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 mm2in 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).

Multicarrier spread spectrum transmission methods based on orthogonal frequency division multiplexing employ a linear transform to spread energy of transmitted symbols over statistically independent Rayleigh fading subcarriers, in order to enable a diversity gain at the receiver. We jointly treat the mapping from bits into transmit symbols and the spreading transform as a code in Euclidean space. We describe criteria to identify good and bad spreading transforms, in terms of the asymptotic bit-error probability (BEP) at high signal-to-noise ratio. Upper and lower bounds on the BEP are derived. It is shown that the BEP performance of the commonly used Hadamard transform is asymptotically bad. Alternative orthogonal transforms with better asymptotic performance are proposed. Simulation results are given for the example of 2-phase-shift keying, block length 8.

We investigate amplitude distributions of transmit signals for multi-carrier spread spectrum (MCSS) transmission methods based on orthogonal frequency division multiplexing (OFDM), also called multi-carrier code division multiplexing (MC-CDM). In particular we compare Fourier spreading with the commonly used Walsh-Hadamard spreading. As a basis for the comparison we describe a general model. Both up- and downlink of multiple access transmission schemes employing either multi-carrier code division multiple access (MC-CDMA) or multi-carrier frequency division multiple access (MC-FDMA) are compared.