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As the IEEE 802.11ac wireless standard evolves, it continues to breed new challenges for those tasked with testing and measuring these wireless systems. With products based on the IEEE 802.11ac standard continuing to add higher density modulation schemes, wider bandwidths, and more multiple-input, multiple-output (MIMO) antenna configurations, testing these products calls for higher-performance test instruments and measurement strategies.

Starting with their release in 1997, the IEEE 802.11 family of wireless communications standards has evolved to deliver higher data rates: from 2 Mb/s in 1997 to nearly 7 Gb/s at present. In 2012, various IEEE 802.11 amendments were consolidated into a single standard that includes direct-sequence-spread-spectrum (DSSS, IEEE 802.11b), orthogonal-frequency-division-multiplexing (OFDM, IEEE 802.11a/g), and high-throughput (HT, IEEE 802.11n) specifications. This paved the way for the imminent release of the very-high-throughput (VHT) IEEE 802.11ac specification. While the HT specification improved the maximum throughput by an order of magnitude over the previous OFDM specification, the VHT (IEEE 802.11ac) specification boosts the maximum throughput by another order of magnitude. As Table 1 shows, the VHT specification builds on technologies developed for previous IEEE 802.11 substandards.

An important result of the IEEE 802.11-2012 amendment was the change in nomenclature from previous amendments. While the wireless industry has long referred to Wi-Fi technologies by their amendment letter (such as IEEE 802.11a or IEEE 802.1g), these technologies were renamed and referred to as DSSS, OFDM, etc. Since 1997, each new amendment to the 802.11 standard has attempted to increase data throughput compared to previous generations. As was seen with HT, increases in data rate have been accomplished through a variety of mechanisms. These include more spatial streams through the use of MIMO, wider channel bandwidths (and hence more data subcarriers), and even higher code rates. Equation 1 shows show to perform a quick calculation of the data rates of OFDM-based signals:

Data rate = Spatial streams x data carriers x symbol rate x bits per symbol x coding rate x duty cycle            (1)

As noted, the VHT specification produces a maximum data rate roughly an order of magnitude higher than HT and two orders of magnitude higher than OFDM. In fact, the improvements in data rate can be easily understood by comparing key parameters for the specifications (Table 2). For variable parameters, Table 2 lists those yielding maximum throughput. The 160-MHz and 80 + 80-MHz options for VHT share the same number of subcarriers/pilots.

As Table 2 shows, the VHT specification allows for channel bandwidths as wide as 160 MHz, or four times greater than the 40-MHz maximum bandwidth offered by HT (or alternately, eight times greater than the 20-MHz bandwidth provided by OFDM). Wider bandwidths allow a greater number of data subcarriers to be handled. Moreover, the VHT specification uses a slightly higher concentration of data carriers per channel bandwidth versus the HT and OFDM specifications. For example, 91.4% (468/512) of the subcarriers in a 160-MHz VHT channel are data subcarriers, versus 75% (48/64) for OFDM.

Table 2 also shows that the increase in spatial streams from OFDM through VHT produces one of the largest contributions to maximum data rate. The increase in spatial streams through the use of MIMO technology increased by a factor of four from the OFDM specification using single-input, single-output (SISO) antenna techniques to the HT specification with 4 x 4 multiple-input, multiple-output (4 x 4 MIMO) antenna methods. Moreover, the VHT doubles the number of spatial streams again, going from 4 to 8.

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