A coded 16 QAM scheme for fast fading mobile radio channels

An adaptive Viterbi algorithm, derived from a dynamic estimate of the fading channel is used for the decoding of a convolutional coded 16 QAM system in a mobile environment. The estimates are obtained by a sequence of known pilot symbols embedded in the data stream, and perform compensation for Rayleigh fading. The likelihood criterion in the Viterbi decoder is also modified by these channel estimates through a metric weighting function. We demonstrate through computer simulations, that our new technique achieves a BER improvement of 7-10 dB at P_e =10^-3 in a fast flat Rayleigh fading environment compared to an uncoded system. The BER performance of our new technique in a co-channel interference (CCI) controlled environment is also studied, and the results show that it may achieve a 40% to 85% improvement in capacity over the standard modem scheme for the new US digital cellular system, π/4-QPSK     

Adaptive differential detection using linear prediction for M-aryDPSK

This paper proposes a novel adaptive differential detection scheme (adaptive DD), which can significantly reduce the irreducible bit-error rate (BER) of M-ary DPSK due to Doppler spread by the adaptive linear prediction of the reference signal. The predictor coefficient is adapted to changing channel conditions by using the recursive least-square (RLS) algorithm. A phase sequence estimation based on the M-state Viterbi algorithm (VA) and another based on the decision feedback algorithm (DFA) are presented. A theoretical BER analysis is presented for adaptive DD-DFA. BER performances of 2 and 4DPSK in Rayleigh fading channels are evaluated by computer simulations. When the RLS forgetting factor of β=1 is used, simulation results show that the irreducible BER of 4DPSK can be reduced to 7.2×10^-5 (3.9×10 ^-4) for VA (DFA) while conventional DD offers 3.9×10 ^-3 when f_DT_b (maximum Doppler frequency times bit duration)=0.01 and average E_b/N₀ (signal energy per bit-to-additive white Gaussian noise (AWGN) power spectrum density ratio)=60 dB, where most errors are produced by Doppler spread. Adaptive DD is also effective in AWGN channels-simulations show that for the case of 4DPSK, a performance gain of 1.2 (0.7) dB is achieved over conventional DD for VA (DFA) at BER=10^-3     

Pilot symbol-assisted coherent multistage interference cancellerusing recursive channel estimation for DS-CDMA mobile radio

A pilot symbol-assisted coherent multistage interference canceller using recursive channel estimation is proposed for DS-CDMA mobile radio cellular system. Since the channel variation caused by fading is recursively estimated at each interference cancelling stage, the accuracy of channel estimation is improved successively. Computer simulation results show that the required E_b/N₀ at the average BER of 3×10^-2 is improved by ~3.5 dB compared to the matched filter receiver for 10 users and two paths with equal power, and where f_dT=10^-3 (f_d: fading maximum Doppler frequency, T: data symbol duration)     

Variable rate QAM for mobile radio

Quadrature amplitude modulation (QAM) schemes which vary the number of modulation levels in accordance with the mobile radio fading channel variations are investigated. Important parameters considered are the fading rate and the block size used. We describe how the adaptive QAM modems can be employed and consider their use in a DECT-like TDD packet structure. System performance in the presence of cochannel interference is also considered. Simulations show that the variable rate system has about 5 dB improvement in channel SNR over a fixed 16-level QAM system for BER's between 10^-2 and 10^-5 and channel SNR's between 25 and 40 dB     

Experiment on coherent adaptive antenna array for wideband DS-CDMAmobile radio

A four-element pilot symbol-assisted coherent adaptive antenna array diversity receiver for 4.096 Mchip/s wideband direct sequence code division multiple access mobile radio is implemented and its performance in a multipath fading environment is evaluated by a laboratory experiment using hardware fading simulators. The receiver comprises an adaptive antenna array using the normalised least mean square algorithm and Rake combiner. It is demonstrated that, for the three-user case, the required average signal-to-interference ratio obtaining average BER of 10^-3 can be reduced by ~8 dB compared to four-branch antenna diversity     

Error rate analysis of differentially encoded and detected 16APSKunder Rician fading

Mobile radio systems require highly bandwidth efficient digital modulation schemes because of the limited resources of the available radio spectrum. A theoretical analysis of bit error rate (BER) is presented for the differential detection of differentially encoded 16-level amplitude/phase shift keying (16DAPSK) under Rician fading in the presence of Rayleigh faded co-channel interference (CCI) and additive white Gaussian noise (AWGN). Differential detection comprises eight-level differential phase detection (DPD) and two-level amplitude ratio detection (ARD). Exact expressions for probability distributions of differential phase noise and amplitude ratio are derived for the BER calculation. The calculated BER performance of 16DAPSK is presented for various values of Rician fading K factor, Doppler spread of diffused component, and Doppler shift of the specular component, and is compared with that of 4-16DPSK. It is shown that 16DAPSK is superior to 16DPSK and requires 1.7 (1.6) dB less E_b/N₀ (SIR) at BER=10^-3 in Rician channels with K=5 dB     

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     

An implementation of a CMA adaptive array for high speed GMSKtransmission in mobile communications

The hardware implementation of an adaptive array as a technique for compensating multipath fading in mobile communications is described. The number of the antenna elements is four. The target communication system is modulated by 256 kbps Gaussian-filtered minimum shift keying (MSK) and has a time-division multiplexing (TDM) architecture with 24 time slots. Based on the digital beamforming concept, all of the signals and the array weights are digital-signal processed. The constant modulus algorithm (CMA) is employed for weight optimizing. In an additive white Gaussian noise channel, this system has 5.6-dB gain in an energy-per-bit-to-noise-density ratio at a bit error rate (BER) of 1.0×10^-3, compared with a single antenna system. The result of the basic field test shows that the gain at a BER of 1.0×10^-3 reaches 22.3 dB in a nonselective, slow Rayleigh fading channel given a 5 Hz maximum Doppler shift     

BER performance of CMA adaptive array for high-speed GMSK mobilecommunication-a description of measurements in central Tokyo

This paper describes the performance of an adaptive array as a countermeasure to multipath fading for a 256 kbps Gaussian-filtered minimum shift keying (GMSK) mobile communication system operating in the 1.5 GHz band. An adaptive array having four antenna elements is implemented using the digital beam forming concept. The constant modulus algorithm (CMA) is employed for the adaptation process to ease the implementation. Measurements in central Tokyo of the bit error rate (BER) performance and an array pattern arising in the multipath environment are presented. Analysis of the array pattern confirms that the array succeeds in directing nulls to the delayed signals. BER performance shows an improvement in E_b/N₀, compared with that of a single antenna system, of 17.5 to 22 dB at a BER of 1.0×10^-2 in a frequency-selective fading channel     

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     

Differentially detected GMSK signals in CCI channels for mobilecellular telecommunication systems

The performance of conventional and decision feedback differential detection receivers for Gaussian minimum-shift keying (GMSK) signals transmitted in the presence of cochannel interference (CCI) and additive white Gaussian noise (AWGN) is evaluated. For the interference, the authors adopt a model which includes N statistically independent static as well as faded CCI. Various bit-error-rate (BER) performance evaluation results have indicated that the receiver under investigation performs better as compared to other more conventional receiver structures. Especially significant BER improvements are obtained for the static CCI channel. For example, it is shown that with a carrier-to-interference ratio of 14 dB, the performance of a 2-b decision feedback differential receiver outperforms a conventional 2-b differential detector by more than 14 dB (at a BER 10^-3). For the faded CCI, the improvement is less: mainly they result in error-floor reductions of about half an order of magnitude. By comparing the performance of static and faded CCI, it was also found that for a given C/I, the performance of the former would depend on the number of interferers, whereas this is not the case for the latter     

Coherent orthogonal filter using pilot symbol-assisted channelestimation for DS-CDMA

A coherent orthogonal filter (COF) using pilot symbol-assisted channel estimation is proposed for DS-CDMA cellular mobile radio. In the proposed scheme, a complex fading envelope in the multi-path environment is estimated using pilot symbols, and tap coefficients of orthogonal filter are controlled for maximising the signal to interference ratio (SIR) of a RAKE combined signal. Computer simulation results show that the required E_b/N₀ of the proposed COF is reduced by ~10.0 dB compared to conventional matched filter receiver at an average BER of 3×10² when there are 10 users and processing gain is 31     

A digital coherent receiver suitable for land-mobile satellitecommunications

Describes an advanced coherent demodulation technique suitable for land-mobile satellite communications. The proposed technique features a combined narrow/wide band dual open loop (DOL) carrier phase estimator, which effectively enables the coherent receiver to track fast phase fluctuations caused by fading, without degradation in phase slip characteristics. Additionally, an open loop phase estimator has inherent quick recovery performance. Its bit error rate (BER) performance is shown to be superior to that for existing detection schemes, achieving a 10^-2 BER at 6.3 dB E_b/N₀ (0.9 dB greater than the theoretical E_b/N₀ condition for perfect carrier phase tracking) for QPSK over a Rician fading channel with the 10 dB Rician factor and the 1/16 baud rate fading pitch. The paper also describes a quick bit timing recovery scheme, with interpolation, featuring an open loop structure. Further, it presents an experimental digital modem developed through the use of digital signal processors     

Soft-detection phase precoding with MPSK-TCM for ISI channel

Trellis-coded modulation (TCM) concatenated with phase precoding is proposed to alleviate the effect of intersymbol interference (ISI) in frequency selective slow fading channels in wireless communications. Simulation results show that gains of 6.5 and 2.5 dB at a bit error rate (BER) of 10^-6 are achieved by the hard detection method for 8PSK-TCM with four and eight state codes, respectively. The comparison is made with respect to a system employing post-equalization with a sub-optimum soft output equalizer and the same TCM schemes. Moreover, soft detection of the precoded signal is also carried out, which results in an additional gain of 4 dB. Finally, the simulation performance is compared with the analytical bound and they match quite well     

Degradation in FH-MFSK mobile radio system capacity due to Rayleighfading and log-normal shadowing

A frequency-hopped multilevel frequency-shift-keying (FH-MFSK) system has been proposed for digital mobile radio communications. The performance of the system is evaluated by studying average probability of error caused by transmission impairments. The degradation in performance due to Rayleigh fading and log-normal shadowing environments is determined. With perfect transmission, where the degradation in the system performance is due to mutual interference between users only, the system can accommodate up to 209 simultaneous users at an average bit error rate of 10^-3. The system capacity decreases to 110 users as a result of additive white Gaussian noise (AWGN), mutual interference, frequency-selective Rayleigh fading, and log-normal shadowing with normalized area mean of 20 dB and standard deviation of 6 dB     

Experimental evaluation of combined effect of coherent RAKEcombining and SIR-based fast transmit power control for reverse link ofDS-CDMA mobile radio

The combined effect of coherent RAKE combining using the weighted multislot averaging (WMSA) channel estimation filter and closed-loop fast transmit power control (TPC) in the 4.096 Mchip/s direct sequence code division multiple access (DS-CDMA) mobile radio reverse link is experimentally evaluated. The WMSA channel estimation filter utilizes periodically transmitted pilot symbols (four pilot symbols are time-multiplexed in each 40-symbol time slot). Its observation period is extended to 2-K slots in order to improve the accuracy of the channel estimation. The fast TPC is based on the measurement of signal-to-interference plus background noise ratio (SIR) using pilot symbols. Laboratory experiments show that the use of the K=2 WMSA channel estimation filter reduces the required E_b/I₀ at the average BER of 10^-3 by approximately 0.5 dB compared to use of the linear interpolation filter, and that the required E_b/I₀ is minimized when the SIR measurement interval is M=10 symbols (one slot TPC delay). It was also clarified that SIR-based TPC works satisfactorily when two users with different information data rates, i.e., SF, independently employ fast TPC. Field experimental results obtained in an area nearby Tokyo showed that the average BER of 10^-3 is achieved at the target E_b/I₀ per antenna of approximately 2.5 dB by using four-finger branch RAKE and two-branch antenna diversity. Although the target E_b/I₀ to achieve same BER, when there is one interfering user with a fourfold greater transmit power than that of the desired user that independently employs fast TPC, is almost the same as that in the single-user case, the mobile transmit power is increased by 1.0-2.0 dB due to the increased MAI. These results indicate that the combination of coherent RAKE combining and fast TPC works well in practical multipath fading channels     

An adaptive modulation scheme for simultaneous voice and datatransmission over fading channels

We propose a new adaptive modulation technique for simultaneous voice and data transmission over fading channels and study its performance. The proposed scheme takes advantage of the time-varying nature of fading to dynamically allocate the transmitted power between the inphase (I) and quadrature (Q) channels. It uses fixed-rate binary phase shift keying (BPSK) modulation on the Q channel for voice, and variable-rate M-ary amplitude modulation (M-AM) on the I channel for data. For favorable channel conditions, most of the power is allocated to high rate data transmission on the I channel. The remaining power is used to support the variable-power voice transmission on the Q channel. As the channel degrades, the modulation gradually reduces its data throughput and reallocates most of its available power to ensure a continuous and satisfactory voice transmission. The scheme is intended to provide a high average spectral efficiency for data communications while meeting the stringent delay requirements imposed by voice. We present closed-form expressions as well as numerical and simulation results for the outage probability, average allocated power, achievable spectral efficiency, and average bit error rate (BER) for both voice and data transmission over Nakagami-m fading channels. We also discuss the features and advantages of the proposed scheme. For example, in Rayleigh fading with an average signal-to-noise ratio (SNR) of 20 dB, our scheme is able to transmit about 2 bits/s/Hz of data at an average BER of 10 ^-5 while sending about 1 bit/s/Hz of voice at an average BER of 10^-2     

An improved π/4-QPSK with nonredundant error correction forsatellite mobile broadcasting

An improved π/4-quadrature phase-shift keying (QPSK) receiver that incorporates a simple nonredundant error correction (NEC) structure is proposed for satellite and land-mobile digital broadcasting. The bit-error rate (BER) performance of the π/4-QPSK with NEC is analyzed and evaluated in a fast Rician fading and additive white Gaussian noise. (AWGN) environment using computer simulation. It is demonstrated that with simple electronics the performance of a noncoherently detected π/4-QPSK signal in both AWGN and fast Rician fading can be improved. When the K-factor (a ratio of average power of multipath signal to direct path power) of the Rician channel decreases, the improvement increases. An improvement of 1.2 dB could be obtained at a BER of 10^-4 in the AWGN channel. This performance gain is achieved without requiring any signal redundancy and additional bandwidth. Three types of noncoherent detection schemes of π/4QPSK with NEC structure, such as intermediate frequency band differential detection, baseband differential detection, and FM discriminator, are discussed. It is concluded that the π/4-QPSK with NEC is an attractive scheme for power-limited satellite land-mobile broadcasting systems     

Upper bounds on the bit-error rate of optimum combining in wirelesssystems

This paper presents upper bounds on the bit-error rate (BER) of optimum combining in wireless systems with multiple cochannel interferers in a Rayleigh fading environment. We present closed-form expressions for the upper bound on the bit-error rate with optimum combining, for any number of antennas and interferers, with coherent detection of BPSK and QAM signals, and differential detection of DPSK. We also present bounds on the performance gain of optimum combining over maximal ratio combining. These bounds are asymptotically tight with decreasing BER, and results show that the asymptotic gain is within 2 dB of the gain as determined by computer simulation for a variety of cases at a 10^-3 BER. The closed-form expressions for the bound permit rapid calculation of the improvement with optimum combining for any number of interferers and antennas, as compared with the CPU hours previously required by Monte Carlo simulation. Thus these bounds allow calculation of the performance of optimum combining under a variety of conditions where it was not possible previously, including analysis of the outage probability with shadow fading and the combined effect of adaptive arrays and dynamic channel assignment in mobile radio systems     

Path diversity for FFH/PSK spread-spectrum communication systems

A fast frequency hopping (FFH) method which uses path-diversity combining is proposed. Diversity techniques are realized when a signal is received from diverse independent paths, each of which carries identical information but suffers from independent fading variations. The improvement of communication performance of FFH systems is possible as the delayed paths are used and path-diversity combination is realized. The advantages of this method, operating in Rayleigh fading channels are confirmed by theoretical analyses. The improvement is in order of 2~3 dB at bit error rate (BER) of 10^-3. This method can be also effective against interferences from other users in a code division multiple access (CDMA) environment. The performance of this system in a CDMA environment is evaluated by theoretical analyses and is shown to be superior to non-combining method. At BER of 10^-3 the required E_b/N₀ of the proposed system is 5 dB lower. If E_b/N_o is fixed, a greater number of users can be accommodated using the proposed system