Converging economic and technology drivers are influencing which architectures and components are used in the design of radio products for wireless basestations. Operators in new, growing markets such as Latin America and Asia, especially China, are depending on cost reductions in the existing digital cellular infrastructure to make services economically viable, while capacity limitations, especially in Japan,and wideband data services are fueling the drive toward third-generation (3G) deployment.
In some markets, 3G will be the only cellular service, bypassing current infrastructure architectures entirely. This combination of demand for lower-cost 2G systems and the initial deployment of 3G systems force infrastructure manufacturers to be highly efficient with their development resources.
Air interfaces dictate architecture and component requirements, especially for data converters. Specific air-interface standards such as GSM and IS95 determine the performance requirements of any given transceiver architecture. Adjacent channel-blocking specifications, minimum sensitivity and operator spectrum allocations are among the concerns that the radio architect must consider when specifying the radio design and data-converter requirements. Viewed from "inside the converter," a radio can be classified by where the channel filtering is performed. In general, converters can be considered to be part of either a single carrier or a multicarrier design.
Typically, the out-of-band interference comes from other operators or services occupying RF spectrum outside the operator's band of interest. Those signals are usually filtered with preselection filters at the RF input frequency or a wideband filter operating at the first intermediate frequency (IF). When the entire operator's band is to be digitized, that is considered a "multicarrier architecture"-because the interference or unwanted signal is digitized along with the desired signal. More often, only the signal of interest is digitized after it is tuned to a lower IF and passed through a narrowband filter-thus a "single-carrier architecture."
Not long ago, system designers and converter manufacturers commonly used the term "wideband radio" to refer to multicarrier receiver or transmitter architectures where the converter was processing multiple carriers simultaneously. Today, 3G discussions are centering on wideband CDMA (W-CDMA) and refer to a single RF carrier-albeit one that may be 5 MHz wide.
Accordingly, ADI has adopted the term "multicarrier" when referring to multiple-carrier radio-digitizing architectures, to avoid confusion with W-CDMA design work.
For single-carrier radios, a typical receiver design may consist of two or three down conversions to provide the sensitivity and selectivity required. With each down conversion, a local oscillator, mixer and filter are required. Here, the RF carrier frequency is subtracted by mixing the captured signal with a synthesized RF carrier, 180 degrees out of phase with the original. Each additional analog down-conversion stage adds complexity, cost and manufacturing difficulties. From the converter's perspective, the required resolution and encode rate of the A/Ds is determined by the dynamic range and bandwidth of a single carrier.
Traditionally, a pair of A/Ds digitize the baseband in-phase and quadrature (I&Q) components following the quadrature demodulation performed at the final IF frequency-10 MHz to 250 MHz. For IS95 (CDMA) and proposed 3G CDMA standards, the sample rate of the dual ADCs is usually set to four times the chipping rate, 4 to 40 Msamples/second. Analog automatic gain control (AGC) and channel filtering have limited the resolution required to less than 8 bits. GSM and D-AMPS channel signals, 200 kHz and 30 kHz respectively, are an order of magnitude lower in bandwidth than CDMA, at 1.25 MHz. Sigma-Delta A/Ds are typically employed with sample rates less than 20 Msamples/s. Additional resolution, 10 to 14 bits, is very affordable at those encode rates and permits additional filtering to be performed in the digital domain.
Classic analog demodulators depend on a high degree of matching between the final mixer, baseband filters and A/Ds to preserve quadrature accuracy. As signal bandwidths increase (CDMA), matching becomes the predominant concern as analog filters become large and expensive.
In the transmitter, dual D/As are also critical. They create the I&Q waveform that drives analog filters and modulators that are upconverted to the final RF frequency for amplification. Transmitted CDMA signals are the linear sum of many voice/data channels sharing the same carrier frequency. Enormous demands are placed on the analog modulators, mixers and amplifiers that must maintain signal fidelity through the entire transmit stage while meeting regulatory requirements for spurious signals.
More processing can be performed in the digital domain with high-speed data converters. Faster A/Ds allow higher analog input frequencies to be accurately digitized. This permits direct IF sampling (DIFS), eliminating the matching problems, numerous components and at least one IF stage. Digital Receive Signal Processors (RSPs), like the AD6620, follow the A/D and shift the burden of quadrature demodulation, tuning and much of the channel filtering to the digital domain-where the results are more repeatable and accurate.
Similarly, high-speed D/As allow direct digital synthesis (DDS) of a modulated IF, eliminating matching problems on the transmit side. DDS is useful in single-carrier applications where analog matching is a problem but typically requires an additional IF stage. In multicarrier architectures, significant cost savings can result.
Essentially, converters for those digital radio applications require specifications beyond just sample rate and resolution. Signal-to-noise ratio (S/N) or the ratio of signal power to noise power is very useful in computing the sensitivity of a receiver. Spurious Free Dynamic Range (SFDR) is a measure of the linearity of the converter. To be useful in radio IF design, both the S/N and SFDR must be specified vs. input frequency.
In single-carrier IF sampling applications where analog AGC is used, the required resolution may be as little as 6 bits at the IF of interest. Typically, it is between 10 and 250 MHz. But the availability of low-cost 10- to 12-bit A/Ds with good S/N and SFDR performance vs. frequency has allowed more of the AGC and channel-filtering functions to be absorbed in the digital RSP. Additionally, high oversampling ratios ease the design of the anti-aliasing filter required in front of the A/D.
Dynamic performance of the A/D is critical at the IF of interest. The AD9226, AD9432 and AD6640 are all 12-bit devices with sample rates greater than 65 Msamples/s and offer increasing S/N and SFDR as a result of the architecture and process technology used (CMOS, BiCMOS, Bipolar, respectively). The AD6600 is an application-specific A/D that includes diversity inputs, AGC and has been optimized for inputs up to 250 MHz.
As dynamic range and sample rates increase, it is reasonable to consider digitizing the entire operator's spectrum simultaneously. Imagine that the basestation has to process 20 carriers. Multiply this by three sectors and again by two for diversity and the cost advantages of software-defined radio- capable of deciphering multicarriers per antenna-become obvious. This architecture eliminates redundant radios in favor of a single high-dynamic-range front end. As digital signal processors become smaller and faster, the incremental cost of adding additional channels behind the A/D is relatively small.
The true software-defined radio can operate in multiple modes-mixed air interfaces-and is reprogrammable for bandwidth, modulation and tuning with only software changes. In reality, the state of the art today is such that the optimum hardware required in the digital receive signal processors varies significantly between the narrowband standards (DAMPS, GSM) and wideband standards such as CDMA. A common analog radio design, up to the A/D, offers significant cost savings, development synergy and size reductions even if the DSP must be altered between air interfaces.
There are additional performance challenges to consider. The adjacent channel and blocking specifications between different air-interface standards can exceed 110 dB of SFDR with 900-MHz GSM being the most demanding. This is known as the "near-far" phenomenon that the basestation receiver must deal with.
In the transmitter, the D/A is also critical for multicarrier DDS. However, the biggest challenge today is the availability of high-power multicarrier power amplifiers (MCPA) with sufficient dynamic range. The linearity requirements of CDMA also present similar challenges, and many MCPA manufacturers are working to meet them.
ADI recently announced the development of the "SoftCell" chip set to enable the next generation of software-defined basestations. SoftCell consists of the latest in DIFS and DDS technology to reduce the cost, size and power of existing multichannel radios. SoftCell is more than a chip set; it represents a new platform for infrastructure design that will enable a new generation of Micro/Pico cell transceivers, in-building wireless basestations, wireless local-loop and phased-array antenna systems. At the heart of RSP is the AD6644 14-bit 80 MSPS A/D. With an S/N of 75 dB and multitone SFDR of 100 dB, weak signals can be captured even in the presence of strong interferers. The AD6624 is a four -channel digital RSP capable of independently tuning and filtering four unique narrowband carriers (GSM, DAMPS, etc.). Non-integer decimation rates allow multiple air interfaces to be supported.
For the transmitter, the AD9754 is the latest in a progression of TxDACs optimized for high dynamic range in a multicarrier environment. Multiple RF carriers can be generated at a low IF frequency and upconverted as a band of carriers to the final RF frequency. The AD6622 is a quad digital transmit-signal processor that can independently filter, tune and combine four carriers per chip (chips can be daisy-chained over the 18-bit parallel bus). Linear I&Q inputs can be FIR filtered using customer-provided coefficients or raw I&Q data streams can be pulse-shaped on board using proprietary pulse-shaping techniques for major air-interface standards.
In transmitters, the AD6622 can be configured to support IS95, including transmit predistortion phase equalizers and 5-MHz W-CDMA signals. Digital RSPs for 1.25- to 5-MHz carriers are planned as part of the SoftCell series.
There seems to be a gathering consensus that the next-generation cellular air interface will utilize some form of CDMA occupying approximately a 5-MHz bandwidth. Decisions regarding precise chipping rates, blocking specifications and especially allocated operator spectrum could take years to finalize.Uncertainty and evolving standards favor the flexibility of a software-defined radio. The SoftCell platform offers a flexible environment for 3G developments, including IS95 and wideband CDMA air-interface standards.