Equalization for DS-UWB Systems---Part II: 4BOK Modulation

Direct-sequence ultra wideband (DS-UWB) transmission is a strong contender for the physical layer of high data-rate short-range UWB systems. Since long delay spreads in UWB channels cause significant intersymbol interference, DS-UWB systems require equalization. In this second part of two papers, we investigate equalization for DS-UWB with 4-ary biorthogonal keying (4BOK), which is one of the two modulation formats that was proposed for standardization by the IEEE 802.15.3a task group. To this end, we first derive expressions for the bit error rate (BER) according to the matched-filter bound for 4BOK DS-UWB, which serve as theoretical performance limits for equalization. We then devise structures and methods for filter optimization for low-complexity linear and nonlinear equalization schemes. In this context, we develop a new equivalent multiple-input multiple-output (MIMO) description of 4BOK DS-UWB, which facilitates the design of efficient equalizers using MIMO filter optimization techniques. Furthermore, we propose the application of widely linear processing to these equalizers. Simulation and semianalytical results show that MIMO equalization is greatly advantageous over more obvious non-MIMO schemes and that the proposed MIMO equalizers allow for power-efficient 4BOK DS-UWB transmission close to the theoretical limits with moderate computational complexity.