How close is the all-optical transceiver for software-defined radio?
Software-defined radio (SDR) is an emerging technology that offers various advantages over conventional radio designs. The SDR approach uses a common hardware platform, programmed by software modules and firmware, to support many communications standards and operating frequencies. The technology is being promoted by the US Department of Defense to replace tens of thousands of single-protocol, single-purpose radios with a common platform. Military and public safety organisations worldwide are also considering this technology to solve interoperability problems enabling, for example, police and firefighters to easily communicate in the field. In addition, the commercial wireless industry views SDR as a means of reducing development cost and time to market.
To fully realize SDR's promise of an all-encompassing communications device capable of transmitting and receiving over a very wide frequency range requires special attention to the radio frequency (RF) transceiver. Although it is relatively easy to use conventional electronics to build SDRs operating at up to 6GHz, it is increasingly difficult to do so at higher operating frequencies. It needs a hardware platform capable of operating up to about 60GHz to take full advantage of the SDR concept.
Ideally, the SDR hardware platform has a continuously-tunable source to act as a local oscillator. It should be possible to modulate this source in any format, perform signal conditioning, and distribute the signal: either by wireless transmission or over an optical network. The prospect of using conventional electronics to develop a commercially-viable source with these properties is limited, so it may be necessary to find another approach.
Optical technology can emulate many of the functions normally associated with RF reception and transmission. For example, it is possible to generate microwave signals optically over a very large frequency range—from MHz to THz—using heterodyne techniques such as optical injection locking,1 optical phase-locked loops,2 and nonlinear up-conversion.3 The electrical RF signal is generated by shining two optical inputs, separated by the desired microwave frequency, on to a square-law photodetector that acts as a mixer and generates their beat frequency.
In general, when no phase or frequency control is applied, the phase noise of the microwave signal is approximately equal to the sum of the optical linewidths. The key to reducing this typically-MHz linewidth down to values that are more practical for a communications system is to ensure that the phases of the optical signals are highly correlated. If they are, a very narrow RF linewidth can be achieved, even from large-linewidth optical sources.3
A carrier is of limited use in a communication system without some form of encoded information. A laser source can be modulated by inserting a baseband optical modulator in the optical path, or by other means.4The modulation formats are varied and may act on the frequency, amplitude, or phase of the signal. In general, the bandwidth of the modulator/demodulator is dictated by the format of the protocol or the radio-spectrum channel allocation and is relatively modest, since the high-frequency carrier is already optically generated. This means that digital demodulation can be done directly by the computer, after optical down-conversion, to make it easier to interface with the SDR software.
Both the receiver and transmitter need RF filtering for signal conditioning. Techniques for electrical filtering in the optical domain have been examined extensively, but the all-optical microwave filters proposed to date are primarily based on incoherent manipulation of optical carriers with only positive taps.5This approach can only create low-pass filters. Many applications, including SDR systems, need bandpass and flat-top filters as well. To overcome this limitation, Yao et. al.5 recently reported a way to build an all-optical microwave bandpass filter with a simple structure.
The large frequency-tuning capability, immunity to electromagnetic interference, and low-loss distribution potential of optical systems, offer many benefits for the development of SDRs. But photonics will only be taken up in the development of SDR technology when the circuitry can be highly integrated. The various RF elements described here have all been developed and optimized using discrete off-the-shelf components. Work has now started on integrating photonic and microwave functions in a single circuit. As this integration progresses, photonic technology will be more widely used in everyday applications. However, challenges such as antenna design remain.
