Hatem Boujemaa

Delay Analysis of Cooperative Truncated HARQ With Opportunistic Relaying

In this paper, we evaluate the delay experienced by Poisson arriving packets for cooperative truncated hybrid automatic repeat request (HARQ) with opportunistic relaying. We derive the theoretical expressions of the expected waiting time and the packets sojourn time in the queue of truncated opportunistic cooperative ARQ, HARQ I with and without packet combining (PC), and HARQ II with code combining (CC). The analysis is valid for block Nakagami- m fading channels and any number of relays for both amplify-and-forward (AF) and decode-and-forward (DF) relaying. In opportunistic AF cooperative HARQ, packet retransmission is done by the relay offering the highest instantaneous signal-to-noise ratio (SNR) of the relaying link (source-relay-destination). In opportunistic DF cooperative HARQ, retransmission is done by the relay offering the highest instantaneous SNR of the relay-destination link among the relays that have correctly decoded the transmitted packet by the source. If no relay has correctly decoded, the retransmission is made by the source. Simulation results are also provided to verify the tightness of the derived expressions. The results indicate a significant improvement of packet delivery delays when compared with the absence of cooperation.

On the RAKE receiver performance

In this paper we study the RAKE receiver performance for the UMTS FDD mode. We begin by investigating the amount of diversity that we can recover from paths with delays separated by less than the chip period. Then, we evaluate the RAKE performance degradation caused by channel estimation noise. Finally, we study the influence of intersymbol interference for small spreading factors.

Exact and asymptotic BEP of cooperative DS-CDMA systems using decode and forward relaying in the presence of multipath propagation

In this paper, we derive the bit error probability (BEP) of cooperative DS-CDMA systems using decode and forward (DF) relaying. We consider a conventional DF protocol where all the relays that have correctly decoded transmit and a selective DF (S-DF) protocol where we activate only the relay that offers the best instantaneous signal to noise ratio (SNR) of the relay to destination link among the relays that have correctly decoded. The derived results are valid for any multipath intensity profile of the channel, any path delays and take into account the correlation of the multipath gains.

Exact symbol error probability of cooperative systems with partial relay selection

In this paper, we derive the exact symbol error probability (SEP) of cooperative systems using amplify and forward (AF) relaying and partial relay selection when the different links are independent and non-identically distributed. In conventional relay selection, the selected relay offers the best instantaneous signal to noise ratio (SNR) of the relaying link (source-relay-destination). However, partial relay selection uses only the SNR of the source to relays links. The exact expression of the moment generating function (MGF) of the SNR at the destination is also derived. Copyright

Enhanced coherent delay tracking for direct sequence spread spectrum systems

An enhanced coherent delay lock loop (DLL) is proposed for code tracking in direct sequence spread spectrum systems on multipath fading channels. By normalising the estimated tracking error for large values of the power of the tracked path, the proposed loop reduces most of the degradation caused by fading. Moreover, by subtracting an estimation of the multipath bias from the output of the coherent DLL, this enhanced scheme reduces most of the degradation caused by multipath interference.

On a maximum likelihood delay acquisition algorithm

In this paper, a maximum likelihood delay acquisition algorithm is proposed. It consists in minimising the mean square error between the received signal and an estimated one based on a number of paths and a set of delay hypotheses. Unlike the conventional acquisition algorithm, the proposed approach is able to detect, with high probability, paths with delays as close as one fourth of the chip period.

Angle and time of arrival statistics for hollow-disc and elliptical scattering models

This paper derives explicit closed-form expressions for the joint/marginal probability density functions of the uplink/downlink multipathspsila time-of-arrival (TOA) and azimuth angle-of-arrival (AOA) in a wireless-communication fading channel. These models assume that the scatterers lie within a hollow-disc and an elliptical space, respectively. These probability density functions (pdf) provide insight into the properties of the spatial wireless channel.

On the performance of a discrete time RAKE

A discrete time implementation of the RAKE receiver (DTR) is proposed. The suggested approach works without knowledge of the path delays and their number. It is based on a global estimation of the multipath channel convolutioned with the shaping filter. Since the DTR performance is very sensitive to the channel estimation quality, we suggest a disregarding method for the least significant estimated channel coefficients while taking into account the known impulse response of the shaping filter. The performance of the DTR and the conventional RAKE are then compared.

BER performance analysis of the discrete time and the continuous time Rake receivers for the UMTS downlink

The paper gives the bit error rate (BER) expressions of the continuous time and discrete time Rake receivers (CTR and DTR) for the UMTS downlink. The former receiver is the most used and consists in combining the output of correlators locked on the most significant paths. Since the conventional synchronization module of the CTR is hot able to distinguish close paths, a discrete time implementation of the Rake receiver which circumvents the problem of path delay estimation was proposed in previous papers. The DTR consists in a matched filter to the discrete time equivalent model of the global channel resulting from the convolution of the impulse response of the transmitter shaping filter and the multipath channel. Using the assumption of Gaussian noise on the receiver output, we derive the BER expressions of both the CTR and DTR. The obtained expressions are valid for any multipath channel and take simultaneously into account path gain estimation noise, the intersymbol interference and the multiuser interference which is a realistic scenario for the UMTS downlink. Based on this theoretical study, a power control policy is also formulated.

Optimized pulse shaping for OFDM multi-user communications over doubly dispersive channels

In multiple access OFDM systems, the division of the different users is achieved by exploiting the orthogonality of the sub-carriers. However, this orthogonality may be lost if pulse shapes used in the modulation and demodulation processes are badly localized in time and frequency and the channel is doubly dispersive. Hence a big amount of self and inter-user interference will arise and severely panelize system performance. This problem becomes more critical in the uplink, where users have different time offsets due to different propagation delays and different frequency offsets due to misalignment in carrier frequencies and different Doppler shifts. In this paper we present a pulse-shaped OFDM based analysis of inter-symbol/inter-carrier interference for synchronous downlink and asynchronous uplink of multi-user systems. We propose an optimization procedure for designing pulses maximizing the SIR. We show that system performances can visibly be improved by inserting frequency guard intervals between adjacent users, while making a convenient trade-off with spectral efficiency.