As processors optimized for high-speed arithmetic, DSPs are technically capable of doing the job on their own; but in today’s marketplace, it is more economical for them to share the work with faster or more flexible chip types: application-specific integrated circuits (ASICs), which use hard-coded logic to perform the arithmetic; and field-programmable gate arrays (FPGAs), whose programmable interconnect and logic functions can be redefined after manufacture. Deciding how best to partition the signal-processing functions of an SDR among these devices is one of the challenges with which cellphone designers must deal.
Although ASICs provide better performance at lower cost, their programmability declines as their level of integration increases. Consequently, radios handling multiple radio interface standards often need multiple ASIC devices. In contrast, several interface standards can be easily integrated into a single DSP or FPGA with no loss of flexibility worth noting.
Architectural blueprint
SDR transceivers implement many functions by running software on general-purpose hardware. The analog hardware for functions like frequency tuning, filtering, modulation, and demodulation is replaced by software that implements those functions digitally. Such an arrangement enables a single radio to reprogram its mixers and filters to handle multiple modulation schemes and to work across many frequency bands.
The conventional dual-mode cellphone is a typical second-generation device. In North America, it would likely work on two kinds of networks–one based on the old AMPS (analog) standard, and one based on the European digital standard known as GSM. To do that, the phone has two separate transmitters and two separate receivers.
In analog mode, outgoing signals emerge from the (analog) signal processor and are fed to a chain of functional blocks of the same general design as in most radio and TV stations–the venerable superheterodyne architecture. From the signal processor, the signal is modulated onto a carrier, translated up to an intermediate frequency (I-F), then up to a higher radio frequency (RF), and finally amplified and sent to the antenna.
In a reverse process, the analog receiver downconverts the received analog signal in two stages, selects the channel assigned to its particular conversation by means of the analog filter, and then passes it on to the signal processor for demodulation.
The digital transceiver is similar except that operations on the received signal are carried out in a DSP instead of in single-purpose analog circuitry. These operations may include decompression and even decryption as well as filtering.
In essence, the dual-mode cellphone employs configurable functions with multiple firmware cores that are activated and deactivated as needed. SDR offers a more elegant approach, using programmable DSPs that first download and then run the functions needed to implement a particular standard.
The first step in transforming a conventional cellphone into an SDR system is to make as much of the circuitry digital as possible. To start with, this means eliminating the baseband analog operations. These are carried out on the input (usually voice) signal while it is still occupying its native region of the spectrum and before it modulates a carrier and is thereby translated up to a higher frequency band.
On the transmit side, this means digitizing the input voice as close to the microphone as possible so that all subsequent signal processing (compression, filtering, modulation) can be done digitally. Now the processing can be made programmable. (Of course, as the diagrams show, at some point the digital signal must be converted back into analog form for transmission, preferably as close to the antenna as possible.)
Similar reasoning holds on the receive side. The goal there is to convert the incoming analog RF signal into digital form as close to the antenna as possible, to process it digitally in a programmable device or devices, and then to convert it back into analog form as close as possible to the earpiece [not shown].