Antonio Salloum

Performance of digital DECT radio links based on semianalytical methods

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 (/spl gamma//sub e/) in the threshold detector. Both the DF equalizer coefficient and /spl gamma//sub 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/sup -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.

Contention-TDMA protocol: performance evaluation

A hybrid access protocol known, as contention time-division multiple access (C-TDMA), is presented and analyzed in a radio cellular multiuser system scenario. C-TDMA shows some features of contention-based (slotted-ALOHA) and reservation-based (packet reservation multiple access, PRMA) protocols. It has been recommended for use in the uplink of future European multimedia distribution systems. A simple Markov model is proposed to describe C-TDMA behavior. A complete statistical analysis of the model has been made in order to evaluate the performance of the protocol. However, due to the long computation time required in the presence of a large number of users, a simpler approach known as equilibrium point analysis (EPA) is used. Moreover, on the basis of the EPA analysis and the C-TDMA design parameters, a fast algorithm has been developed to improve the achievable throughput of C-TDMA. Results in terms of throughput and delay under variable traffic conditions indicate that C-TDMA is able to grant optimum throughput/delay figures for typical multiuser systems. Moreover, for a digital speech scenario, a performance comparison with PRMA demonstrates that C-TDMA yields equivalent performance to PRMA in terms of number of users supported by the system with a limited packet dropping rate.

Worst case equalizer for noncoherent HIPERLAN receivers

Coherent detection of HIPERLAN Gaussian minimum-shift keying signals calls for complex and expensive receivers. However, when the channel delay spread is limited to at the most 50% of the symbol time and a reliable line-of-sight component of the radiated signal exists (Rician fading model), noncoherent detectors are capable of achieving a good performance. Based on the above motivations, we compare four different demodulation techniques, namely the following: (1) one-bit differential detector; (2) discriminator detector; (3) limiter discriminator detector; and (4) limiter discriminator integrator detector (LDID). The intersymbol interference introduced by these demodulators is nonlinear (with respect to the data symbols) and a decision-feedback equalizer (DFE) based on a mean square-error criterion may not be appropriate. Moreover, at this high speed, a DFE may be very complex to implement. Hence, we propose a new DFE design method that increases the eye-diagram aperture by removing the worst case interference. Performance of the above demodulators in the presence of a simple nonlinear DFE (with feedback part only) is computed in terms of the bit-error rate (BER) by means of the saddle-point approximation. This procedure, for static channels, turns out to be a very general tool with a simple and robust implementation. The same method can be applied, for multipath fading channels, to the BER evaluation as part of a semianalytic approach. The main conclusion from this work is that for LDID demodulators and in the presence of Rician fading channels with an average normalized root mean square delay spread of 0.3 and dual antenna diversity, the new equalizer lowers the outage probability from 60% to 10% at a BER of 10/sup -4/.

Further results on differential detection of GMSK signals

We evaluate the performance, in terms of bit-error rate, of 1- and 2-bit differential detectors for Gaussian minimum-shift keying (GMSK) signals by using a very fast numerical technique which makes use of the saddle point approximation. Moreover, the performance of each detector is optimized by using a closed-form expression of the optimum threshold in the slicer.

Performance of the multitrellis Viterbi algorithm for sparse channels

Reports the error probability, P/sub e/, of a novel receiver structure for data transmission over multipath channels characterized by a long impulse response but with only few coarsely located coefficients, as can be found in HF and mobile radio systems. The structure makes use of an optimized Viterbi algorithm, named the multitrellis Viterbi algorithm (MVA), whose complexity depends only on the number of nonzero coefficients of the channel. Performance of the MVA is compared with that obtained by other classical algorithms; namely the decision feedback equalizer (DFE) and the reduce-state sequence estimator (RSSE). When the channel has minimum phase, performance of the MVA is comparable to that of the RSSE and both save few dB of signal to noise ratio (at P/sub e/=10/sup -3/) with respect to the DFE. However, for non-minimum phase channels, while the P/sub e/ of the RSSE deteriorates drastically, the P/sub e/ of the MVA is not affected by the phase response of the channel.

Access Protocols for Cellular High-Speed Data Services

In this contribution, novel access techniquesfor the future cellular information networks areproposed. To facilitate a fast deployment, the suggestedaccess techniques have been implemented over well-known technological platforms, namely DECT (DigitalEnhanced Cordless Telecommunications) containing aFDM/TDMA (Frequency Division Multiplex/Time DivisionMultiple Access) structure. Two situations areconsidered: i) no limits in the use of the all carriers atterminal (ideal case) and ii) maximum number ofcontiguous/simultaneous carriers at terminal (realisticcase). First, to grant a quick access channel, a random access mechanism with reservation is proposed.It is called contention-TDMA (C-TDMA). Next, in order tostudy the feasibility of better collision resolutionalgorithms for C-TDMA, a centralized approach called Centralized C-TDMA Demand Assignment(CC-TDMA-DA) has also been studied. For the ideal case,numerical results indicate that C-TDMA and CC-TDMA-DAyield similar throughput values in typical cellular scenarios. Also, CC-TDMA-DA tends to bepreferable for traffic sources with long messages andweak delay constraints, while C-TDMA performs better forbursty traffic sources with hard time requirements. Additional numerical results for the realisticcase have demonstrated that system performancedeteriorates little with respect to the ideal case interms of collisions, delays and throughputfigures.

Uplink access protocol for multimedia systems based on DECT/ATM layer

In this paper, we present a new, simple and efficient access protocol for the return channel of the local multipoint distribution system (LMDS). The protocol is called contention-TDMA (C-TDMA) and it is able to support multimedia, multirate and multiuser services in typical LMDS scenarios. Moreover, it is based on a very cheap and already existing radio technology: DECT (digital enhanced cordless telecommunications) adapted to be ATM compatible. Three channel selection mechanisms for C-TDMA have been tested by using computer simulations. Numerical results indicate that, in general, C-TDMA guarantees a reduced number of collisions, small message delays and high throughput levels.

An all digital receiver for DECT radios

Most DECT radio receivers, up to now, have been analog and detection was via a limiter-discriminator. We present a receiver including an all-digital part after the analog differential detection scheme. The tasks of the digital signal processing unit are: (i) to give an estimate of both the optimum sampling phase and the non-linear channel coefficients, (ii) one-tap decision feedback (DF) equalizer, and (iii) evaluation of the approximate optimum bias level of the threshold detector. We consider static channels with impulse responses generated by the simulation program SIRCIM. For channels with large delay-spread, use of the optimum threshold together with the DF equalizer gains about 2 dB at a BER=10/sup -3/ with respect to a receiver without an equalizer.

Multitrellis Viterbi algorithm for indoor systems using narrowbeam antennas

In earlier work, the performance (in terms of data eye closure) of a threshold detector in 2- and 4-PSK modulation schemes has been analyzed for wireless indoor systems using narrowbeam antennas. Here, assuming the channel is known, a very efficient implementation of the Viterbi algorithm (VA) is included in the receiver, and a bit-error rate (BER) criterion is used to evaluate the receiver robustness to channel conditions. In contrast to the earlier work, it is seen that 4-PSK signals with VA detectors are indeed more robust than 2-PSK signals to channel conditions.