the link performance are achieved relative to the LLRs that do not take into account the CEE. This indicates that separating the detector design from channel estimation does not significantly impact the system performance, which leads to simplifications in the overall receiver implementation.

A log-max generalized bit log-likelihood ratio (GLLR) was derived in an earlier work of Wanget al.for soft decision decoding of quadrature amplitude modulation that takes into account channel estimation error. The results in the above paper show that the GLLR metric outperforms the conventional LLR metric that does not account for channel estimation error. In trying to reproduce the results of the above paper, our simulations indicate that their results were obtained without proper scaling of the transmit constellation. Conversely, through simulations, we demonstrate that by properly scaling the constellation to have unity average energy, the GLLR metric does not result in a noticeable advantage in the bit error rate over the conventional LLR metric for practically meaningful values of the channel estimation error.

Since multi-antenna (MIMO) systems are becoming more popular thanks to their inherent potential for capacity improvement, interference from MIMO transceivers is an increasingly serious concern. Spatial multiplexing schemes are particularly vulnerable to multi-user interference. Fortunately, this interference can be mitigated, when the channel matrices show a sufficient spatial separation. In this paper, we quantify the separability of multi-user MIMO channels using actual measurements in a scenario where a single outdoor base station transmits to two indoor mobile receivers. To quantify the spatial distance between the two users, we compare the spatial correlation matrices using two simple measures: (i) matrix collinearity, and (ii) the condition number ratio. Both measures are directly linked to MIMO system performance. Our measurement-based evaluations demonstrate that the downlink channels of different users can have a significantly different spatial structure, even when the users are in the same room. This leads to the following conclusions: (i) new multi-user MIMO models are needed to describe the spatial characteristics of different users, and (ii) spatial interference can be well managed by appropriate scheduling and precoding algorithms.

In this paper, we show that the use of cyclic delay diversity (CDD) on the downlink (DL) can improve the performance of uplink (UL) diversity methods such as antenna selection in a time-division duplexing (TDD) system. An analytical framework for showing why CDD transmission on the DL improves UL diversity transmission in TDD system is provided. The analysis is used to quantify the gain in the UL received signal-to-noise ratio (SNR) when DL CDD is used. The analytical results show that for a 2times2 MIMO system that uses DL CDD, UL transmit diversity based on antenna selection improves the average UL SNR by 0.41 dB relative to the case when CDD is not being applied at the BS transmitter and improves the 1% CDF UL SNR by 1.58 dB. Measurement-based analysis validates the analytical results using measured frequency response of a MIMO system using a wideband channel sounder.

This paper describes a frame work for evaluating the coverage performance of an orthogonal frequency division multiplexing (OFDM) system as defined in the IEEE 802.16e/WiMAX standard. Both the downlink and the uplink coverage are presented under a system with multiple cells and sectors. The spatial area coverage for a single frequency reuse and reuse of 3 are evaluated. Our results show that a WiMAX system can be uplink coverage limited for base station receivers with only two antennas. It is shown that there is a tradeoff between the uplink sector throughput and coverage performance. On the downlink, it is shown that a combination of cyclic delay diversity (CDD) with segmented frequency reuse of 3 significantly improves coverage performance. Also, shown is a comparison of downlink coverage performance between maximum ratio combining (MRC) and minimum mean square (MMSE) receivers at the subscriber station. It is shown that for a single frequency reuse an MMSE receiver with CDD at the transmitter are needed for adequate downlink coverage performance.

The effect of channel estimation error on the performance of a wideband code division multiple access link using a pilot channel multiplexed with the data channel is analyzed. The gain achievable by improving the channel estimation algorithm is studied. Analysis and simulation results are presented for a data-aided channel estimation scheme that effectively extends the duration of the pilot signal. Using the maximum likelihood detection principle, it is shown that making tentative minimum mean-squared error decisions on the data bits using estimates obtained from the pilot channel can extend the number of samples used in the channel estimation, thus improving the channel estimate quality. Simulations show that a gain of up to 1.5 dB in block-error rate performance is achievable using the proposed channel estimation scheme for both Convolutional and Turbo codes. It is shown that the performance gain depends on the information bit rate, the channel characteristics, including the Doppler frequency, and the multipath fading channel power delay profile. Simulation results also show that for weak paths, data-aided channel estimation benefits significantly from diversity combining.

The forward link of an IS-2000 CDMA system may become Walsh code limited under certain scenarios. To enhance the capacity in such scenarios, alternative sets of orthogonal functions called the quasi-orthogonal functions (QOF), which possess optimal minimax cross correlation with Walsh code sets of variable length, have been incorporated in IS-2000. We discuss a method of significantly enhancing the maximum throughput in evolutions of IS-2000 systems. This method uses aggregation of multiple quasi-orthogonal functions with a relatively smaller constellation alphabet size for a single user with a joint multi-channel detector. This method is compared with the alternative method for enhancing the maximum throughput using aggregation of a relatively smaller number of Walsh functions, but with a higher constellation alphabet size (multi-level modulation). Discussions on the trade-offs and recommendations as to the better method for enhancing the throughput in evolutions of IS-2000/3G systems are then provided.

The performance of a DS/CDMA system using M-ary orthogonal modulation with noncoherent demodulation is evaluated. The system operates in a multipath fading channel. A RAKE receiver structure with equal gain combining is used for demodulation. An approximation to the bit error probability is given which depends only on the first- and second-order moments of the multipath energies. The analysis results are compared with the results from computer simulations. It is seen that the approximation is accurate for multipath energies with realistic coefficient of variation. The system performance is also evaluated in terms of the capacity, which is defined as the number of users that can be supported at a given bit error probability. The approximation is used to evaluate the capacity reductions due to power variations caused by multipath fading.>

Mobile radio fading analyses typically assume a single frequency carrier transmitted through the propagation medium. The results are then used for the case of narrowband transmission. New phenomena occur with wideband transmission. This paper presents an analysis of the mitigation of the Rayleigh fading effect for wideband direct sequence code division multiple access (DS/CDMA) signaling. There are measurements that show this phenomenon. The reduction of the Rayleigh fading effect is an advantage of CDMA over narrowband transmission systems and it eases the burden of CDMA power control. A general expression for the coefficient of variation of the received wideband signal power is derived. It is shown how the coefficient of variation of the received power decreases as the bandwidth spreading increases. This paper also analyzes the time correlation needed for other analyses (e.g., CDMA power control).>

For narrowband signals the envelope of the received signal is well characterised by a Rayleigh random process. Rayleigh fading causes large variations in the signal envelope. For wideband signals, the effect of Rayleigh fading is mitigated. This paper considers the effect of receiver processing on the mitigation effect for wideband DS/CDMA signals. The mitigation effect is shown here by a reduction of the coefficient of variation of the received power as a function of bandwidth spreading. The parameters chip rate, processing gain and the number of multipath components tracked are investigated for their effect upon the mitigation-the mitigation effect is sensitive to combinations of these parameters.>