In this paper, we present a pilot design for an OFDM cellular system based on Costas arrays. The design provides pilot symbols for both common and dedicated pilots for MIMO transmission. The pilot symbols are designed so that pilots from different cells intersect minimally. The design results in a large number of available distinct pilot symbol patterns with minimally overlapping resource elements that may be assigned to different cells which simplifies cell planning. The design is described in the context of IEEE 802.16m, but is applicable to any OFDM system. Performance of the pilot design in noise and interference limited environments is presented and is shown to be on par with currently used pilot designs while providing a larger number of patterns.

In this paper, we explore a method for interference suppression in EGPRS (also commonly known as EDGE) systems, wherein 8PSK is used as the modulation for the desired signal and the interferer is modulated using GMSK. Previous methods for single antenna interference cancellation (SAIC) have concentrated on the case when both the desired and interfering signals are GMSK modulated, or have relied on highly complex methods such as joint demodulation. We derive a simple method that exploits the redundancy inherent in the interferer signal to suppress it. The interference suppression method causes a well- modeled nonlinear distortion to the 8PSK signal. This non-linear distortion is handled using a modified equalizer that deals with a time-varying input symbol constellation applied to a matrix channel. Simulation results using the method show great promise.

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.

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.

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.

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.

Both IS-136 and PDC digital cellular systems employ forward error correction (FEC) encoding followed by a form of DQPSK modulation. To optimize FEC decoder performance at the receiver, soft information from the DQPSK demodulator is required. For frequency nonselective fading channels, which occur often in these systems, coherent demodulation optimizes performance. In this paper, we derive the maximum a posteriori (MAP) bit estimate for DQPSK modulation in non-ISI channels, assuming knowledge of the channel response. The MAP estimator forms a bit log-likelihood ratio, which provides the optimal "soft information" for MLSE or MAP convolutional decoding. Both single and multiantenna receivers are considered. MAP estimation requires exponentiation and logarithm operations, as well as knowledge of the noise covariance. To reduce complexity, two approximate forms are developed which avoid the exponentiation and logarithm operations. If the noise has the same power on each antenna and is uncorrelated between antennas, then knowledge of the noise covariance is also not needed. Performance of the optimal and approximate schemes is evaluated via simulation for single and multi-channel receivers under static and frequency non-selective Rayleigh fading conditions. The approximate approaches are shown to be within 0.5 dB of the optimal approach and to outperform existing approaches.

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.

Tail bits and tail-biting are the two commonly used methods for terminating the trellis of convolutional codes. In this paper, we evaluate the relative performance of these two options by comparing their performance in a variety of cases. Suboptimal decoding algorithms for the tail-biting case are proposed. Puncturing is proposed as an alternative method to prevent the rate loss when tail bits are used. The performance of several puncturing schemes are evaluated. In addition, we explore the bit error rate of these schemes as a function of position of bits in a block.

Achievable spectral efficiency in land-mobile radio (LMR) systems is limited by adjacent-channel interference (ACI) specifications. Spectral efficiency can be improved by using digital modulation schemes. With linear modulation schemes, intermodulation products produced by non-linear power amplifiers lead to increased ACI. In this paper, a new family of linear modulation schemes called envelope-constrained modulation schemes is introduced to improve the spectral efficiency of LMR systems using quasilinear power amplifiers. These modulation schemes exhibit low envelope variations and thus ease the linearity requirements on the power amplifier. Also, power savings can be realized by incorporating coded modulation. An analytical evaluation of the performance is performed to illustrate the gain achievable using the new modulation schemes.

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.>