By capturing the imagination of home, enterprise, and mobile users, wireless-local-area-network (WLAN) technologies have attracted both chip and equipment companies. The rapid and unrelenting evolution of WLAN technology has strained the standards-setting bodies and interoperability forums. Product OEM designers and their semiconductor-design counterparts also are struggling to deal with this market's pressures.

Within the past few years, IEEE 802.11-based products have proven to be low-cost, easy-to-use devices for enabling wireless-Ethernet and -Internet connectivity. The 11-Mbps 802.11b network is now available in a variety of form factors to suit computers, PDAs, cell phones, and gaming devices.

For enterprise applications, interest in higher-performance WLANs originally led to the 802.11a standard. This standard transmits data at up to 54 Mbps. However, its effective range is less than the range of 802.11b networks. The 802.11a networks also have met with muted market acceptance because they use the 5.2-GHz spectrum. This spectrum is incompatible with the 2.4-GHz spectrum that's used by the many previously installed 802.11b devices. With an 802.11a-only network card, users can't readily communicate with the already deployed 802.11b access points.

The latest WLAN technology to emerge is 802.11g. The 802.11g standard provides several important improvements for WLAN users. For instance, it includes the faster data rates and robust orthogonal frequency division multiplexing (OFDM) of 802.11a. But it operates in the same 2.4-GHz unlicensed ISM frequency band as 802.11b products. Obviously, 802.11g makes backward compatibility possible with 802.11b-based products. The radios operate in the same frequency spectrum. In addition, such compliance is mandatory according to the 802.11g standard that was established by the IEEE this past summer.

For OEMs and ODMs, the existence of 802.11 in a, b, and g versions leads to a plethora of new-product alternatives. This trend is reinforced by their customers' reluctance to discard recently deployed systems. Potential products include g-only, dual-mode (a + b), and multimode devices (a + b + g). Each of these approaches has its own cost, performance, and time-to-market tradeoffs.

The multimode products are the likely long-term market winners. They'll provide the best overall user experience and performance. For example, they'll offer seamless roaming. Such roaming will be supported by the dynamic selection of 802.11a, b, or g, depending on the system capabilities, channel loads, and the make-up of information being exchanged by the user. Multimode products enable customers to take advantage of the larger coverage area that's offered by 802.11g and the higher user density supported by 802.11a. They also support a gradual meshing of enterprise end points, thereby enabling service at 2.4 and 5.2 GHz.

To productize these new technologies, semiconductor solution providers are following varied paths. Radio system architectures continue to play a pivotal role in determining these solutions' overall cost, performance, robustness, size, and power-consumption characteristics. Meanwhile, semiconductor companies are struggling to deliver the best possible technology to wireless-system integrators. But those integrators are reluctant to pay a premium for multimode solutions. The only way to ensure success is to identify and address the architectural design issues that span the radio-frequency IC (RFIC), baseband digital-signal-processor (DSP), and media-access-controller (MAC) functional blocks. Together, those blocks collectively comprise a WLAN chip set.

When designing multimode 802.11 solutions, it's no longer viable to independently optimize the RF and DSP sub-blocks. In older wireless systems, the data rates were low enough that second-order RF impairments didn't require compensation. But today's radio systems use very dense I/Q constellations to achieve the required high bit rates. The employed radio architectures therefore need to deliver signals with higher signal-to-noise ratios (SNRs) to the data detector. Using active-distortion-cancellation techniques, the RF + DSP subsystem can meet these stringent requirements for throughput and range.

The performance that is demanded by these new bandwidths and modulation schemes is putting quite a squeeze on radio designs. For proof of this statement, look at the evolution of WLANs. In the early days, WLANs migrated from 1 to 2 Mbps to 5.5 to 11 Mbps. In the process, the RFIC architectures evolved from discrete components to a combination of discrete and integrated circuits. They could then support zero-intermediate-frequency (ZIF), or direct-conversion, radios.

Yet constellation sizes largely remained the same: BPSK (1 bps/Hz) and QPSK (2 bps/Hz). Instead of the earlier direct sequence spread spectrum (DSSS) modulation, the design challenges on the DSP side grew to include support for complementary code keying (CCK) modulation. To reach 54-Mbps data rates, OFDM was introduced. It uses 64-QAM constellations (6 bps/Hz). This evolution split the design community into two camps. One camp supported ZIF as the optimal radio architecture, while the other favored very-low IF (VLIF).

The radio subsystem's SNR is a crucial design constraint because of the combination of the bit rate and the dense constellation of current Wi-Fi radios. The final SNR that's seen by the data detector is determined by:

• The aggregated noise that's injected at the transmitter-thermal noise, phase noise, 1/f noise, quantization noise, and local-oscillator (LO) leakage
• The Rayleigh fading and path loss that occur during transmission through the wireless medium
• The noise injected at the receiver (thermal noise, phase noise, 1/f noise, etc.) combined with distortions due to frequency and DC offsets and I/Q imbalance