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.

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 performance of STAR by comparing the results achieved with generic and measured channels for an average multipath power profile of [0, -4, -8] dB and a vehicular speed below 30 Km/h. The results suggest that losses due to operations with real 5 MHz channels are only 1 dB in SNR and 20-30% in capacity with DBPSK and single transmit and receive antennas. The corresponding SNR threshold for operation with real channels is about 5 dB.

Many research efforts were recently spent to address the challenging problem of DOA and angular spread estimation. However, to the best of our knowledge, no previous work has thoroughly investigated their specific impact on the performance of DOA-based antenna-array beamforming. In this contribution, we address this issue in the particular context of wideband CDMA using pilot-assisted or blind antenna-array receivers. In the process, we also assess whether the generalized channel-matched beamforming (i.e., without DOA estimation or a priori knowledge of the spatial structure of the channel) offers a better alternative. Link-level simulation results in terms of required SNR at target BER of 1% suggest that wideband CDMA array-receivers, whether pilot-assisted or blind, are extremely sensitive to angular spread mismatches and that the benefits of exploiting the spatial structure of the channel (i.e., estimation of DOA, angular spread, etc..) translate, at best, in negligible SNR gains when accurate channel identification is already implemented. These results call for implementing channel-matched instead of conventional DOA-based beamforming.

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.

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.