We are experiencing continuing and substantial growth in the IEEE 802.11 WLAN marketplace. Much of this growth is being fueled by the introduction of dual-band radios. These radios are capable of operating under the IEEE 802.11a protocol at 5 GHz and the IEEE 802.11b and (proposed) 802.11g protocols at 2.45 GHz. They can operate at data rates that range from the legacy 1 and 2 Mbps to the state-of-the-art 54 Mbps. As a result, dual-band radios are capable of ubiquitously communicating with all known 802.11 access points.

Though this capability should make dual-band radios attractive to designers, their optimization, testing, and debugging poses some unique problems. For example, the high data rates use complex 64-QAM modulation, which requires special test equipment and techniques for troubleshooting. This modulation requires a high degree of linearity in both the receiver and transmitter in order to prevent excessive error vector magnitude (EVM). The front-end circuitry also is a factor. It now contains separate 2-GHz and 5-GHz RF signal paths in addition to common paths. Because of the large assortment of possible data rates, unique issues emerge concerning the minimization of test time in high-volume applications.

A dual-band 802.11a/802.11g radio has nine unique modulation modes. Each of these modulation modes requires a different closed-loop power-control setting for optimum performance:

• 2-GHz CCK modes (BPSK or QPSK)
• 2-GHz 6- and 9-Mbps OFDM (BPSK)
• 2-GHz 12- and 18-Mbps OFDM (QPSK)
• 2-GHz 24- and 36-Mbps OFDM (16 QAM)
• 2-GHz 48- and 54-Mbps OFDM (64 QAM)
• 5-GHz 6- and 9-Mbps OFDM (BPSK)
• 5-GHz 12- and 18-Mbps OFDM (QPSK)
• 5-GHz 24- and 36-Mbps OFDM (16 QAM)
• 5-GHz 48- and 54-Mbps OFDM (64 QAM)

In addition, these settings can vary somewhat over the frequency band. It is therefore prudent to provide the capability of storing this data for multiple channels. Practically, it is usually adequate to store this data at three channels: one at the low end of the band, one in the center, and one at the high end. The designer can then use a best-fit curve to interpolate the data for other channels.

Fortunately, the 802.11b CCK modes are a bit less complicated to test than the OFDM modes. Taking into account this consideration, however, it remains clear that the dual-band network-interface-card (NIC) radio is approximately 2.5 times more complicated to test than a single-band radio. As a result, it is a considerable challenge to design an automatic-test-equipment (ATE) suite that yields reasonable execution time (say 2.5 to 3 min. per radio).

Another considerable challenge is the measurement of error vector magnitude. Take the case of the 54-Mbps data rate with 64-QAM modulation. In this instance, the 802.11 specifications require that a minimum EVM level of −25 dB be maintained in the transmitter. This requirement is quite an important one. In a typical system, documented evidence shows some degradation of receiver sensitivity if this transmitter EVM level is not maintained. This therefore becomes a potential interoperability issue. In order to avoid system-range issues, strict adherence to this EVM specification is mandatory.

Competent laboratory-grade vector signal analyzers (VSAs) are available that can measure EVM. But they are high-ticket items costing in excess of $80,000. While this cost is perhaps acceptable in an engineering-laboratory situation, it is certainly not desirable in a production test environment. Here, more than 20 test stations may be needed to ship the necessary volume of NICs.

Fortunately, the major test-equipment suppliers and the NIC manufacturers are working to address this issue through an ongoing dialog. They are being prompted by the already large volume of 802.11x NICs currently being manufactured, as well as those planned for the near future. Hopefully, a breakthrough in the price point will emerge in the medium term. Certainly, complex modulation formats such as 64-QAM are here to stay. In fact, they are destined to grow in usage.

Until these production-test issues are resolved, however, engineers will have to take innovative approaches. For example, it is possible to avoid testing a large percentage of NICs for EVM in production by first correlating the EVM with the spectral mask. In this method, a reasonably large sample of NICs is tested. The NICs are then used to correlate 54-Mbps EVM readings with the resultant transmitter spectral mask. Using this data, the actual production radios are aligned based on the spectral-mask measurement. Hopefully, a small sample from each lot also will be re-checked for EVM. This step will ensure that the correlation is holding constant and that the variance in these two measurements is low.

The advantage of this approach is that a substantially lower-cost instrument—namely a spectrum analyzer—may be used for high-volume testing. This same instrument is needed anyway. It is already being used for the proper alignment of lower-data-rate constellations, so it incurs no incremental cost.