This paper describes a low complexity demodulator for MIMO, which approximates the optimal joint detector. Our demodulator is called SLIC, which stands for serial localization with indecision and color suppression. The main features of SLIC are: 1) a serial structure, where the search for the eventual solution is incrementally localized in each stage; 2) each stage uses a sparse set of centroids as a proxy for constellation symbols; 3) centroids represent subsets of constellation symbols, and the subsets overlap by design. The use of centroids creates an intentional residual signal, which is left to a later stage to resolve, and is suppressed as a colored noise in the current stage. We present performance results with different subset designs for 16, 64 and 256QAM. Overall, we show that SLIC is an attractive tradeoff of complexity and performance in the MIMO scenario.

In this paper, we study the performance of transmit diversity schemes in a wideband code-division multiple-access (WCDMA) system. We consider a space-time transmit diversity (STTD) scheme using the Alamouti code and a simple switch transmit diversity (STD) scheme in which the transmitter selects instantaneously the best antenna for transmission. Receiver impairment is generally modeled as a colored, complex Gaussian process, which gives rise to temporal and spatial correlations. First, we extend the G-Rake formulation to process a STTD signal, accounting for the space-time code structure as well as the temporal and spatial correlations of the impairments. The performance of STTD and STD for a WCDMA system is then evaluated. We show that STTD improves the coverage of low-to-medium rate services significantly for users at cell edge compared to the scheme without transmit diversity. Such improvements are observed even when the channels already have diversity through multipath and multiple receive antennas. Thus, STTD is well suited for broadcast when feedback from mobiles is not available. For high-data-rate availability, however, using one feedback bit to allow the STD scheme to transmit only on the better antenna achieves significantly better performance compared to STTD. This is because sending the desired signal through one radio channel makes it easier for linear equalizers to restore orthogonality. For mobiles dominated by self-interference due to loss of orthogonality, better equalization results in much improved data rates.

The system benefits of using an advanced dual-branch diversity receiver at the mobile terminal are studied in a Wideband Code-Division Multiple-Access (WCDMA) cellular system. The receiver considered is an extension of the generalized Rake (G-Rake) receiver which provides a form of array processing. In addition to exploiting the temporal correlation in receiver impairments and the orthogonality between spreading codes, spatial correlation is also accounted for in the formulation of finger combining weights. We show that this dual-branch G-Rake receiver, even with a pessimistic antenna configuration, has a potential to increase the system capacity, relative to the conventional single-branch Rake receiver, by 94% for voice communications. For data communications, high-data-rate availability can be significantly improved. This allows a HSDPA user at the cell border to enjoy a data rate close to 4 Mbps in dispersive channels. For end-user performance of TCP/IP applications, users with a dual-branch G- Rake receiver experience significantly higher packet bit rates and shorter download times.

This paper deals with finder placement for Rake receivers, which is a key aspect in the design of receivers for spread-spectrum communications. Finger placement is typically performed using a path searcher, which estimates the power/delay profile (PDP) of the received signal to determine where signal energy is present. A commonly used finger placement approach is to place fingers at delays corresponding to the peaks of the PDP, with a restriction that fingers he no closer than some minimum spacing. Another approach is to place fingers on an equally-spaced grid of delays. For the typical urban (TU) channel model, we quantify performance of the two approaches in terms of acquired signal-to-noise ratio (SNR) and modem bit- error-rate (BER). We show that performance depends primarily on minimum finger spacing and number of fingers. The grid approach performs well when there are enough fingers to collect most of the signal energy. For a channel model in which path delays vary dynamically, we find that the grid approach provides robust performance.

This paper describes the physical layer for a high penetration paging (HPP) mode, that is a highly integrated adjoint to a cellular system. Through the use of very low rate coding and periodically inserted synchronization bursts, the HPP mode enables a cellular user to locate and maintain a minimal communication link with a cellular network at 15 to 20 dB below the normal operating signal to noise ratio (SNR). In this paper, the ANSI-136 air interface standard is used as the underlying cellular system. The same concept can be extended to other systems as well.

We give a general procedure for constructing shaping codes from costly constrained graphs. The codes have finite-state encoders and sliding-block decoders. The shaping codes are in the spirit of Calderbank and Ozarow (1990), where shaping is achieved by nonequiprobable signaling on T subconstellations of a signal constellation /spl Omega/. We show how nonequiprobable signaling relates to a costly constrained channel. Khayrallah and Neuhoff developed a procedure for constructing codes with rates close to the capacity-cost function of the general cost-constrained channel, and we use their procedure to construct asynchronous (variable-length to fixed-length) shaping codes. Comparisons are made with shaping codes having comparable gain. The codes presented here are less complex and have lower peak-to-average power and lower constellation expansion ratio.

Robust source coding provides both compression and noise mitigation without channel coding. Two methods of robust source coding are presented. The first is DPCM incorporating a nonlinear filter; the second is predictive trellis-coded quantization (PTCQ), whose prediction filter is also nonlinear. Findings show that the incorporation of the nonlinear filter provides the ability to conceal the effects of noise and minimize the propagation of error. A comparison is made between the performance of the two methods in a noisy environment. It is demonstrated that the PTCQ scheme provides a method of least distortion at low BER. However, at higher BER the DPCM scheme outperforms PTCQ. Robust source coding of images employing PTCQ or DPCM with the nonlinear prediction filter exhibits low sensitivity to bit errors and yields, visually, a better reconstructed image after transmission over very noisy channels.

We address the problem of finite-state code construction for the costly channel. This channel model is a generalization of the hard-constrained channel, also known as a subshift. Adler et al. (1986) developed the powerful state-splitting algorithm for use in the construction of finite-state codes for hard-constrained channels. We extend the state-splitting algorithm to the costly channel. We construct synchronous (fixed-length to fixed-length) and asynchronous (variable-length to fixed-length) codes. We present several examples of costly channels related to magnetic recording, the telegraph channel, and shaping gain in modulation. We design a number of codes, some of which come very close to achieving capacity.

We address the problem of data compression and transmission applied to images. The noisy channel requires the use of a compression scheme with low sensitivity to bit errors. We present a DPCM scheme with a modified feedback loop containing a nonlinear filter in addition to the familiar linear predictor. This greatly improves the channel response of DPCM, without significantly degrading its compression performance.

We investigate the expansion factor /spl Delta/(C) of an (n, k, d) linear block code. /spl Delta/(C) indicates how well the code preserves and expands the distance relations of the input. We relate /spl Delta/(C) to n, k, d, the total weight, and the weight distribution, and we find bounds on /spl Delta/(C).>

We investigate the bit-error rate (BER) of linear block codes. The BER is a function of the encoder realization of the code. We express the BER in terms of the weight profiles of the subspaces of the code. We also discuss the relation between the Hadamard transform and the BER.>