A SAW-based commutation signaling modem for broadband indoor wireless communication

Commutation signaling is a bandwidth expanding modulation scheme that is robust to multipath induced intersymbol interference making it suitable for wireless digital communications. By using multipath diversity combining, commutation signaling exploits the time diversity that is inherent in a multipath propagation environment. This paper considers a surface acoustic wave (SAW) implementation of a commutation signaling modem for broadband indoor wireless communication. The modem employs differential encoding and a form of direct sequence spread spectrum modulation with the following specifications: data rate 40 Mb/sec, chip rate 200 MHz, and IF frequency 1 GHz. The differential coherent detector is a key element of a low cost, low complexity commutation signaling modem. A commutation signaling differential coherent detector has been implemented using SAW and RF integrated circuit (RF IC) technologies. The SAW devices have been fabricated on 128 degrees-rotated Y-cut, X-propagating lithium niobate using approximately 1 micron line widths. RF IC technology is used to implement the high-speed bilinear multipliers needed for differential coherent detection as well as the low-impedance buffers used to drive these multipliers.