Software-defined radio comes of age

The architecture of a software defined radio can be divided into three distinct elements: a digital signal processing section; a section responsible for the conversion between RF and digital; and the antenna. While the performance of the antenna and RF to digital conversion play a key part in determining the capabilities of an SDR platform, the flexible digital signal processing is what qualifies a radio as being a software defined radio. FPGAs, DSPs, and general purpose processors are the three leading technologies that can provide the flexibility and processing power needed for SDR systems.

The hardware architecture, pictured in Figure 1, groups the hardware components into three blocks representing the antenna, RF-to-digital and processing subsystems. No hardware component in the architecture is specialized to any particular waveform. While the architecture places no limitation on the achievable waveforms, any given implementation of the architecture can only support some waveforms.

Each implementation supports a limited range of RF frequencies, bandwidths, and amount of computational power. For example, in order for a platform to be software upgradeable from 2G to 3G cellular standards, the implementation must be able to receive a 5 MHz wide band in the appropriate frequency ranges and have enough computational power to perform the 3G processing.

The rightmost block in figure 1 represents the antenna subsystem. The interfaces to the antenna block are RF transmit and receive analog lines and a digital control interface. With these interfaces, the architecture can accommodate traditional passive antennas (for which the digital interface has no function) as well as advanced systems such as electrically controllable antenna arrays. The architecture does not specify a particular type of digital connection (e.g. RS-232), as this is a detail of the implementation.

The next block to the left in figure 1, labeled RF-to-digital, is the only layer of the system that contains radio-specific analog components. On the receive side, its sole function is to generate a digitized representation of a downconverted slice of the radio spectrum. On the transmit side, it generates an upconverted radio signal from a digitized representation. This block does not perform waveform specific processing such as demodulation or equalization.

The name of the third block, motherboard, is borrowed from the PC world because software radios look much more like computers than like legacy radios. Like a PC motherboard, this layer contains memory and processor components, and provides I/O to a network, to the user, timing support, and similar functions.

Applications

SDR technology can be applied to wide range of markets. Fundamentally, SDR technology can be used in any device that uses RF for communication, which encompasses a wide range of products including cellular basestations, military communications systems and public safety radios.

Technology in cellular basestations

Cellular standards evolve slowly, from analog in the 1980s to digital in the 1990s, and possibly to 3G sometime this decade. While the underlying processing, communications and DSP technology evolves rapidly, cellular service is limited to once-a-decade upgrades because the high capital costs of infrastructure upgrades are prohibitive.

For example, AT&T and Cingular are upgrading their networks from time-division multiple access to (TDMA) to the global system for mobile communications (GSM). This “upgrade” actually involves building out a new GSM network in parallel to their existing TDMA network, an initiative that costs each carrier upwards of $4 billion and requires a 10-year deployment to achieve a reasonable return on investment.

A wireless network infrastructure using software radio technology can be software upgraded to new standards, thus deploying new standards more quickly and at lower cost than today's approach. Carriers can then increase revenue by rapidly implementing new revenue generating services as well as new systems that use spectrum more efficiently. A further benefit of SDR is reduced operating expenses — many of the maintenance and upgrades today that require truck rolls to tower sites could be serviced as remote software changes in an SDR system.

The architecture for a SDR base station is essentially a basic SDR with an array of processing elements that can be scaled to handle more capacity, or more complex waveforms. Using current x86 general purpose processors as an example, it is now possible to provide one GSM channel with 8 time slots for every 1 GHz of processing. Standard networking equipment such as gigabit Ethernet now has the bandwidth to supply digitized spectrum, and allows the use of standard PC servers with x86 processors to act as the cluster of processing units.

The radio section of a software radio basestation is responsible for converting a wide band of radio spectrum to a digital IF. This equipment is available today in the form of multi-carrier power amplifiers, wide-band upconverters and downconverters, and high-speed A/D and D/A converters. This provides a digital interface that is completely independent of the air standard, and able to support multiple channels of different standards in a band. When coupled with a SDR backend, it is possible to change air standards simply through a software upgrade.