Rate regulated TDMA system for cochannel interference limited wireless ad hoc networks

The time slotted system with rate regulation, which adapts the transmission rate to the interference for each time slot, is proposed to enhance the throughput while meeting the SINR requirement for cochannel interference limited wireless ad hoc networks. We show that the throughput of the rate regulated TDMA system is larger than that of the rate regulated TDMA/CDMA system. Furthermore, we also propose a scheduling algorithm to maximize the total throughput of the rate regulated TDMA system.     

Distributed cochannel interference control in cellular radiosystems

Distributed power control algorithms that use only the carrier-to-interference ratios (C/I ratios) in those links actually in use are investigated. An algorithm that successfully approximates the behavior of the best known algorithms is proposed. The algorithm involves a novel distributed C/I-balancing scheme. Numerical results show that capacity gains on the order of 3-4 times can be reached also with these distributed schemes. Further, the effects of imperfect C/I estimates due to noise vehicle mobility, and fast multipath fading are considered. Results show that the balancing procedure is very robust to measurement noise, in particular if C/I requirements are low or moderate. However, for required high C/I levels or for a rapidly changing path loss matrix, convergence may be too slow to achieve substantial capacity improvements     

Spatial-temporal equalization for IS-136 TDMA systems with rapiddispersive fading and cochannel interference

In this paper, we investigate spatial-temporal equalization for IS-136 time-division multiple-access (TDMA) cellular/PCS systems to suppress intersymbol interference and cochannel interference and improve communication quality. This research emphasizes channels with large Doppler frequency (up to 184 Hz), delay dispersion under one symbol duration, and strong cochannel interference. We first present the structure of the optimum spatial-temporal decision-feedback equalizer (DFE) and linear equalizer and derive closed-form expressions for the equalizer parameters and mean-square error (MSE) for the case of known channel parameters. Since the channel can change within an IS-136 time slot, the spatial-temporal equalizer requires parameter tracking techniques. Therefore, we present three parameter tracking algorithms: the diagonal loading minimum MSE algorithm, which uses diagonal loading to improve tracking ability, the two-stage tracking algorithm, which uses diagonal loading in combination with a reduced complexity architecture, and the simplified two-stage tracking algorithm, which further reduces complexity to one M×M and one 3×3 matrix inversion for weight calculation with M antennas. For a four-antenna system, the simplified two-stage tracking algorithm can attain a 10^-2 bit error rate (BER) when the channel delay spread is half of the symbol duration and the signal-to-interference ratio (SIR) of the system is as low as 5 dB, making it a computationally feasible technique to enhance system performance for IS-136 TDMA systems     

Single-antenna co-channel interference cancellation for TDMA cellular radio systems

Co-channel interference cancellation is particularly challenging in the downlink of cellular radio systems because usually only one receive antenna is available at the mobile terminal. This tutorial provides an overview of promising a single-antenna co-channel interference cancellation techniques. Focus is on the downlink of time-division multiple access systems. The results may, however, be extended to related applications, including interference suppression in multiple-input multiple-output systems.     

Separation of cochannel signals in TDMA mobile radio

We propose a sequential algorithm that separates cochannel time-division multiple-access (TDMA) signals that encounter multipath interference and noise. The receiver employs a multistage architecture where each stage consists of a beamformer and an equalizer that isolates one source, compensates for intersymbol interference (ISI), and demodulates the data. A problem encountered with such bursty sources is that the beamformer/equalizer trained for a particular time slot may not be appropriate for all the data contained in that slot. This occurs because a cochannel source typically overlaps only part of the time slot of interest and may not overlap the training sequence at all. The algorithm presented overcomes this problem by processing the data forward and backward in a sequential noncausal manner. Computer simulations using signals with the IS-54 format are presented to demonstrate the properties of the sequential algorithm     

Coverage probability of cellular networks using interference alignment under imperfect CSI

Interference alignment (IA) is well understood to approach the capacity of interference channels, and believed to be crucial in cellular networks in which the ability to control and exploit interference is key. However, the achievable performance of IA in cellular networks depends on the quality of channel state information (CSI) and how effective IA is in practical settings is not known. This paper studies the use of IA to mitigate inter-cell interference of cellular networks under imperfect CSI conditions. Our analysis is based on stochastic geometry where the structure of the base station (BS) locations is considered by a Poisson point process (PPP). Our main contribution is the coverage probability of the network and simulation results confirm the accuracy.     

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.     

Single antenna interference cancellation (SAIC) for cellular TDMA networks by means of joint delayed-decision feedback sequence estimation

In this letter, single antenna co-channel interference cancellation for cellular time-division multiple access (TDMA) networks by means of joint delayed-decision feedback sequence estimation is studied. The performance is increased by a novel adaptive state allocation technique.     

Inter-cell interference modeling for cellular networks

We develop and analyze several inter-cell interference modeling methods for various fading scenarios in wireless systems. The models can analyze multiple interfering signals under different fading scenarios. Incoherent addition of multiple interfering signals is assumed. In this paper, we present two approaches, i.e., the approximate and exact analysis methods to calculate the probability density function (PDF) of the power of the interference signals. We propose the use of the generalized Gram-Charlier Series to analyze the error of the reference model. Although the computational complexity of exact analysis is high, it can be used to ensure the accuracy of the approximate analysis method. Hermite polynomials are used to simplify the integration operation into the summation operation, thus reduces the computational complexity of the exact method dramatically. The approximate method is simple although it may lead to increased errors. The methods proposed are useful in designing and analyzing practical systems.     

An adaptive canceller of cochannel interference for spread-spectrummultiple-access communication networks in a power line

The authors propose and investigate an adaptive canceller of intersymbol and cochannel interference due to channel distortion and cross-correlation among pseudonoise sequences assigned to individual users of a DS-SSMA (direct-sequence spread-spectrum multiple-access) system. In order to implement a local area network (LAN) by using a power line installed in a building wall as a transmission channel, the authors have investigated utilization of DS-SSMA which has advantages such as robustness against narrow-band interference and noise and realization of asynchronous code division multiple access. In a power line, however, restriction of transmission bandwidth for communications makes it difficult to suppress cochannel interference and the channel is also time-varying due to fluctuation of loads. Since the proposed canceller adaptively eliminates cochannel interference as well as intersymbol interference, it can facilitate synchronization and increase the number of the simultaneously accessing users on a power line with restricted processing gain. The error probability in the output of the canceller is theoretically calculated for the steady-state case by using a Markov model. Computer simulations illustrate stable convergence properties of the canceller     

Subspace based estimation of the signal to interference ratio for TDMA cellular systems

The Signal‐to‐Interference Ratio (SIR) has been highlighted in the literature to be a most efficient criterion for several methods aiming at reducing the effects of cochannel interference, e.g., diversity reception, dynamic channel allocation and power control. In this paper we address the problem of how to obtain fast and accurate measurements of this parameter in a practical context. We develop a general SIR estimation technique for narrow‐band cellular systems that is based on a signal subspace approach using the sample covariance matrix of the received signal. Simulation results using the frame structure in the GSM system show that the SIR can be estimated to within an error of 0.3 dB after only 200 ms, or within an error of 0.1 dB after only 0.6 seconds.     

Subspace based estimation of the signal to interference ratio for TDMA cellular systems

The Signal‐to‐Interference Ratio (SIR) has been highlighted in the literature to be a most efficient criterion for several methods aiming at reducing the effects of cochannel interference, e.g., diversity reception, dynamic channel allocation and power control. In this paper we address the problem of how to obtain fast and accurate measurements of this parameter in a practical context. We develop a general SIR estimation technique for narrow‐band cellular systems that is based on a signal subspace approach using the sample covariance matrix of the received signal. Simulation results using the frame structure in the GSM system show that the SIR can be estimated to within an error of 0.3 dB after only 200 ms, or within an error of 0.1 dB after only 0.6 seconds. This revised version was published online in July 2006 with corrections to the Cover Date.     

Channel assignment algorithms satisfying cochannel and adjacent channel reuse constraints in cellular mobile networks

Improved channel assignment algorithms for cellular networks were designed by modeling the interference constraints in terms of a hypergraph. However, these algorithms only considered cochannel reuse constraints. Receiver filter responses impose restrictions on simultaneous adjacent channel usage in the same cell or in neighboring cells. We first present some heuristics for designing fixed channel assignment algorithms with a minimum number of channels satisfying both cochannel and adjacent channel reuse constraints. An asymptotically tight upper bound for the traffic carried by the system in the presence of arbitrary cochannel and adjacent channel use constraints was developed by Deora (1995). However, this bound is computationally intractable even for small systems like a regular hexagonal cellular system of 19 cells. We have obtained approximations to this bound using the optimal solutions for cochannel reuse constraints only and a further graph theoretic approach. Our approximations are computationally much more efficient and have turned out to track very closely the exact performance bounds in most cases of interest.     

Pilot interference cancellation technology for CDMA cellular networks

This paper considers the link-level and network-level performance of code division multiple access (CDMA) pilot interference cancellation (pilot IC) technology, a low-complexity advanced receiver technology being considered for use in commercial third generation (3G) CDMA cellular systems. The concept behind this technology is to estimate and cancel at the handset receiver the interference effects associated with CDMA downlink pilot signals broadcast from the base stations of the network. The canceling of interference at the receiver improves the signal-to-interference/noise ratio (SINR), which enables increased cell capacity or throughput. In this paper, we derive SINR expressions for evaluating the probability of error performance of both the RAKE and pilot IC handset receivers, under conventional random spreading code assumptions. The approach can easily and accurately model a wide variety of transmitter, channel, and receiver conditions, including the effects of channel estimation. We also utilize radio network simulations to illustrate and quantify the capacity gains available for 3G CDMA networks through the use of pilot IC handsets. Network simulations are also used to examine the reduced level of soft-handoff found to be possible in pilot IC-based networks and the increased flexibility available in setting pilot power levels. We further consider the impact of using stronger pilot signals for improving the demodulation performance of sensitive higher-order modulation constellations that are needed to support spectrally efficient high-rate data services.     

Performance of trellis coded-8PSK with cochannel interference

The performance of trellis coded-8PSK in the presence of cochannel interference is analyzed and compared with the performance of uncoded QPSK. The analytical expressions are derived and supported by simulating the bit error rate performance of different TC-8PSK systems. The analytical and simulation results show that TC-8PSK is slightly more robust to cochannel interference than uncoded QPSK     

On modeling coverage and rate of random cellular networks under generic channel fading

In this paper we provide an analytic framework for computing the expected downlink coverage probability, and the associated rate of cellular networks, where base stations are distributed in a random manner. The provided expressions are in computable integral forms that accommodate generic channel fading conditions. We develop these expressions by modeling the cellular interference using stochastic geometry analysis, then we employ them for comparing the coverage resulting from various channel fading conditions namely Rayleigh and Rician fading, in addition to the fading-less channel. Furthermore, we expand the work to accommodate the effects of random frequency reuse on the cellular coverage and rate. Monte-Carlo simulations are conducted to validate the theoretical analysis, where the results show a very close match.     

Bit-error rate of lightwave systems at the zero-dispersionwavelength

The author derives approximate expressions for the bit-error rate of very long lightwave systems with optical amplifiers that are operated at the zero-dispersion wavelength. In this case, the nonlinear interaction of the signal wave with the spontaneous emission noise of the amplifiers corrupts the signal. Its spectrum broadens by several orders of magnitude, and the instantaneous optical power assumes the appearance of bandlimited thermal or spontaneous emission noise. This observation suggests that the bit-error rate should be computed under the assumption that the optical signal and the optical spontaneous emission noise obey Gaussian statistics. The approximate expressions for the bit-error rate are derived under this assumption. The formulas are written in such a way that the signal and the noise may be either completely polarized or completely unpolarized     

Analysis of TCP performance under joint rate and power adaptation in cellular WCDMA networks

To improve the spectral efficiency while meeting the radio link level quality of service requirements such as the bit-error-rate (BER) requirements for the different wireless services, transmission rate and power corresponding to the different mobile users can be dynamically varied in a cellular wideband code-division multiple-access (WCDMA) network depending on the variations in channel interference and fading conditions. This paper models and analyzes the performance of transmission control protocol (TCP) under joint rate and power adaptation with constrained BER requirements for downlink data transmission in a cellular variable spreading factor (VSF) WCDMA network. The aim of this multilayer modeling of the WCDMA radio interface is to better understand the interlayer protocol interactions and identify suitable transport and radio link layer mechanisms to improve TCP performance in a wide-area cellular WCDMA network.     

Error rate performance for s.f.s.k. with cochannel and intersymbol interference

The probability of error of s.f.s.k., with matched-filter detection in the presence of additive white Gaussian noise, intersymbol interference and cochannel interference has been studied. Results are presented as a function of the signal/interference ratio for a system using a third-order Butterworth filter.