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Volume 139
Issue 4
IEE Proceedings I (Communications, Speech and Vision)
Volume 139, Issue 4, August 1992
Volumes & issues:
Volume 139, Issue 4
August 1992
Voice activity detection using a periodicity measure
- Author(s): R. Tucker
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 139, Issue 4, p. 377 –380
- DOI: 10.1049/ip-i-2.1992.0052
- Type: Article
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p.
377
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The paper describes a voice activity detector (VAD) that can operate reliably in SNRs down to 0 dB and detect most speech at −5 dB. The detector applies a least-squares periodicity estimator to the input signal, and triggers when a significant amount of periodicity is found. It does not aim to find the exact talkspurt boundaries and, consequently, is most suited to speech-logging applications where it is easy to include a small margin to allow for any missed speech. The paper discusses the problem of false triggering on nonspeech periodic signals and shows how robustness to these signals can be achieved with suitable preprocessing and postprocessing.
Spectral efficiency of M-QAM in frequency reuse radio systems
- Author(s): S.W. Wales
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 139, Issue 4, p. 381 –385
- DOI: 10.1049/ip-i-2.1992.0053
- Type: Article
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p.
381
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The spectral efficiency of M-QAM modulation has been investigated for frequency reuse radio systems under the propagation conditions of the land mobile radio environment. The results indicate that, under certain circumstances (of propagation and coverage requirements), increasing the number of levels can lead to an improvement in the spectral efficiency of interference limited systems, but that, as better coverage or service quality is sought, these spectral efficiency improvements diminish.
Convolutionally encoded FFH-MFSK: demodulation and frequency tracking over frequency-varying jammed channels
- Author(s): J.A. Gibbs and S.J. Braithwaite
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 139, Issue 4, p. 386 –394
- DOI: 10.1049/ip-i-2.1992.0054
- Type: Article
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p.
386
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(9)
A novel soft decision metric for the Viterbi demodulation and decoding of convolutionally encoded fast-frequency-hopped M-ary frequency shift keying is described. The effectiveness of two possible hop-by-hop AGC methods is evaluated for use with metric calculation methods to optimise the worst-case partial-band-jamming performance. The metric is shown to improve the worst-case jamming bit-error-rate (BER) performance for M≥8 in these situations and to alter the worst-case jamming scenario from one of sparsely separated tones to either full-band AWGN or densely spaced jamming tones. This metric also makes large signal sets (M≥16) more attractive for anti-jam M-ary frequency-shift keyed systems. The worst-case partial-band tone jamming BER performance is consistently close to the broadband AWGN case, irrespective of the receiver bandwidth. This is the reverse with conventional linear metrics. The Viterbi frequency-tracking method is shown to provide fast tracking of frequency offsets whilst degrading the BER performance of 8-FSK in AWGN or worst-case partial-band tone jamming by 0.5 dB for a tracking range of 16 possible carrier-frequency positions.
A chirp modem incorporating interference excision
- Author(s): E.P. Darbyshire and G.F. Gott
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 139, Issue 4, p. 395 –406
- DOI: 10.1049/ip-i-2.1992.0055
- Type: Article
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p.
395
–406
(12)
The paper describes an experimental 75-baud chirp modem designed to operate at high frequency (HF), which is able to reject interference from other HF users by adaptive filtering at the receiver. It also has inherently good performance in multipath preparation and in white noise, and is thus capable of robust digital communication. The system has been tested over a 170 km point-to-point HF skywave link, in which short chirp messages (approximately 30 s duration) were alternated in time with frequency-exchange keyed (FEK) messages. The total duration of the tests was 72 hours, and included day and night operation. In these tests, the chirp system had an overall performance which was superior to that of FEK. FEK occasionally suffered from long periods (e. g. 1 hour) of severe failure, essentially due to interference, whereas the chirp modem achieved more continuous useful operation.
Scheduling delay protocols integrating voice and data on a bus LAN
- Author(s): S. Bolis ; E.G. Economou ; G. Philokyprou
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 139, Issue 4, p. 407 –412
- DOI: 10.1049/ip-i-2.1992.0056
- Type: Article
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p.
407
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Two new protocols integrating voice and data on a multiaccess broadcast bus LAN with reverse round robin service are presented. The protocols are based on a collision free scheduling delay access mechanism. According to this mechanism, each station computes its transmission time relying on the activity of the bus. In the first protocol, we use this mechanism for both voice and data traffic. In the second protocol, the data traffic is based on the above mechanism, while, for the voice traffic, we additionally exploit the periodicity of voice. Separation of the two traffic types is achieved by the requirement for sharing the channel alternately in voice and data rounds. The transmission of data packets and voice call packets for setting up calls is assigned to the data round while the transmission of voice packets is assigned to the voice round. The proposed protocols were studied via simulation tools. They satisfy the delay constraint of voice traffic and, at the same time, guarantee a sufficient bandwidth for data traffic. Results obtained under various traffic conditions are given.
Decoding delays of orchard codes
- Author(s): J. Lee and A. Shiozaki
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 139, Issue 4, p. 413 –417
- DOI: 10.1049/ip-i-2.1992.0057
- Type: Article
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p.
413
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To evaluate an orchard code, a standard measure of how well the orchard code works is necessary. In the paper we propose the decoding delay to replace the commonly used scheme length as a criterion for evaluating orchard codes. The ever approachable lower bound of the decoding delay is derived to be 2τ for τ-error correcting orchard codes. With the help of the concept of the decoding delay, the performances of several orchard codes are compared. Also given are some very interesting new orchard codes.
Examination of an adaptive dfe and MLSE/near-MLSE for fading multipath radio channels
- Author(s): P.K. Shukla and L.F. Turner
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 139, Issue 4, p. 418 –428
- DOI: 10.1049/ip-i-2.1992.0058
- Type: Article
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p.
418
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This paper examines the performance over an HF channel of two adaptive equalisers; the decision feedback equaliser (DFE) and the maximum likelihood sequence estimator (MLSE), or an approximation to it (near-MLSE). It is shown that the superiority of the more complex MLSE/near-MLSE over the DFE is not great, and that this advantage is only due to error propagation in the DFE. In the absence of any special arrangement by which the receiver can re-align itself (e.g. periodic training), 16-QAM appears to be the highest feasible signal constellation size. With the periodic insertion of known symbols into the data stream, it is shown that performance can be maintained near the level for which decision errors do not affect the tracking algorithm. With the facility of a request-for-training link to the transmitter, it is shown that performance can be significantly improved for all constellation sizes, with only a small loss in the useful data rate.
Synchronised linear almost periodically time-varying adaptive filter
- Author(s): Heping Ding and M. Moustafa Fahmy
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 139, Issue 4, p. 429 –436
- DOI: 10.1049/ip-i-2.1992.0059
- Type: Article
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p.
429
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Conventional adaptive filters only suit stationary and slowly time-varying nonstationary inputs. Based on the previous work on linear almost periodically time-varying adaptive filtering, the paper develops such an adaptive filter adopting the output-modulator structure. In addition to being capable of dealing with almost cyclostationary inputs (an important class of nonstationary signals), this scheme is also able, by means of adaptive frequency tracking, to remain synchronous with the varying input statistics.
Transmission of digitally encoded speech at 1.2 Kbaud for PCN
- Author(s): L. Hanzo ; R. Salami ; R. Steele ; P.M. Fortune
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 139, Issue 4, p. 437 –447
- DOI: 10.1049/ip-i-2.1992.0060
- Type: Article
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p.
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The transmission of digitally encoded speech at 1.2 Kbaud for mobile personal communication networks (PCN) is investigated both analytically and by computer simulations. Speech was encoded at 4.8 Kbit/s using a low complexity transformed binary pulse excited LPC codec. A 64-level QAM modem was used having three subchannels that operated with different BERs. The sensitivity of the encoded speech bits to transmission errors was identified, and the bits classified into three groups. Each group was then individually coded by BCH codecs of differing power. The output of the BCH coders were Gray coded onto the three QAM channels. By this arrangement the protection given to the speech bits was dependent on their vulnerability. The six-bit QAM symbols were transmitted at 1.2 Kbaud over Rayleigh fading channels with pedestrian mobiles travelling up to 4 mile/h. For microcells using a propagation frequency of 1.9 GHz and operating with channel SNRs in excess of 26 dB we achieved good communications quality speech when second-order diversity and AGC were used.
Teletraffic performance of microcellular personal communication networks
- Author(s): R. Steele and M. Nofal
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 139, Issue 4, p. 448 –461
- DOI: 10.1049/ip-i-2.1992.0061
- Type: Article
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p.
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Teletraffic performance of a personal communication network (PCN) based on city street microcells catering for pedestrian mobile users is analysed. By opting for microcells, the network capacity dramatically increases as well as the spectrum efficiency, but the network must be able to accommodate substantial increases in the number and rate of call handovers. A teletraffic analysis is performed to determine the new and handover call rates as well as the channel holding time of the PCN. Next, a teletraffic model is established to determine the teletraffic performance parameters, such as the blocking probabilities of new and handover calls, the overall grade of service, the carried traffic, the channel utilisation, and the spectrum efficiency. Techniques to reduce the premature termination of calls in progress are proposed and modelled. These include reserving a set of channels exclusively for handovers at each microcell fixed station, and deploying a macrocell to overlay a microcellular cluster, where the macrocell base station is assigned channels for handover requests from mobiles who cannot be serviced by their microcell fixed stations. A large spectral efficiency of 1000 Erlang/MHz/km2 is achieved at nominal blocking probability of 2%, while the probability of premature termination is reduced to less than 10−4 in the presence of high call rates in excess of 0.06 calls/s/microcell. The results presented will assist network planners in predicting teletraffic performance.
Performance off 8-PSK trellis codes over nonlinear fading mobile satellite channels
- Author(s): B. Vucetic and J. Nicolas
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 139, Issue 4, p. 462 –471
- DOI: 10.1049/ip-i-2.1992.0062
- Type: Article
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p.
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In this paper a new analytical bound for large effective code length codes (ECL) is derived. The accuracy of the analytical bounds is validated by simulation. Based on the new bound a trellis code design procedure for fading channels is defined. A quasi-analytical technique for BER estimation on nonlinear channels is proposed. This method is used to estimate the performance degradation of nonlinear communication systems with the pilot tone.
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