Considerations on the design of transceivers for wireless optical LANs

Abstract

This paper discusses practical issues and results on the implementation of low-cost transceivers for several wireless optical LANs, covering most physical layer elements. Target optical LANs under consideration have bit-rates varying between 1Mbps and 25Mbps, and input photodiode capacitance may vary between 10pF and 50pF. PPM modulation, either in a 4-PPM or 16-PPM format, is typically used in the physical layer. Differential front-end topologies are presented, both in discrete and integrated (CMOS) implementations, targeting different LANs. Strategies for bandwidth improvement, interference reduction and dynamic range improvement have been used in some of these topologies. These strategies lead to a switched-gain transceiver with a transimpedance gain*bandwidth of 25THzΩ, achieved with a 10pF capacitance photodiode in the input. Both non-sectored and sectored transceivers are discussed. The increased complexity brought about by sectored receivers will be made clear as the design of a signal-to-noise ratio estimator and constraints on power consumption of the basic front-end design are discussed. Considerations on practical usage of MAP detection confronted with adaptive threshold detection are further presented. Clock recovery issues are also mentioned. Final comments on practical issues and evolution of these low-cost systems are presented.