Effect of Nonlinear Amplification on Walsh Encoding/Spreading with Turbo Coding

Multi-user communication schemes such as code division multiple access (CDMA) require users to operate in various noisy environments. Typically, these systems use both spread spectrum (SS) and forward error correction (FEC) to achieve an acceptable bit error rate (BER). Walsh spreading, based on the Hadamard matrix, is one type of spreading used in CDMA schemes. In particular Walsh spreading is used in both IS-95 and UMTS (3G WCDMA) cellular telephone standards. Walsh-encoded symbols can also be considered an orthogonal modulation alphabet. This paper presents the BER performance of an end-to-end transceiver in the presence of nonlinear amplification with both Walsh modulation and turbo coding. Nonlinear amplification is due to the high-power amplifiers in the cellular base station or satellite downlink. In order to achieve very low BER with reasonable delay, an end-to-end transceiver is implemented in real-time hardware on a field-programmable gate array (FPGA). The high throughput afforded by the hardware implementation allows us to characterize the large system trade space defined by the Walsh order, amplifier back-off, and signal-to-noise ratio.