Time-reversal signal processing can boost wireless comms

Abstract

One way to increase the data rate of a communications system is to use multi-antenna arrays at both transmitter and receiver. Such methods, known as MIMO (multiple input–multiple output), enable different bitstreams sent from the transmitting array to be decoded at the receiving end, provided the medium creates sufficient scattering.1 It is commonly thought that spacing between the receiving antennas must be larger than λ/2 because the diffraction limit prevents two antennas from resolving two independent bitstreams if they are less than a half-wavelength apart. With these two conditions fulfilled, the global maximum error-free data rate, or ‘Shannon capacity,’ is multiplied by the number of transmitting antennas. However, from a practical perspective, a large number of antennas make the λ/2-spacing requirement difficult to satisfy. This situation typically arises with antennas in laptops and telecommunications wavelengths on the centimeter scale (e.g., Bluetooth or Wi-Fi). We propose a solution to overcome the diffraction limit and thus increase the number of receiving antennas that can be set out in a small region of space. Our approach relies on a MIMO communications scheme using an eight-antenna time-reversal mirror (TRM)2 as the transmitter and an eight-microstructured antenna array as the receiver (Figure 1). Both transmitter and receiver arrays operate at 2.45GHz (λ=12cm)with a 150MHz bandwidth. The distance between two adjacent receiving antennas is λ/30. In our procedure, the TRM is used as follows. One antenna in the receiving array transmits a 10ns microwave training pulse that propagates in a chamber where it undergoes strong reverberation (Figure 1A). The resulting signals are recorded at each antenna of the TRM, flipped in time, and used to precode the transmitted symbols. When the resulting sequences are sent back by the TRM, the time-reversed wave retraces its former Figure 1. Experimental setup. (A) A time-reversal mirror comprised of eight commercial dipolar antennas and a subwavelength receiving array consisting of eight microstructured antennas are housed within a 1m3 reverberating chamber. (B) Details of one antenna show the core of a coaxial line extruding 2mm from an insulating layer, and a microstructure consisting of a random distribution of thin copper wires. (C) Eight-element subwavelength array surrounded by copper wires.