Effects of PRMA on objective speech quality

When transmitting 32 kbit/s adaptive differential pulse code modulation (ADPCM) speech using Reed-Solomon error correction coding and 16 level quadrature amplitude modulation (16-QAM), a 20 slot packet reservation multiple access (PRMA) assisted cordless telecommunications (CT) scheme supported 36-38 speech users with negligible objective and subjective speech degradation. The average number of users per slot was nearly doubled due to deploying PRMA and toll quality speech was transmitted in a user bandwidth approximately 11.6 kHz. For a channel signal-to-noise ratio (SNR) in excess of 25 dB, a Rayleigh fading channel and mobile speeds above 2 mph the speech segmental SNR degradation was less than 0.3 dB.<>     

On the use of Reed-Solomon codes in improving digital FMtransmission performance in Rayleigh fading

Reed Solomon codes with interleaving are used in a binary FM transmission affected by Rayleigh fading as encountered in land mobile radio. A new decoding algorithm is proposed which can be implemented in simple hardware. The error performance of the binary FM system with the RS code with one and two degrees of interleaving is measured. A significant reduction of error rate floor as a function of fading speed is noted and discussed     

The capture effect in multiaccess communications-the Rayleigh andlandmobile satellite channels

The capture phenomenon and its consequences in nonbitsynchronous mobile packet radio networks for binary phase-shift-keying (BPSK) and differential phase-shift-keying modulation are investigated. Exact values of the bit error probability for given signal-to-noise ratios of colliding BPSK signals are derived. Packet error rates, which are needed for analysis of slotted random multiple access methods, are obtained by simulation. Two kinds of mobile radio channels are considered: the Rayleigh fading channel and the land mobile satellite channel. In the latter, because of shadowing, the probability that one of several colliding data packets is correctly received can be on the same order as the probability that a single packet that is not experiencing a collision is correctly received. The influence of Reed-Solomon codes on packet error probabilities is also studied. A slotted ALOHA system using the land mobile satellite channel is analyzed. It is found that with significant shadowing, the overall system throughput may reach the point-to-point throughput. Also, the code rate cannot be optimized in a straightforward manner by assuming one single transmission at a time     

A packet reservation multiple access assisted cordlesstelecommunication scheme

A packet reservation multiple access (PRMA) assisted microcellular cordless telecommunications (CT) system is investigated for office type environments in absence of cochannel interference. The objective speech quality is found as a function of channel signal-to-noise ratio (SNR) for transmissions over narrowband Rayleigh fading channels, parameterized with the number of PRA users. A moderate complexity 16-ary CT scheme constituted by a 4-bit/symbol 32-kbps adaptive differential pulse code modulation (ADPCM) speech codec, Reed-Solomon forward error correction (FEC) codec, and diversity-assisted 16-level star quadrature amplitude modulation (16-StQAM) modem is proposed. The 264-kBd 20-slot PRMA scheme supports 36 users while maintaining virtually imperceptible speech degradation for channel SNRs in excess of about 25 dB, and for a mobile speed of 2 mi/h if the signal-to-interference ratio (SIR) is above 30 dB     

Mutual interferences in a binary FH-FSK spread spectrum system for mobile radio

A binary frequency-hopped frequency shift keying (FHFSK) spread spectrum system for mobile radio communication as well as an address assignment scheme for such a system and an evaluation of error probabilities due to mutual interferences in binary FH-FSK in function of the number of simultaneous users are presented. In the absence of any form of synchronization between sequences, the system's performance is not as good as frequency-hopped differential phase shift keying (FH-DPSK) for any decoding scheme; decoding with intermediate decisions on interval pairs (i, i + L/2) in a sequence of length L gives approximately the same performance as when using only the combiner circuit and maximum likelihood decoding. With synchronization between sequences at the chip level, however, the results show binary FH-FSK performance comparable to that of frequency-hopped multilevel frequency shift keying (FH-MFSK) and thus, substantially better than that of FH-DPSK.     

Subjective Quality of the Same Speech Transmission Conditions in Seven Different Countries

Because of the diversity of subjective testing methods, it is not possible, in general, to compare speech quality measurements from different laboratories. A recent experiment had the aim of determining whether comparable results can be obtained when the same test is performed in several different countries. Tape recordings of speech samples in the native languages of Britain, Canada, France, Italy, Japan, Norway, and the United States were processed by 38 communications circuits in the Communications Methods Research Department of Bell Laboratories. Recordings of the processed speech were returned to each location for evaluation, on a five-point scale, by native listeners. The communications circuits included white noise and speechmodulated noise references, pulse code modulation (PCM) references and four adaptive differential PCM (ADPCM) test circuits, with bit rates between 16 and 48 kbits/s, each with four binary error rates. An important part of the data analysis was estimation of the quality of the 16 ADPCM conditions at locationB, given measurements of ADPCM quality at locationAand knowledge of reference-circuit quality measured atAand atB. The subjective testing data indicate that a reasonably accurate estimate of ADPCM quality at locationBis the quality measured atAadjusted by an additive constant which is the difference between average reference-circuit measurements atBand atA. The accuracy of this estimate is equivalent to that of a subjective test at locationBwith about seven evaluations of each transmission condition. Three physical measurements are less reliable guides to subjective quality. They lead to estimates with less accuracy than the average of three subjective ratings per circuit.     

An error protected 16 kbit/s voice transmission for land mobileradio channel

Robust speech coding and an efficient error correction coding is indispensable for toll-quality voice transmission in mobile radio because of degradation due to multipath fading. A decision procedure for the bit-selective forward-error-correction (BS-FEC) scheme is proposed as a promising solution to this problem, and backward-type prediction speech coding is shown to be more robust against transmission errors than forward-type prediction. Combinations of BS-FEC (channel coding) and backward-type prediction speech coding are designed, and improvements in the SNRs of transmitted voice signals are examined. Simulation results show that BS-FEC can provide good speech quality even at a bit error rate (BER) of 10^-2 in Rayleigh fading environments, at the cost of a slight degradation in the SNR (signal-to-noise ratio) at low BERs     

Performance evaluation of cellular mobile radio systems with interference ing of dominant interferers

This paper develops an analytical framework for characterizing the average symbol error rate and outage performance of a smart antenna system in cellular mobile radio environments. Specifically, the carrier-to-interference ratio statistics with N remaining (uncancelled) "weakest" cochannel interference (CCI) signals from a total of N/sub I/ signals are derived, given that both the desired user signal and the CCI signal amplitudes are subjected to Rayleigh, Rice, Nakagami-m, or Nakagami-q fading. General expressions for the outage probability and the average symbol-error rate performance of different digital modulation schemes in the presence of CCI signals are derived. Selected numerical results are presented to demonstrate the utility of the analysis in assessing the selective interference ing performance in different fading environments.     

Time Diversity with Adaptive Error Detection to Combat Rayleigh Fading in Digital Mobile Radio

A new diversity technique is proposed to combat Rayleigh fading in digital mobile radio systems transmitting speech signals. The speech signals are μ-law PCM encoded (mu = 255, 8 kHz sampling, 8 bits/code word, 64 kbit/s data rate), and alternate data words are used to form two streams called "odd" and "even." The even stream is delayed by τ seconds and the streams are interleaved prior to radio transmission using two-level PSK modulation. At the receiver the odd data stream is delayed by τ and interleaved with the even stream. Consequently, if an error burst occurs, the effect of the reshuffling of the data stream is, in general, to place words with bit errors in juxtaposition to those correctly received. After μ-law PCM decoding of the words, a statistical error detection strategy is evoked to identify the erroneous samples. These samples are replaced by adjacent sample interpolation to give the recovered speech sequence. No recourse to channel protection coding is made. In our experiments a Rayleigh fading envelope was generated from a hardware simulator and stored in a computer, along with four sentences of speech. The system was then simulated and the recovered speech perceived. The objective performance measures were segmental SNR for the audio signal, and BER. Different error detection strategies were examined and restrictions on τ investigated. For a mobile speed of 30 mph, SNR values of 32, 21, and 16 dB were obtained for BER values of 0.1, 1, and 2 percent, corresponding to SNR gains over an uncorrected system of 3, 9, and 11 dB, respectively.     

Results on turbo-codes for speech transmission in a joint detectionCDMA mobile radio system with coherent receiver antenna diversity

Turbo-codes which are applicable to speech transmission in digital mobile radio systems are treated. Three turbo-codes of different complexity are presented. The proposed turbo-codes are suitable for the application to speech transmission in the joint detection code-division multiple access (JD-CDMA) mobile radio system with coherent receiver antenna diversity (CRAD) which are described concisely. The performance of the designed turbo-codes in terms of bit and frame error rates are shown in the case of additive white Gaussian noise (AWGN) channels, flat Rayleigh fading channels, and in the uplink of the aforementioned JD-CDMA mobile radio system     

An adaptive coding scheme with code combining for mobile radiosystems

The authors propose and study an adaptive error-control coding scheme for binary digital FM (BFM) mobile radio transmission. The scheme employs code combining through packet retransmissions. The number of transmissions of a packet is in proportion to the channel fading/noise levels, which is in contrast to time diversity techniques where a fixed number of repetitions of a data packet is performed even in the absence of channel errors. Furthermore, the receiver uses received signal envelopes as channel state information, which significantly improves the throughput and bit error rate (BER) performance. Performance of the proposed scheme is analyzed for frequency-flat Rayleigh fading channels with additive white Gaussian noise (AWGN), co-channel interference and random FM noise     

Performance analysis of punctured convolutional codes andturbo-codes

Digital radio transmission techniques offer the prospect of improved reception compared with analogue signals and are being introduced for radio broadcasting in the short-wave bands. The coding scheme adopted plays an important part in achieving a high quality in the presence of noise and fading, which can be particularly severe for Digital Radio Mondiale (DRM) receivers. This paper compares the performance of turbo-codes and punctured convolutional codes over the radio broadcast transmission channels proposed in ITU-R Circular Letter 10/LCCE/39. The results show that the bit error ratio for binary communication over both additive white Gaussian noise (AWGN) and Rayleigh fading channels is low for turbo-codes in comparison with that for punctured convolutional codes having the same code rate. This result holds over a wide range of bit energy to noise power ratios. The results, which are evaluated in terms of the efficiency for Gaussian and Rayleigh fading channels, show that system efficiency increases with decreasing code rate     

Improved ADPCM voice signal transmission employing click-noisedetection scheme for TDMA-TDD personal communication systems

In personal communication systems, better voice quality and lower terminal price are essential requirements, and adaptive differential pulse-code modulation (ADPCM) codecs are one of the voice codec candidates that have good voice quality and reasonable cost. However, in slow fading environments, burst errors induce click noise in ADPCM voice signal transmission. This paper proposes a novel error-concealment scheme to improve the ADPCM voice signal transmission quality of time division multiple access time division duplex (TDMA-TDD) personal communication systems in slow fading environments. The proposed scheme employs an improved error-detection technique with PCM differential detection and overflow detection coupled with a cyclic redundancy check (CRC) code. To compensate for residual click noise, an expanded CRC error-detection signal is used. Erroneous PCM signals are adaptively suppressed according to the click-noise detection information. Hardware simulation results show that these techniques significantly improve ADPCM voice signal transmission quality by 0.5 points and 1.0 compared to that of the conventional ADPCM muting scheme and no processing, respectively, with the mean opinion scoring (MOS) test in the slow fading environments typical of personal communication systems     

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     

Partial response continuous phase modulation and DPCM coding for speech transmission in cellular mobile radio systems

The performance of a speech transmission scheme with application to cellular digital mobile radio systems is considered. The source coder is embedded differential pulse code modulation (DPCM) and the modulation schemes belong to the class of partial response continuous phase modulation (CPM). Both quantizing noise and transmission errors contribute to the overall mean square error. The performance measure is the audio signal-to-noise ratio (SNR). It is seen that in a fading environment space diversity is very effective in bringing down the threshold of channel SNR to maintain the required audio SNR. The number of channels the system can support is evaluated under various conditions.     

Optimizing information efficiency in a direct-sequence mobile packet radio network

We consider a direct-sequence multihop packet radio network using slotted ALOHA in a Rayleigh fading environment. We are interested in optimally choosing the transmission range, code rate, and slotted ALOHA transmission probability to be used by each node. We use a new performance measure, information efficiency, to analyze the network and show that the information efficiency of the low-cost mobile packet radio network can be improved approximately 20 fold by using the optimum parameters. We suggest a practical trellis-coded modulation scheme which comes close to realizing the theoretic limits found.     

Pseudo-analog speech transmission in mobile radio communicationsystems

A low-complexity pseudo-analog speech transmission scheme is proposed for portable communications. It uses a speech coder based on adaptive differential pulse code modulation (ADPCM) in combination with a multilevel digital modulation technique such as M-ary DPSK or M-ary FSK and features low quantization noise, bandwidth efficiency, and robustness to transmission errors. A nonsymmetric M -ary DPSK scheme called skewed M-ary DPSK is proposed to enhance the noisy channel performance. Comparison to conventional analog FM and a digital speech transmission scheme using adaptive predictive coding and forward error correction (FEC) based on convolutional coding shows that the pseudo-analog system has the best objective signal-to-noise ratio performance under most channel conditions. Informal subjective evaluations rate the digital system superior to the pseudo-analog scheme for bad channels and conversely for good channels. It is concluded that the pseudo-analog system can be designed with low delay and high speech quality for good channels with high spectral efficiency     

Analysis of a direct-sequence CDMA mobile radio system with reducedset of code sequences

A code-division multiple-access scheme that requires a reduced set of code sequences, in which the size of the set is much less than the number of users in the system, is proposed for mobile radio communications. A direct-sequence spread-spectrum signaling is adopted to exploit the property of high time resolution and combat fading effects through multipath diversity reception. Theoretical performance of the proposed system is analyzed by deriving the collision-free path distribution and determining the proper size of the set without reducing the effect of multipath diversity. In connection with the proposed system, two combining schemes are investigated to characterize the effects of the primary and secondary user interferences and evaluate the behavior of the error rate when compared to the original CDMA system     

The Hybrid Transmission Scheme of a 900 MHz Digital Land Mobile Radio System

The hybrid transmission scheme of MATS-D uses different transmission principles on both directions of a duplex radio link. Based on the analysis of the radio transmission characteristics and of multiple-access methods, the two directions are designed independently from each other. In the downlink (from base station to mobile station) a combination of TDM and CDM is applied. In the uplink (from mobile station to base station) single channel per carrier FDMA with generalized tamed frequency modulation (GTFM) is employed. Both transmission schemes have been implemented. Measurements with fading generators as well as field tests were carried out. Experimental results are presented for the performance of both transmission schemes, expressed in terms of the bit error rate versus the signal-to-noise ratio and the signal-to-interference ratio.     

DS/CDMAM-ary orthogonal signalling in a shadowed Rayleigh channel for mobile communications

In this paper a modified RAKE receiver is studied for a frequency selective mobile radio channel. The reverse link (Mobile to base station) is analysed, assuming lognormal shadowing and Rayleigh fading andK asynchronous users, withM orthogonal sequences per user. The analysis is based on the consideration of the quadrature components of the signal and noise, taking advantage of the multipath effects. The performance evaluation is carried out in terms of both the bit error rate and outage probability in order to qualify completely the proposed receiver. The positive results assure the possibility of applying this system in a microcellular mobile radio environment.