Soft details define wireless basestations

Soft details define wireless basestations
Wireless telephone system operators are under continuing pressure to improve their infrastructure by changing the basestation operations. It may be necessary to repair errors, to improve system operations or plug some security deficiency that has been detected. Occasionally, it is a large-scale change to upgrade to a new air interface format. Software-defined radio (SDR) technology facilitates all of these changes by using general-purpose hardware with detailed functionality determined by software.

RF front-end performance has long been a limiting factor on basestation efficiency, configuration flexibility and ability to take full advantage of the benefits of SDR technology. The cost of this equipment has also been a major factor in the capital expense required to field improvements for extended coverage and support of new applications requiring digital data transfer.

System security is also a significant wireless system consideration from a number of perspectives. After a major first-generation problem with theft of services, operators demand that all of the service supplied be accounted for to ensure that revenues are fully protected. They also want assurance that all the terminals accessing the network are behaving properly and following the rules, such as power control, for system capacity optimization. System subscribers require that their information is secure from any attempt to intercept or divert it. Regulators want assurance that all radio users are operating in conformance with the regulations. They are particularly concerned that newly downloaded software does not introduce problems.

Hypres Inc. (Elmsford, N.Y.) is developing products to respond to the need for improved basestation and system capability with a dramatic improvement in front-end performance, using superconducting microelectronic (SME) technology. Digital Real-time Traffic Flow Management (RTFM) logic using SME promises to bring major performance improvements to wireless infrastructure.

The accompanying illustration shows the approach to a basestation front end using digital RTFM. On the receive path, the signal comes from the antenna system directly to the high-performance analog-to-digital converter. This device delivers a 10- to 12-bit resolution at 100 Msamples per second with a 100 dB spur-free dynamic range to the correlation-based receiver. Here the circuits develop a correlation between the known waveform and the digital representation of the received signal. Correlation is a well-known technique, and frequently used in applications where the need for minutes of processing time is not critical. With the very high clock rates used in these circuits it is possible to iterate the correlation enough times in a few microseconds to maintain the power level of the signal of interest, but substantially reduce the power of noise and interfering signals. This process provides an effective reduction of the signal-to-noise ratio, resulting in very high receiver sensitivity.

Good receiver sensitivity offers a number of benefits to the network operator. It permits wider cell-site spacing, reducing capital expenditure. It also enables handsets to operate at lower power levels, reducing the noise floor, increasing channel capacity and requiring fewer hand-offs.

On the transmit path, the digital signals representing all of the channels to be transmitted are combined, and converted to an analog exciter signal. The high-precision DAC ensures that there will be a clean RF waveform. The signal is then amplified to the desired transmit level by a power amplifier (PA).

In a conventional basestation a very linear, and hence expensive, PA is required to avoid introducing spurious signals. With the performance of digital RTFM implemented in SME, it is possible to introduce a predistortion feedback loop. The signal spectrum generated by the PA is analyzed, and an inverted correction generated. By feeding that correction back into the transmitter input, it is possible to compensate for PA errors, and operate on a less linear portion of the amplifier's output curve.

As the PAs constitute a major cost element in a basestation, obtaining equivalent performance with less expensive units is a major benefit. This capability opens a new trade space for wireless system designers.

Because the superconductive phenomenon takes place only at very low temperatures, use of SME technology requires a cryocooler, restricting its use to applications where its 100- to 150-W power draw is acceptable, such as basestations. Recent advances in cryocoolers have made very small units available, and extremely high reliabilities have been demonstrated over a 15-year product life.

Because the cryocooler is inherently a physically closed environment, it has interesting possibilities as location for a security module. Functionally adjacent to the receiver and transmit functions, this position has some interesting possibilities for system security.

The security module can control frequency hopping without the phase delays inherent in putting that function further back in the processing sequence. By installing cryptographic keys in the unit during manufacturing, a remote controlling site can secure interaction with the security module. It can cut off transmissions or issue other commands to the transmitter. It can receive information about ambient signals from the receiver, and perform a monitoring function. A number of novel system capabilities are currently under study.

Hypres is currently focusing on development of RF facilities for military applications, but there is little inherent difference between those and commercial basestations. This may be a win-win situation, as commercial sales will not have to bear the burden of development cost, while the military benefits from the economies of scale available with the much larger production runs associated with commercial apparatus. Hypres anticipates that product evaluation for wireless infrastructure application will take place in 2004.

Peter G. Cook is president of PGCC Inc. (Mesa, Ariz.).P> See related chart

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