Rajaram Ramesh

Delay tracking for direct sequence spread spectrum systems in multipath fading channels

This paper considers the problem of multipath delay tracking in direct sequence spread spectrum systems operating in multipath fading channels. Due to the pulse shaping of the PN code chips, tracking multipath delays is a challenging problem, for which we introduce three novel techniques. In the first technique, maximum likelihood estimation, we search all possible combinations of delays and select the set of delays that minimize a metric derived from the error between the received signal and an estimated signal based on these postulated delays. To reduce complexity, we introduce the ordered maximum likelihood technique, in which the above mentioned metric is minimized iteratively assuming that the channel has one path, then two paths, etc. At each iteration, the delay estimates derived from previous iterations are fixed. Therefore, in each iteration only one delay estimate is produced. Another technique presented in this paper is envelope tracking with subtraction. In this technique, we select peaks of the correlation function between the received direct sequence spread spectrum signal and the local replica. After selecting each peak, the contribution due to the corresponding channel path is subtracted from the correlation function.

Improved channel estimation with side information

In this paper, we describe a technique to incorporate side information into the channel estimation operation at the receiver in a GSM-based system. The technique has a complexity comparable to that of the standard method. Simulations indicate that in terms of coded bit error rate (BER), our technique produces gains of about 0.8 dB in a static channel, and 1 dB in non-dispersive Rayleigh and Rician fading channels. Similar gains are achieved for uncoded BER and probability of detected error. Such gains are desirable in a cellular system. They are considerable in a satellite system, where link margin is at a high premium.

System and method for personalizing electronic mail messages by rendering the messages in the voice of a predetermined speaker

Electronic messages are personalized by rendering the messages to recipients in the voice of a predetermined human speaker. This is accomplished by associating with the message payload a set of basis vectors comprising speech parameters from the predetermined human speaker for use in speech synthesis.

Adaptive joint detection of cochannel signals for TDMA handsets

In mobile communication systems, downlink (forward link) system capacity is limited by the ability of mobile receivers to recover the desired signal in the presence of cochannel interference (CCI). Joint detection of the desired and cochannel signals is a useful approach to improving receiver performance, thus increasing system capacity. In this paper, we show that a practical single-antenna joint-detection receiver can provide significant gains in system capacity for the time-division multiple-access (TDMA) standard Telecommunications Industry Association/Electronic Industry Association/Interim Standard-136 (TIA/EIA/IS-136 or IS-136). For a sectorized system, joint detection provides a capacity gain of 47% in a typical urban environment. When used in conjunction with transmit beamforming, the synergy between the two approaches leads to a capacity gain of over 200%. In determining these gains, practical aspects of the IS-136 system are considered, namely, unsynchronized networks, limited receiver complexity, and adaptability. A semiblind acquisition process, which uses the training sequence of the desired user only, is employed, because the desired and interfering base stations are not synchronized. The receiver complexity is controlled by processing only one sample per symbol period, even though it is shown that multiple samples per symbol period should ideally be used. Finally, because receiver performance may be limited by its own intersymbol interference instead of CCI, an adaptive joint-detection process is used which selects between joint demodulation and single-user equalization for each slot.

Maximum SNR prefiltering for MIMO systems

In this paper, we derive a class of FIR prefilters for MIMO systems that maximize the signal-to-noise ratio (SNR) defined as the sum of SNRs for the output branches with uncorrelated noise. This definition is motivated by an analysis of the pairwise error probability. The maximum-SNR (MSNR) class of prefilters includes decision feedback equalization (DFE) and channel shortening equalization (CSE) prefilters. The feedback taps are completely suppressed by the CSE prefilter and not suppressed at all by the DFE prefilter. The MSNR class includes hybrid prefilters which are a combination of CSE and DFE prefilters. Hybrid prefilters allow middle ground for suppressing feedback taps which is desirable for equalization using decision feedback sequence estimation (DFSE). We show simulation results for joint DFSE demodulation of two 8-PSK modulated cochannel EDGE users using a single antenna receiver. We find that a hybrid prefilter performs best for this example.

Maximum likelihood sequence estimation in the presence of constant envelope interference [cellular radio networks]

Gaussian models of signal disturbance are widely used in conventional designs of mobile receivers. However, such models do not accurately reflect the statistics of co-channel interference in narrow-band cellular networks. In this paper, we investigate the potential of suppressing co-channel interference using a simple non-Gaussian disturbance model in equalization. In particular, the model exploits the constant-envelope nature of GMSK-modulated interference, which commonly appears in GSM/EDGE networks. We show that the metric of the corresponding MLSE equalizer is simply the sum of the conventional Euclidean-distance metric and a correction term that depends on the estimated amplitude of the interference. The new metric sagaciously biases the decision regions in accordance with the non-Gaussian statistics. Using the GSM/EDGE air interface, we demonstrate that a large gain (up to 18 dB) in C/I can be achieved by the new metric with little increase in receiver complexity.

Iterative channel and parameter estimation for noise with non-Gaussian distributions

Traditional designs of mobile communication receivers commonly rely on a (colored or white) Gaussian model of the signal disturbance. Such a model does not accurately reflect the statistics of co-channel interference, a dominant source of signal disturbance that limits the capacity of most cellular networks. Consequently, the performance of receivers designed based on the Gaussian model is often far from optimum in a heavily loaded network. In this paper, we explore the potential of suppressing interference in channel estimation by using a non-Gaussian noise model of the signal disturbance. An EM-based, iterative algorithm is derived for jointly estimating the channel response and the parameters that characterize the non-Gaussian noise model. As an example, we also consider a special case where the non-Gaussian model has a constant envelope characteristic. Using the GSM/EDGE air interface, we demonstrate that significant performance gains can be obtained with the proposed channel estimation algorithm when a dominant GMSK-modulated interference exists. The proposed algorithm can also be used in combination with equalizers designed for non-Gaussian noise models to provide further performance gains.

Traffic analysis of partially overlaid AMPS/IS-136 systems

We consider the problem of calculating the traffic allowable for a certain grade of service in a cellular network employing both AMPS and IS-136 channels. The dual-mode capability of the IS-136 users enables the system to assign them to AMPS channels if IS-136 channels are blocked, hence the two pools of users cannot be treated independently. An analytical method for the calculation of the traffic is derived and the actual capacity improvements obtained by a partial deployment of IS-136 are shown. We also consider the case of reconfigurable transceivers at the base station and derive the traffic allowable. It is seen that a significant increase in traffic is achieved by this option.

A Physical Layer Abstraction for Maximum Likelihood Demodulation of MIMO Signals

We develop a physical layer abstraction to model the performance of MIMO with maximum likelihood demodulation (MLD) in a system simulation. MLD is inherently a non-linear process, and thus it is difficult to find a linear abstraction to its performance. The abstraction is based on bounds on performance obtained from the QR & QL decompositions of the channel. The accuracy of the abstraction is demonstrated using link simulations. In addition, the abstraction has low complexity. The model is used in a system simulation to evaluate the gains obtained with MLD.

Performance of a spectrally efficient modem for land-mobile radio

The spectral efficiency in land mobile radio (LMR) systems is limited by stringent adjacent-channel interference protection ratio (ACIPR) specifications of at least 60 dB. High spectral efficiency can be achieved using digital modulation techniques. Linear modulation schemes exhibit high spectral efficiencies but require linear power amplifiers. Power amplifiers typically used in LMR are nonlinear which degrade the ACIPR for conventional linear modulation schemes. The paper presents simulated performance results of a trellis-coded linear modulation scheme that is designed to be less sensitive to the nonlinearity exhibited by the power amplifier. Simulation results indicate that an ACIPR of 60 dB can be achieved. An E/sub b//N/sub 0/ of 14 dB is required to achieve a bit error rate of 3% with 3 users per 25 kHz channel, with an average data rate of 9.6 kbits/second/user.<<ETX>>