Karim Cheikhrouhou

Impact of synchronization on performance of enhanced array-receivers in wideband CDMA networks

The synchronization performance of the receivers may severely impact the capacity or spectrum efficiency of wideband code division multiple access (CDMA) networks. Evaluations of the uplink capacity for improved spatio-temporal array receivers (STAR) and the two-dimensional RAKE (2-D-RAKE) with both perfect and active synchronization indicate that synchronization with the early-late gate component of a RAKE-type receiver may constitute a bottleneck to performance improvement. Enhancement of synchronization remains a key issue. Results also suggest that STAR offers a promising alternative to the 2-D-RAKE with early-late gate, offering an average increase in spectrum efficiency up to 100% in the presence of synchronization errors at both data rates of 9.6 and 128 kb/s. This gain further increases in high Doppler and fast multipath delay drifts. Data oversampling above the chip-rate favors STAR even more. Significant simplifications are introduced in the formulation of the STAR receiver which result in a complexity comparable to the 2-D-RAKE.

Enhanced interference suppression for spectrum-efficient high data-rate transmissions over wideband CDMA networks

Recently, we developed an efficient multiuser upgrade of the single-user spatio-temporal array-receiver (STAR), referred to as interference subspace rejection (ISR) (Affes, S. et al., IEEE J. Selec. Areas Comm., vol.20, no.2, p.287-302, 2002). The resulting STAR-ISR receiver offers a number of implementation modes covering a large range in performance and complexity for wideband CDMA networks. Here we apply STAR-ISR to high data-rate (HDR) transmissions by extending operation to delay spreads larger than the symbol duration. Low processing gain situations render symbol timing extremely difficult, especially with RAKE-type receivers. For HDR links of 512 Kbps in 5 MHz bandwidth, simulations indicate that the simplest mode of STAR-ISR outperforms the 2D-RAKE-PIC by a factor of 4 to 5 in spectrum efficiency while requiring the same order of complexity. This gain increases by up to a factor of 7.5 in high Doppler.

Impact of synchronization on receiver performance in wideband CDMA networks

Synchronization may severely impact the capacity or spectrum efficiency of wideband CDMA networks. Evaluations of the uplink capacity for improved STAR and the 2D-RAKE with both perfect and active synchronization indicate that synchronization with the early-late gate component of a RAKE-type receiver may constitute a bottleneck for any performance improvement. Enhancement of synchronization remains a key issue. Results also suggest that STAR offers a promising alternative to the 2D-RAKE with early-late gate, offering on average an increase in spectrum efficiency of more than 100% in the presence of synchronization errors at both data rates of 9.6 and 144 kbps.

Design verification and performance evaluation of an enhanced wideband CDMA receiver using channel measurements

The spatio-temporal array receiver (STAR) decomposes generic wideband CDMA channel responses across various parameter dimensions (e.g., time delays, multipath components, etc.) and extracts the associated time-varying parameters (i.e., analysis) before reconstructing the channel (i.e., synthesis) with increased accuracy. This work verifies the channel analysis/synthesis design of STAR by illustrating its capability to extract accurately the channel parameters (time delays and drifts, carrier frequency offsets, Doppler spread, etc.) from measured data and to adapt online to their observed time evolution in real-world propagation conditions. We also verify the performance of STAR by comparing the results achieved with generic and measured channels for an average multipath power profile of [] dB and a vehicular speed below 30 km/h. The results suggest that losses due to operations with real channels are only 1 dB in SNR and –% in capacity with DBPSK and single transmit and receive antennas. The corresponding SNR threshold for operation with real channels is about 5 dB.

Adaptive MIMO-diversity selection with closed-loop power control over wireless CDMA rayleigh-fading channels

This contribution considers adaptive (multiple input multiple output) MIMO-diversity selection at the receiver jointly with closed-loop power control (PC) to efficiently combat fading with only few transmit (Tx) and receive (Rx) antennas. Hence we avoid resorting to antenna selection among large MIMO-arrays without PC which would be otherwise required to keep complexity low. With low Doppler closed-loop PC significantly increases the transmission capacity and reduces the MIMO-array size.

Area-efficient advanced multiuser WCDMA receiver

The authors explored the benefits of employing STAR as a wideband CDMA receiver. It is shown that it inherently addresses data recovery and channel characterization through its analysis/synthesis paradigm. Simulation results carried out on real-world channel measurements showing the robustness and quick adaptation rate of STAR in terms of time-delay tracking performance and carrier frequency offset recovery were presented. Then an overview of the preferred hardware embodiment along with hard values in terms of hardware resource utilization, showing that a 24-user STAR-enabled 3G base station receivers can be realized within a single XC2V6000 FPGA was given.

Synchronization sensitivity of enhanced CDMA array-receivers to fast channel time-variations and to shaping-pulse design

We previously reported on the severe impact synchronization errors may have on the spectrum efficiency of wideband CDMA networks and on the significant performance gains of STAR (spatio-temporal array receiver) over the 2D-RAKE at similar order in complexity. Here we further enhance the 2D-RAKE performance by incorporating a high-resolution early-late gate with a clamping state and further reduce the complexity of STAR. New complexity and performance assessments indicate that STAR, at a comparable computational load, increases its performance advantage over the enhanced 2D-RAKE in the presence of fast channel time-variations even with pulse distortions or at different rolloff factors.

Efficient wireless network deployment by cognitive transceivers with multimodal pilot-use modems

In this paper, we develop new cognitive antenna-array transceivers with a multimodal modem that, according to channel conditions, exploits the pilot signal differently yet optimally for improved adaptive channel identification and data combining. This new concept allows better throughput optimization of wireless networks while reducing the Tx pilot power at the BS. By keeping the Tx pilot power unchanged at the BS, this increased power efficiency can alternatively translate into an increased cell-size coverage, i.e., in a reduced deployment cost with relatively less BSs in a given area.

A cognitive multi-antenna transceiver with a multimodal-pilot-use modem for increased wireless network throughput, coverage and power efficiency

In this paper, we develop new cognitive antenna-array transceivers with a multimodal modem that, according to channel conditions, exploits the pilot signal differently yet optimally for improved adaptive channel identification and data combining. This new concept allows better throughput optimization of wireless networks while reducing the Tx pilot power at the BS. By keeping the Tx pilot power unchanged at the BS, this increased power efficiency can alternatively translate into an increased cell-size coverage, i.e., in a reduced deployment cost with relatively less BSs in a given area.

Impact of the Angular Velocity on the Signals Spectrum and Performance of Antenna-Array Receivers

Tremendous effort was previously made to estimate the location of moving sources with large angular velocities. However, to the best of our knowledge, no previous work has thoroughly investigated the effect of the combined angular velocity and mobile speed on the spectrum of the signal received by an antenna-array. In this contribution, we address this issue in the particular context of wideband CDMA transmission. We first show both by theoretical development and by simulations that the maximum frequency shift is the sum of the conventional Doppler term and a new term due to angular speed and that it is a linear function of both. We also show that the additional frequency shift due to the angular speed increases channel identification errors and thereby degrades the performance of antenna-array receivers. This performance loss becomes even higher at a larger number of antennas in the array.