J.M. Torrance

Latency and networking aspects of adaptive modems over slow indoors Rayleigh fading channels

Adaptive modulation exploits the time-variant channel capacity fluctuation of fading channels using a range of different modem modes. Specifically, no information is transmitted when the instantaneous channel signal-to-noise ratio (SNR) is low, and, hence, during this period the data must be buffered, which results in delay or latency, When the instantaneous channel quality improves, 2-, 4-, 16-, and 64-level modem modes are invoked, which allows the transmission buffer to be emptied. It is shown that channel capacity gains are achieved at the cost of some latency penalty. The latency is quantified in this treatise and mitigated by frequency hopping or statistical multiplexing. The latency is increased when either the mobile speed or the channel SNR are reduced, since both of these result in prolonged low instantaneous SNR intervals. It is demonstrated that as a result of the proposed measures, typically more than 4-dB SNR reduction is achieved by the proposed adaptive modems in comparison to the conventional benchmark modems employed.

Interference aspects of adaptive modems over slow Rayleigh fading channels

Adaptive modulation can achieve channel capacity gains by adapting the number of bits per transmission symbol on a burst-by-burst basis, in harmony with channel quality fluctuations. This is demonstrated in the paper for target bit error rates of 1 and 0.01%, respectively, in comparison to conventional fixed modems. However, the achievable gains depend strongly on the prevalent interference levels and hence interference cancellation is invoked on the basis of adjusting the demodulation decision boundaries after estimating the interfering channels magnitude and phase. Using the modem-mode switching levels of Table X and with the aid of interference cancellation, target BERs of 1 and 0.01% can be maintained over slow-fading channels for a wide range of channel signal-to-noise ratios (SNR) and signal-to-interference ratios (SIR).

Adaptive modulation in a slow Rayleigh fading channel

A novel, uneven protection phase shift keying technique is proposed for the encoding of the required modulation scheme in an adaptive modem arrangement, which exhibits an improved performance in comparison to previously proposed schemes. The performance is derived numerically and a system dependent optimisation is presented. A benefit of 5 dB is achieved in a Rayleigh channel.

Latency considerations for adaptive modulation in an interference-free slow Rayleigh fading channel

Adaptive modulation exploits the time-variant channel capacity fluctuation of fading channels using a range of different modem modes according to the equation of the transmission scheme and the results presented in the table. This is achieved at the cost of some latency penalty, which is quantified and mitigated by frequency hopping.

Statistical multiplexing for mitigating latency in adaptive modems

The latency of instantaneously adaptive modulation is mitigated using statistical multiplexing. The proposed scheme exhibits typically higher integrity than fixed modems, or requires an approximately 2 dB lower channel SNR.

A low-delay multimode speech terminal

The intelligent, adaptively reconfigurable wireless systems of the near future require programmable source codecs in order to optimally configure the transceiver to adapt to time-variant channel and traffic conditions. Hence we developed a programmable 8-16 kbits/s low-delay speech codec, which is compatible with the G728 16 kbits/s ITU codec at its top rate and offers a graceful trade-off between the speech quality and bit rate in the 8-16 kbits/s range. The issues of robustness against channel errors strongly influenced the algorithmic design of the 8-16 kbits/s speech codec, and hence special attention is devoted to these issues. Source-matched Bose-Chaudhuri-Hocquenghem (BCH) codecs combined with unequal protection pilot-assisted 4- and 16-level quadrature amplitude modulation (4-QAM, 16-QAM) are employed in order to transmit both the 8 and the 16 kbits/s coded speech bits at a signalling rate of 10.4 kBd. In a bandwidth of 1728 kHz, which is used by the Digital European Cordless Telephone (DECT) system 55 duplex or 110 simplex time slots can be created. Good toll quality speech is delivered in an equivalent bandwidth of 15.71 kHz, if the channel signal-to-noise ratio (SNR) and signal-to-interference ratio (SIR) are in excess of about 18 and 26 dB for the lower and higher speech quality 4-QAM and 16-QAM modes, respectively.

Multi-level modulation in the indoors leaky feeder environment

Leaky feeders offer the prospect of high average signal to noise ratio (SNR) in an indoors environment. The signal is transmitted from a series of slots acting as antennae and superposition of the paths results in fades of up to 25 dB below the mean signal level. A model is presented for the propagation path between the leaky feeder base station (BS) antenna and a single omnidirectional receiver antenna, which was verified using measurements. The bit error rate (BER) performance of such a system is investigated for 1, 2 and 4 bits per symbol, coherent and non-coherent modulation schemes. These are compared with the performance of a more conventional indoor BS antenna.

Interference cancellation for adaptive multi-mode terminals

Adaptive modulation can achieve channel capacity gains by adapting the number of bits per transmission symbol on a burst-by-burst basis, in harmony with channel quality fluctuations. This is demonstrated for target bit error rates of 1 and 0.01%, respectively, in comparison to conventional fixed modems. However, the achievable gains depend strongly on the prevalent interference levels and hence interference cancellation is invoked on the basis of adjusting the demodulation decision boundaries after estimating the interfering channels magnitude and phase. Using given modem-mode switching levels and with the aid of interference cancellation, target BERs can be maintained over slow-fading channels for a wide range of channel signal-to-noise ratios (SNR) and signal-to-interference ratios (SIR).