Zhenning Shi

Joint iterative decoding of serially concatenated error control coded CDMA

Joint iterative decoding of multiple forward error control (FEC) encoded data streams is studied for linear multiple access channels, such as code-division multiple access (CDMA). It is shown that such systems can be viewed as serially concatenated coding systems, and that iterative soft-decision decoding can be performed successfully To improve power efficiency, powerful FEC codes are used. These FEC codes are themselves serially concatenated. The overall transmission system can be viewed as the concatenation of two error control codes with the linear multiple access channel, and soft-decision decoders are used at each stage. A variance transfer function approach applied to the analysis of this system captures the role of the component decoders in an overall iterative decoding system. We show that this approach forms a methodology to study the effects of the component codes as well as that of the iteration schedule. Analysis and simulation examples are presented for transmission systems that operate close to the Shannon limit and illustrate the accuracy of the analysis.

Training symbol based coarse timing synchronization in OFDM systems

In orthogonal frequency division multiplexing (OFDM) systems, coarse frame timing can be acquired from one or more training symbols preceding every OFDM burst. The existing literature studied the case where there was only one training symbol consisting of identical segments. We generalize the timing synchronization methods to take advantage of multiple training symbols and only require the segments to be highly correlated but not necessarily identical. We construct a series of component timing metrics, one for each pair of the highly correlated segments, and combine them linearly to minimize the false alarm probability while keeping the asymptotic missed detection probability to the same level as other techniques. The OFDM data symbols in a downlink burst can have different power levels to reach the users at different distances. We take that into account and yield more realistic results than those in existing literature which only considered the equal power case. The performance of the proposed method is analyzed in three scenarios generalized from the IEEE 802.11 and IEEE 802.16 standards. Numerical results are presented to confirm the robustness of the proposed method in various channel conditions.

Successive multiuser detection and interference cancelation for contention based OFDMA ranging channel

In this letter, we propose a successive multiuser detector (SMUD) for contention based OFDMA ranging channel compliant to the IEEE 802.16 (WiMAX) standard. A ranging channel consists of a set of subcarriers in specific time slots shared by multiple users, so the multiple access interference (MAI) limits the performance of ranging detectors. Different from existing methods that treat the MAI as noise, the proposed SMUD successively detects the channel paths of active ranging signals and cancels their interference for further detection. This approach significantly suppresses the MAI and improves both user detection and parameter estimation performance.

On uplink interference scenarios in two-tier macro and femto co-existing UMTS networks

A two-tier UMTS network is considered where a large number of randomly deployed Wideband Code Division Multiple Access (WCDMA) femtocells are laid under macrocells where the spectrum is shared. The cochannel interference between the cells may be a potential limiting factor for the system. We study the uplink of this hybrid network and identify the critical scenarios that give rise to substantial interference. The mechanism for generating the interference is analyzed and guidelines for interference mitigation are provided. The impacts of the cross-tier interference especially caused by increased numbers of users and higher data rates are evaluated in the multicell simulation environment in terms of the noise rise at the base stations, the cell throughput, and the user transmit power consumption.

Iterative Turbo Channel Estimation for OFDM System over Rapid Dispersive Fading Channel

Current OFDM systems assume the channel is not time varying within one OFDM frame, and use channel estimates obtained from preamble or pilots for data symbols within the same frame. This performs poorly in rapid dispersive fading channel with high mobility. This paper proposes a novel iterative turbo channel estimation to improve the performance of OFDM system over fading channel with both time and frequency selectivity. Unlike the conventional channel estimation techniques only using preamble or pilots, the proposed channel estimation technique makes use of preamble, pilots and soft coded data information in a turbo iterative approach. Mean square error analysis and simulation results show that the proposed channel estimation technique can approach the performance of perfect channel state information (CSI) with fewer pilots insertion.

Successive multiuser detection and interference cancelation for contention based OFDMA ranging channel [transactions letters]

In this letter, we propose a successive multiuser detector (SMUD) for contention based OFDMA ranging channel compliant to the IEEE 802.16 (WiMAX) standard. A ranging channel consists of a set of subcarriers in specific time slots shared by multiple users, so the multiple access interference (MAI) limits the performance of ranging detectors. Different from existing methods that treat the MAI as noise, the proposed SMUD successively detects the channel paths of active ranging signals and cancels their interference for further detection. This approach significantly suppresses the MAI and improves both user detection and parameter estimation performance.

EXIT Chart Analysis of an Iterative Receiver with Channel Estimation

In this paper we derive a novel extrinsic information transfer (EXIT) function for a channel estimator that uses both pilot symbols and soft decoding information. We consider a CDMA system with forward error correction that employs iterative multiuser detection based on soft interference cancellation and single user decoding. The J function is useful in convergence analysis as it provides a mapping between mutual information and variance. J functions are derived for both the flat-fading Rayleigh channel and channel estimate, allowing the a priori inputs to the channel estimator, interference canceller and decoder to be modeled. Algorithms for obtaining the EXIT functions for the three receiver blocks are proposed. We then combine the EXIT functions to create an EXIT chart that can be used to predict the behaviour of the overall receiver. Simulations are run for a 3GPP-like receiver in a mobile environment with a vehicle speed of 100 km/h. It is found that under these conditions, the detector trajectory closely matches our proposed EXIT chart.

Optimal power allocation and code selection in iterative detection of random CDMA

Iterative joint detection of random CDMA is considered. It is shown that optimal power levels and optimal code can be found that achieve the capacity of this channel. Even few power levels can significantly improve the achievable capacity. Furthermore, it is shown that very low rate codes are optimal, but that practical codes, again, can achieve a large portion of the channel capacity.

Rate optimization for IDMA systems with iterative joint multi-user decoding

In this letter, we develop a rate allocation scheme for interleave-division multiple-access (IDMA) systems with iterative decoding. We use a fully-analytical approach to predict the performance of the scheme. Then, we propose a modified linear programming method to find the best rate profile for the scheme. Numerical results show that with just repetition coding and optimal rate allocation, the performance of the scheme is only about 5 dB away from the capacity for a wide range of SNR, provided that the number of users is sufficiently large. Compared with power allocation schemes for IDMA, the proposed rate allocation scheme achieves a similar performance at a moderate spectral efficiency and the requirement of sophisticated power amplifiers can be relaxed.

Iterative Channel Estimation for IDMA Systems in Time-Varying Channels

In this paper, we develop low-complexity iterative channel estimation techniques for emerging IDMA systems. The channel estimators make use of pilots as well as soft decoded data information. We derive a lower bound for channel estimation error that reflects the reliability of soft decoded data. We show that the estimators perform close to a minimum variance unbiased estimator as the mean square error (MSE) approaches the lower bound. Numerical results on the MSE and BER performance also show that the proposed channel estimators are able to track the time-varying channel states.