Performance of digital DECT radio links based on semianalyticalmethods

We report a very efficient semianalytical approach for the performance evaluation of differential detection schemes for GMSK signals of the DECT standard. Precisely, for a given channel, the performance is determined by means of an analytical procedure which includes the saddlepoint approximation. We consider both static channels (with impulse response generated by the simulation program SIRCIM) and two-ray Rayleigh and log-normal fading channels. As a departure from previous works, our receiver includes an all-digital part after the analog differential detection scheme. The digital part includes: (1) a block for the estimation of both the optimum sampling phase and the nonlinear channel coefficients (by making use of the DECT training sequence), (2) a one-tap decision feedback (DF) equalizer, and (3) a block for the evaluation of the approximate optimum bias level (γ _e) in the threshold detector. Both the DF equalizer coefficient and γ_e are based on the nonlinear channel coefficients estimate. For channels with a normalized delay spread up to 0.2, the use of the optimum threshold together with the DF equalizer permits a gain of about 2 dB at BER=10^-6 with respect to a receiver without equalization and a zero-level decision threshold. In addition, we discover that, in indoor environments, the 2-bit GMSK detector performs roughly the same as the 1-bit detector. The threshold optimization is also effective in the presence of channels affected by fading. To support this statement, we report the performance of the 1-bit differential detection scheme combined with antenna selection diversity in the presence of a two-ray log-normal and Rayleigh fading channel     

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     

The performance of the PACS radio link and its improvements atvehicular speeds

The personal access communication system (PACS) radio interface is the leading low-tier candidate for standardization in North America. This radio interface was originally conceived to serve pedestrian and fixed-distribution applications; there has been significant recent interest in extending this technology into high-mobility environments. In such environments, rapid channel variations significantly degrade the performance of the preselection diversity scheme proposed for use in PACS handsets. The effects of time-delay spread on the PACS radio link was also included in our investigation. It is found that a received signal strength indicator (RSSI) with a short measurement length can better cope with high fading rates than can a quality measure in a preselection diversity system, although a quality measure has better performance than RSSI at low speeds in the presence of time-delay spread. In the preselection diversity scheme, using short-measurement RSSI provides relatively good performance in both low- and high-mobility environments. However, its performance degrades rapidly for RMS delay spreads larger than about 9% of a symbol time. Postselection diversity using two complete receiver chains is more robust than preselection diversity, both to high fading rates and to delay spread. Postselection diversity is relatively insensitive to changes in the fading rate and can tolerate an RMS delay spread up to 12.5% of a symbol time     

Adaptive frequency-domain equalization and diversity combining forbroadband wireless communications

We introduce a new kind of adaptive equalizer that operates in the spatial-frequency domain and uses either least mean square (LMS) or recursive least squares (RLS) adaptive processing. We simulate the equalizer's performance in an 8-Mb/s quaternary phase-shift keying (QPSK) link over a frequency-selective Rayleigh fading multipath channel with ~3 μs RMS delay spread, corresponding to 60 symbols of dispersion. With the RLS algorithm and two diversity branches, our results show rapid convergence and channel tracking for a range of mobile speeds (up to ~100 mi/h). With a mobile speed of 40 mi/h, for example, the equalizer achieves an average bit error rate (BER) of 10 ^-4 at a signal-to-noise ratio (SNR) of 15 dB, falling short of optimum linear receiver performance by about 4 dB. Moreover, it requires only ~50 complex operations per detected bit, i.e., ~400 M operations per second, which is close to achievable with state-of-the-art digital signal processing technology. An equivalent time-domain equalizer, if it converged at all, would require orders-of-magnitude more processing     

Trellis-coded CPFSK and soft-decision feedback equalization for micro-cellular wireless applications

In this paper, trellis-codedM-ary CPFSK with noncoherent envelope detection and adaptive channel equalization are investigated to improve the bit error rate (BER) performance of microcellular digital wireless communications systems. For the same spectral efficiency, the trellis-coded modulation (TCM) schemes studied outperform minimum shift keying (MSK) with noncoherent or differentially coherent detection in Rayleigh fading channels. For the case of frequency-selective fading channels, adaptive channel equalization is applied to mitigate the time-variant intersymbol interference (ISI). A new equalizer structure is proposed which, in its feedback path, makes use of fractionally spaced signal samples instead of symbol-spaced hard decisions on transmitted symbols. Computer simulation results indicate that the soft-decision feedback equalizer (SDFE) can significantly improve the system's performance.     

Performance analysis of adaptive DFE using set-membership binormalized data-reusing LMS algorithm for frequency selective MIMO channels

This paper addresses the concern of complexity involved with adaptive equalization in wireless systems operating over time-varying and frequency selective multiple-input multiple-output (MIMO) channels. Here, we propose a decision feedback equalizer using binormalized data-reusing least mean square (BNLMS) algorithm with set-membership filtering for MIMO channels. The performance of the equalizer is investigated for a MIMO receiver in a multi-path fading environment as experienced in the indoor and pedestrian environment. The equalizer performance is also studied for channels having higher delay and Doppler spread. The convergence issues, BER performance and tracking capabilities are examined through computer simulations. Moreover, the computational complexity issue for this MIMO equalizer is compared with other existing data-selective algorithm based techniques.     

Adaptive DFE for GMSK in indoor radio channels

We simulate the performance of an equalized Gaussian minimum shift keying (GMSK) signal in an indoor radio environment with fading, noise, imperfect carrier recovery, cochannel interference (CCI), and intersymbol interference (ISI). We show that data rates of 20 Mb/s at bit error rates (BER) ⩽10^-4 are possible with root mean square (RMS) delay spreads up to 25 ns using a simple limiter-discriminator-integrator (LDI) receiver and a (6, 4) decision feedback equalizer (DFE). In environments with larger RMS delay spreads, coherent detection is required for the same performance. We show that using a decision-directed second-order digital carrier synchronizer with time varying loop filters, frequency offsets up to 200 kHz can be corrected with negligible performance degradation. This paper utilizes a DFE structure which compensates for both modulator and channel ISI, and yet requires no power-intensive multiplication operations in the feedback section. A DFE (8, 8) with two-level switched (selection) diversity is shown to allow 20 Mb/s data transfer at a BER⩽10^-4 for RMS delay spreads under 150 ns, with CCI. A light BCH (26, 31) code allows error-free reception of over 90% of packets with RMS delay spreads under 150 ns, and up to 70% of packets with RMS delays of 150 ns     

Joint equalization and interference suppression for high data ratewireless systems

Enhanced Data Rates for Global Evolution (EDGE) is currently being standardized as an evolution of GSM in Europe and of IS-136 in the United States as an air interface for high speed data services for third generation mobile systems. In this paper, we study space-time processing for EDGE to provide interference suppression. We consider the use of two receive antennas and propose a joint equalization and diversity receiver. This receiver uses feedforward filters on each diversity branch to perform minimum mean-square error cochannel interference suppression, while leaving the intersymbol interference to be mitigated by the subsequent equalizer. The equalizer is a delayed decision feedback sequence estimator, consisting of a reduced-state Viterbi processor and a feedback filter. The equalizer provides soft output to the channel decoder after deinterleaving. We describe a novel weight generation algorithm and present simulation results on the link performance of EDGE with interference suppression. These results show a significant improvement in the signal-to-interference ratio (SIR) performance due to both diversity (against fading) and interference suppression. At a 10% block error rate, the proposed receiver provides a 20 dB improvement in SIR for both the typical urban and hilly terrain profiles     

Bit-error rate of TDMA links under cochannel interference inoverlaid cellular networks

This paper presents a series of numerical experiments conducted to investigate the error performance of a simulated digital time-division multiple-access (TDMA) radio link between a portable handset and the base station serving it, subject to interference coming from other portable handsets. The experiments take radio channel impairments such as fading, shadowing, and distance loss into account. Analytical expressions for the bit-error probability in uncoded digital radio transmission bursts under interference from nearby analog FM or digital minimum-shift keying (MSK) transmitters are reviewed. Numerical methods are used to extend analytical estimates of symbol error probability of a link with a single interferer, without channel impairments, to the case where the interference is due to a random distribution of transmitters whose signals are subject to fading, shadowing, and distance power loss. Specifically, three links, each based on one of three different modulation methods, MSK, quadrature phase-shift keying (QPSK) (proposed in the North American Digital Cellular Standard IS-54), and Gaussian MSK (GMSK) [used in the Global Standard for Mobile communications (GSM)] are simulated, and the bit-error rate (BER) results reported. The BER results generated thus indicate the sensitivity of the digital radio link to the user density for a given radio frequency (RF) bandwidth and, where applicable, to fading depth and postmodulation intersymbol interference (ISI) removal method. Possible extensions and applications of the simulation model to the problem of resource sharing between coexisting networks are suggested     

An MLSE receiver using channel classification for Rayleigh fadingchannels

We propose a channel classification method that identifies the delay path profile of Rayleigh fading channels, which can be used in conjunction with conventional maximum-likelihood sequence estimation (MLSE) receivers. The proposed method determines the appropriate number of delay taps for the MLSE trellis, based on a decision variable obtained from multiple traffic bursts. By formulating the decision variable using the F-distribution, we derive exact expressions for misclassification probability. A new MLSE receiver structure that utilizes the channel classification method is described, and bit error rate (BER) simulation results are presented to demonstrate the performance improvement. The particular application which is discussed is the IS-136 TDMA standard     

Performance Improvement of IS-95 CDMA Uplink Using Decision Feedback and Sub-Optimal Viterbi Decoding

This paper presents a methodology designed to improve the effectiveness of a non-iterative decision feedback (DF) receiver/decoder for IS-95 Code Division Multiple Access (CDMA) uplink in additive white Gaussian noise (AWGN) and Rayleigh fading. The effectiveness of the DF receiver/decoder is linked to the interleaver specification and the decoding delay of the convolutional decoder. Using sub-optimal convolutional decoding the average decoding delay is reduced resulting in more effective decision feedback decoding (DFD). Simulation results of average decoding delay, bit error rate (BER) and frame error rate (FER) are presented for coherent and noncoherent detection of unfaded single-path and Rayleigh fading multipath signals. Instead of the usual performance degradation these results show that the DF receiver/decoder benefits from some forms of sub-optimal Viterbi decoding. The additional performance gain can further improve the quality of service and/or capacity of a cellular IS-95 system.     

Differential detection of π/4-shifted-DQPSK for digital cellularradio

The detection of π/4-shifted-DQPSK modulation using a tangent-type differential detector with an integrated symbol timing and carrier frequency offset correction algorithm is discussed. π/4-shifted-DQPSK modulation has been proposed for use in a high-capacity, TDMA-based digital cellular system being developed in the US; differential detection could potentially allow the production of low-complexity mobile units. Results obtained using the proposed IS-54 TDMA frame structure for base to mobile transmissions are presented. Theoretical and simulation bit-error-rate (BER) results are presented for static and Rayleigh fading channels. BER results are provided as a function of E_b/N₀ and C/I, where the interferer is a second π/4-shifted-DQPSK signal. Additional results are provided which show the BER sensitivity to Doppler frequency shifts, time delay spread, and carrier frequency offsets     

Comparison of diversity with simple block coding on correlatedfrequency-selective fading channels

We investigate the effects of correlation on the performance of diversity systems in wideband wireless radio environments. Specifically, the average bit error rate (BER) performance of M-ary differential phase shift keying (MDPSK) on correlated frequency-selective slow Rayleigh fading channels is analyzed. A two-branch diversity receiver with postdetection equal gain combining is considered. Nyquist pulse shaping is used and differential detection is employed at the receiver. The effects of cochannel interference on the system performance are assessed using a Gaussian interference model. To further enhance the system performance, the effects of combined diversity and forward error correction (FEC) coding on the average BER are investigated. Results using short cyclic block codes with perfect bit interleaving are obtained. The effects of the root mean square (RMS) delay spread, the amount of correlation, and the level of modulation, M, on the average BER are investigated in detail for both coded and uncoded systems. The results show that dual branch diversity combining with a correlation coefficient of 0.5 outperforms (in terms of BER) short block codes with perfect bit interleaving, and that combined diversity and coding strategies are effective in combatting the effects of frequency-selective fading     

Multicarrier 16QAM transmission with diversity reception

A 96 multicarrier 16QAM transmitter and a diversity receiver for 3.072 Mbit/s data transmission are described. Pilot symbol aided (PSA) coherent detection is applied. A laboratory experiment demonstrates that an irreducible bit error rate (BER) of 10^-3 can be achieved at the RMS delay spread τ_rms=6.3 μs under frequency selective Rayleigh fading when two branch maximal ratio combining (MRC) diversity is used     

The effects of time-delay spread on unequalized TCM in a portableradio environment

This paper studies the effects of time-delay spread on trellis-coded modulation (TCM) in portable radio channels, where equalization is not employed to mitigate frequency-selective fading. The average irreducible bit error rate (BER) of three different TCM schemes are analytically formulated first and then numerically evaluated by simulation. The results for a delay spread lower than 0.2 of the symbol period indicate that the performance of TCM schemes with interleaving/deinterleaving is much better than that of QPSK, and better TCM schemes for flat fading also give better performance under low delay spread. Analytical results indicate that a good TCM scheme in frequency-selective fading channels should have both a large Euclidean distance and a high degree of built-in time diversity. If higher time-delay spread is encountered, TCM does not have advantages over QPSK. We also compare TCM performance with and without diversity. It is found that diversity greatly improves the performance under low delay spread, while the diversity gain quickly diminishes as the RMS delay spread approaches 0.2 of the symbol period     

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.     

Performance comparison of space diversity and equalizationtechniques for indoor radio systems

By means of analytical and numerical methods, we derive the bit error rate (BER) of π/4-DQPSK systems in frequency-selective fading channels. For a theoretical analysis of the system, a simplified two-ray channel model has been used. However, both Rayleigh and lognormal distributions for the ray envelope have been considered. The system performance in the presence of antenna diversity and in combination with a new nonlinear equalizer has been evaluated. In particular, it is seen that in flat fading environments, space diversity may improve the performance by more than 10 dB at a BER=10^-3. However, for channels with a large delay spread, nonlinear intersymbol interference (ISI) is the predominant disturbance, and the performance can only be enhanced by the nonlinear equalizer     

基于FPGA的自适应均衡器的研究与设计

近年来,自适应均衡技术在通信系统中的应用日益广泛,利用自适应均衡技术在多径环境中可以有效地提高数字接收机的性能.为了适应宽带数字接收机的高速率特点,本文阐述了自适应均衡器的原理并对其进行改进.最后使用FPGA芯片和Verilog HDL设计实现了自适应均衡器并仿真验证了新方法的有效性.     

Field test measurement for 128 kbit/s QPSK signal transmission and delay time spread in 1.5 GHz-band land mobile radio

High-speed digital signal transmission is a promising new field of communication services. In land mobile radio communication, transmission quality severely suffers from frequency selective fading under multipath radiowave transmission environments. Field test measurement results for 128 kbit/s QPSK signal transmission and delay time spread in the Tokyo metropolitan area are presented. The two-branch diversity reception method was adopted to combat fading. Bit error rate (BER) performance is obtained as a function of delay time spread.<>     

MMSE Equalization of Interference on Fading Diversity Channels

Adaptive equalization is used in digital transmission systems with parallel fading channels. The equalization combines the diversity channels and reduces intersymbol interference due to multipath returns. When interference is present and correlated from channel to channel, the equalizer can also reduce its effect on the quality of information transfer, important applications for interference cancellation occur in diversity troposcatter systems in the presence of jamming, diversity high frequency (HF) systems which must cope with interfering skywaves, and space diversity line-of-sight (LOS) radio systems where adjacent channel interference is a problem. In this paper we develop the general formulation for minimum mean square error (MMSE) equalization of interference in digital transmission diversity systems. The problem formulation includes the use of available receiver decisions to assist in MMSE processing. The effects of intersymhol interference are included in the analysis through a critical approximation which assumes sufficient processor capability to reduce ISI effects to levels small enough for satisfactory communication. The analysis also develops he concept of additional implicit or intrinsic diversity which results from channel multipath dispersion. It shows how the MMSE processor sacrifices diversity to suppress interference even when the interference arrives in the main beams of the receiver antenna patterns. The condition of near synchronous same-path interference is also addressed. Because the spatial angle of arrival of the interference may result in delay differences between interference signals in different antenna channels, interference delay compensation may be required. We show that this effect is compensated for with a small number of appropriately spaced equalizer taps.