Comparing 802.11ac and 802.11ad QAM and OFDM Implementation
Having laid the groundwork regarding QAM and OFDM we can now compare and contrast 802.11ac and 802.11ad based on the underlying technology.
802.11ac supports 468 data subcarriers per OFDM symbol. 802.11ad supports 336 data subcarriers. Each 11ac subcarrier can be modulated to 256-QAM while the maximum modulation for an 11ad subcarrier is 64-QAM. 802.11ac specifies up to 8 simultaneous MIMO spatial streams, 802.11ad uses only a single stream for transmission. At 5.8 GHz a single 11ac OFDM symbol occupies 4 microseconds; at 60 GHz a single 11ad symbol occupies only 242 nanoseconds. The fact that the carrier frequency in 802.11ad is 10 times faster than 11ac allows the OFDM symbol time to be close to 16 times faster! Although each 11ad OFDM symbol has fewer data subcarriers with a lower maximum QAM modulation rate the symbols can be transmitted 16 times faster than 11ac.
Compared to 802.11ad, 11ac has optimized the smaller channels (160 MHz) in 5 GHz by offering 256-QAM modulation and more data subcarriers. 802.11ad has a massive 2.16 GHz channel width and high frequency carrier (60 GHz) to provide throughput comparable to the 11ac maximum. The wide (2.16 GHz) 802.11ad channel width coupled with fast symbol time (16 times faster than 11ac) gives 11ad it’s capabilities.
Remember that the 1 spatial stream in 802.11ac provides roughly 866 Mbps and it requires EIGHT, 256-QAM 11ac spatial streams to get to 6.93 Gbps. The SINGLE 11ad spatial stream can provide 7 Gbps if it can utilize 64-QAM modulation. When considering 802.11ac or 802.11ad (or any standard) you have to see beyond the promises of the standard and look at the practicalities of the real-world. In the same way that you’re not going to get 600 Mbps throughput with 802.11n you’re not going to get the maximum capabilities of 11ac or 11ad. In the same way that we’ll never see 4-stream 802.11n it’s unlikely that we’ll ever see 8-stream 802.11ac client devices.
Instead of comparing standards based on the most extreme capabilities they present, let’s look at what the lower limits are – those you can count on. Only the marketing folks like to think in terms of the glorious best-case upper limits of a new wireless standard. We’ll pose the question, “At what point do we expect to see the real-world start to limit the stipulated capabilities of the wireless standard?” When evaluating technology it's important to identify real-world limitations to the stipulated capabilities of the applicable standard. We’ll find that while the upper limits of the 802.11ac and 802.11ad standard are similar the practical limits are very different. We’ll find that the design goals for 11ac and 11ad are very different and they each apply to a specific set of requirements. Ultimately you’ll find that 802.11ac and 802.11ad are not interchangeable standards when it comes to meeting the requirements of a specific project.