Modern CDMA systems achieve band-width expansion through a combination of direct sequence spreading and channel coding. The goal of the coding-spreading tradeoff is to find the coding and spreading factors that maximize the spectral efficiency (measured in bits/chip) of the system. Motivated by recent results (1), this paper considers the coding-spreading tradeoff for CDMA systems using practial low rate binary convolutional codes and random binary spreading sequences. The receiver consists of a linear multiuser detector front-end followed by autonomous single-user decoders. The spectral efficiencies obtained with the conventional matched filter (MF) and linear minimum mean-squared error (LMMSE) receivers are shwon for single-cell and multicell scenarios, and for hard and soft decision decoders. Comparing the spectral efficiencies of the two detectors at their corresponding optimum coding-spreading operating points, the results shwo a large gap in the single-cell scenario and a considerably smalelr gap in the multicell scenario. These results indicate that linear multiuser detection may offer little improvmeent in spectral efficiency over conventional matched filter detection in multicell CDMA systems with soft decision decoding.
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