Optical free space feedforward non-linearity correction system
Recent years have seen unprecedented growth in the popularity and deployment of mobile phones. As this continues, so the strain on existing mobile cellular radio network has also increased, leading to the need to investigate new technologies to relieve this pressure. The problem is being further exacerbated by the introduction of the 3rd generation of mobile communications, otherwise known as UMTS (Universal Mobile Telecommunications System), with the aim of offering multimedia services on pocket sized portable receivers. A major cost of the mobile radio network, in terms of both financial and social/environmental aspects, is the need apparent need for more base transceiver stations (BTS), due to the increased number of services, and the density of them. Therefore, judicious use of fewer, but more "intelligent" base stations, thereby reducing the overall system costs, and extra flexibility in the design of mobile cells would be desirable. This can be achieved by having the BTS antennas remotely positioned from the BTS by transmitting the radio signals down an optical fibre or, as in this project, over free space. The main application for this is in densely urban heavy use areas, where there is extensive reuse of both cell and cell cluster. This, along with building shadowing, would require a BTS on every corner, and where extra cell design flexibility would be desirable. Also, in remote rural areas, where various natural features, such as rivers or mountains can cause similar cell design problems, there is a need for this flexibility. The problem with this requirement is that the electrical to optical conversion process, involving a laser diode driver unit, is inherently non-linear, and, unless this is resolved, the desired signal will become unusable due to distortion. To overcome these nonlinearities, a novel correction may be used, based on an optical feedforward correction technique. The prototype system employs off-the-shelf components, and has one Fabry Perot laser diode (FP-LD) providing two signals (via a beam splitter), for a main path and one for the error path loop. The error path signal is detected by a receiver circuit, then mixed with a reference signal to produce a 'pure' error signal, which then modulates the second FP-LD. In contrast with previous fibre feedforward systems, where the two LD outputs are then combined in the optical fibre pre-reception, this system has to combine the signals post-reception. After the main signal and error signal are received and recombined, the non-linearities of the main path are predominantly cancelled by those present in the error path signal, leaving only the desired signal, free of non-linearities.