Implementing CDMA Reverse Link Interference Cancellation. John E. Smee, Jilei Hou, Joseph B. Soriaga. QUALCOMM Inc., 5775 Morehouse Drive, San Diego, ...
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Implementing CDMA Reverse Link Interference Cancellation John E. Smee, Jilei Hou, Joseph B. Soriaga
QUALCOMM Inc., 5775 Morehouse Drive, San Diego, CA, 92121 USA {jsmee, jhou, jsoriaga}
convolving a multipath channel estimate with the transmit and receive filter responses. An analytical framework with closed-form solutions was developed to model the residual power remaining after subtractive cancellation incorporating channel variation, channel estimation errors, filter mismatch, and chip asynchronism [3]. Simulations were used to further model practical reconstruction on sub-chip multipath channels, comparing pilot based estimates with estimates from the re-encoded traffic channel itself. One practical approach adopted in our results is to cancel pilot and overhead channels based on pilot channel estimates while canceling each decoded traffic packet based on its re-encoded data. Results quantify how the power of the traffic channel and the processing gain from accumulating over chips within the fading coherence interval give high cancellation efficiency. Extensive 57-sector 19-cell network simulations for CDMA2000 EV-DO were used to evaluate IC with path loss, shadowing, inter-cell interference, hybrid-ARQ, multipath fading channels, and the MAC-layer dynamics of power control, rate allocation, and rise-over-thermal control [4]. Each of the 10 data users per sector employed standard power control targeting 1% packet error rate after the transmission of 4 subpackets. The resulting rise-overthermal and user data rate distributions demonstrate how network stability, link-budget, latency, and fairness were maintained relative to a non-IC system. With 2 receiver antennas per sector, reverse link throughput gains of over 65% were achieved by IC implementations that are realizable in today' s baseband processing technologies.
Abstract - We describe ongoing research and product development focused on the theory, system design, and capacity gains from incorporating reverse link interference cancellation into commercial CDMA basestations. The work considers receiver architecture design tradeoffs, link cancellation efficiency in multipath fading channels, and the multi-cell network operation and performance. Index Terms - CDMA, multiuser detection, interference
cancellation, reverse link, uplink, hybrid-ARQ. EXTENDED SUMMARY
CDMA has seen wide commercial deployment due to its combination of high capacity and system robustness and its evolution from supporting voice calls to supporting wireless internet connectivity, VoIP, video telephony, and broadcast services. With each reverse link user employing channel coding, successively decoding and cancelling spread-spectrum users has been shown theoretically to achieve capacity for a multiple access system [1]. From a practical standpoint, continual advances in ASIC, FPGA, DSP, and RAM technologies have now progressed far enough to allow commercial deployment of CDMA basestation interference cancellation (IC) without modifications to the user terminals or standards. In current basestation receivers, each user is typically demodulated from a shared receiver sample buffer and then decoded; incorporating IC involves the additional steps of re-encoding, re-modulation, signal reconstruction, and subtraction from the shared buffer. CDMA reverse links such as EV-DO RevA and WCDMA EUL have incorporated Hybrid-ARQ to efficiently translate unpredictable time-varying SINR into predictable packet error rate performance with varying time-span per packet. The design tradeoff for sizing the front-end demodulation/cancellation buffer favors spanning the maximal number of subpackets per packet so that all subpackets of a packet can be re-demodulated to benefit from IC with older subpackets having higher SINR as more packets decode [2]. Based on this shared buffer that spans multiple decode attempts per packet and allows iterative cancellation between users, H-ARQ improves the robustness of IC by inherently factoring in the unpredictability of signal fading and user decoding events. With Nyquist rate samples stored at twice the chip rate, the transmitted chip streams can be reconstructed based on
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REFERENCES [1] A. J. Viterbi, "Very low rate convolutional codes for maximum theoretical performance of spread-spectrum multiple-access channels," IEEE JSAC., vol. 8, no. 4, pp. 641-649, May 1990. [2] J. E. Smee, J. Hou, J. B. Soriaga, "Receiver architectures and design tradeoffs for CDMA interference cancellation," to appear Asilomar 2006. [3] J. Hou, J. E. Smee, J. B. Soriaga, J. Chen, H. D. Pfister, "Link level modeling and performance of CDMA interference cancellation," to appear Globecom 2006. [4] J. B. Soriaga, J. Hou, J. E. Smee, "Network level performance of the EV-DO CDMA reverse link with interference cancellation," to appear Globecom 2006.
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