A low cost multi-standard near-optimal soft-output sphere decoder: Algorithm and architecture

We present an algorithm and architecture of a soft-output sphere decoder with an optimized hardware implementation for 2×2 MIMO-OFDM reception. We introduce a novel table look-up approach for symbol enumeration that simplifies the implementation of soft-output decoders. The HW implementation is targeted towards WLAN (IEEE 802.11n) with stringent latency and throughput requirements. The current implementation supports all modulation schemes (BPSK,QPSK,16-QAM, 64-QAM) and shows near-optimal real-time performance. To achieve this, the sphere decoder computes in the worst-case Euclidean distances of 4.1 Giga QAM symbols per second. This challenging requirement is met by a scalable, multi-standard HW architecture which can be tuned to other applications such as LTE, WiMax with no re-design effort. The current instance for WLAN occupies an area of only 0.17 mm2 in 45 nm CMOS technology while providing a guaranteed throughput of 374 Msoftbits/s at 312 MHz clock rate (i.e. outputting 2×6 softbits worst-case every 10 clock cycles).