Although DSP speed is improving every year, single-chip performance is still a limiting factor for SDR applications, which–remember–must execute 100 billion instructions per second if they are to take on I-F circuitry functions. One path to high speed would be to optimize large arrays of DSPs for parallel processing, but the increases in size, weight, power consumption, and price would be unacceptable.
The best way to build an SDR is with a programmable DSP, claims Panos Papamichalis, director of the Imaging and Audio Laboratory in the DSP R&D Center at Texas Instruments, and also 2000-2001 President of the IEEE Signal Processing Society. But given the speed and power constraints of today’s DSPs, an ASIC-based solution will be a more realistic approach in the short term, he says. Later, a mixture of DSPs and ASICs (probably in the form of one or more ASIC coprocessors integrated onto a DSP chip) will take over. According to Papamichalis, the fastest processors currently available from his company are in the C6000 family, which offers speeds of up to 600 MHz and an execution rate of eight instructions per cycle (or 4800 million instructions per second).
Fast as they are, those devices, as we have seen, are too slow by a factor of about 20 to implement even I-F functions, not to mention RF. In practice, slower DSPs are favored today for telecommunications because of their lower cost and power consumption. The relatively low power requirement of TI’s C5000 family makes up for speeds of up to 300 MHz and the only 600 million instructions executed per second.
Dedicated filters
Because filters are critical to the performance of an SDR, gains can be achieved through the use of dedicated digital filtering chips. These devices can perform the filtering function at a small fraction of the complexity and cost of a programmable DSP. True, they are not programmable, but since the frequency bands in cellphone standards are fixed, they are nevertheless useful in that application.
Filters affect an SDR’s signal-processing speed, sensitivity, dynamic range, and ability to avoid interference. Their importance is reflected in their physical presence–filters constitute a third of the volume of a conventional dual-mode cellphone, being used in all three sections: RF, I-F, and baseband. Existing SDRs do not eliminate analog filters altogether. In fact, because the systems operate over wide frequency ranges, they require filters made of new combinations of materials with electrical properties far beyond those of conventional inductors and capacitors. The novel materials and modern filter fabrication techniques will lead to smaller, yet more flexible and adaptive wideband filters. In this area, superconducting and microelectromechanical devices (MEMS) may play important roles.
Regarding output amplifiers, silicon is the material of choice for moderate-performance RF amplifiers in the cellular and PCS bands, all of which fall below 2 GHz. A new technology involving silicon-germanium transistors promises an extremely low-cost, low-voltage approach for analog RF power amplifiers as well as receiver front-ends operating well into the millimeter-wave region (up to at least 40 GHz). Another development in RF technology is the design of ultralinear power amplifiers to process multiband signals from multiple transmitters simultaneously and add them coherently with high fidelity. SDR designs based on these amplifiers will improve power efficiency and consume less space than traditional amplifiers. But such multiband amplifiers may still be very expensive to produce.
Adapting to change
So far we have stressed the value of embodying radio functionality in software as a means of protecting cellphone subscribers and network operators against unpredictable changes in the technological landscape. The concern is with insurance against obsolescence, if you will.
But SDR can do more. It can make all kinds of radios, not just cellphones, perform better by helping them adapt in real time to the rapidly changing characteristics of the wireless environment, which is an especially severe problem for mobile users. Specifically, software can add new functions to a radio without affecting its original functions or correctness. For example, it can implement RF power control to make a radio work better in a fringe reception area, or it may include additional code to mitigate interference in a congested radio environment. Conversely, specific functions and unnecessary code can be removed from existing transceivers to make them more efficient without affecting the remaining functions.
A major drawback of such adaptivity is that it typically introduces extra latency, or delay, into transmissions, since it takes some finite time to recognize and possibly react to a changed link characteristic. That latency can affect delay-sensitive traffic like voice and video.
Because they can monitor, identify, and make use of unused or underutilized frequency channels, multiband SDRs can help network operators use bandwidth more efficiently. For example, if the paging channel is not being used, these radios can use it to transmit other kinds of information like user data.