Real-World Expectations for 802.11ac and 802.11ad
802.11n, 802.11ac and 802.11ad use the term “Modulation Coding Scheme” (MCS) to refer to the combination of pre-modulation mathematical coding and modulation method used. A value called the “MCS Index” is carried in each packet’s preamble to tell the recipient how demodulate the received packet. The very first bits of a packet preamble are compatible with 802.11b, g, n and ac to everyone knows “what’s coming next”. The standards assign completely different definitions to each MCS index (i.e. “MCS 1” doesn’t mean the same thing for 11n, 11ac and 11ad). In every case, “MCS 0” is the lowest connection rate with the simplest pre-coding and modulation and the index numbers go up from there.
We need to introduce two other types of modulation (besides QAM): BPSK and QPSK; the simplest modulation schemes. Where QAM modulation implies using both phase and amplitude to represent bits BPSK and QPSK use only phase. BPSK (Binary Phase Shift Keying) uses a single, simple carrier frequency that is either phase-shifted or not to represent bits. QPSK (Quadrature Phase Shift Keying) uses two overlapping carriers at the same frequency, each of which is essentially BPSK shifted – double the data rate of BPSK. The hierarchy of complexity in modulation goes: BPSK (single carrier), QPSK (two overlapping waveforms), QAM (shift both phase and amplitude). BPSK is the most noise-resistant modulation method because it’s simply a single carrier – the phase either shifts or it doesn’t for 1 and 0.
For MCS 0: 11n uses BPSK to provide 6.5 Mbps per spatial stream in a 20 MHz channel; 11ac/BPSK/6.5 Mbps per stream; 11ad/BPSK/27.5 Mbps. The fact that the lowest connection rate for 802.11ad is 27.5 Mbps compared to 6.5 Mbps for 11n and 11ac is a key differentiator. Even if 11n or 11ac were able to acquire 3 BPSK spatial streams they would not exceed 11ad’s lowest BPSK connection rate. Every standard makes the assumption that the lowest connection rates will tolerate the assumed worst-case environment. Even in the assumed worst-case for noise and interference 11ad offers 27.5 Mbps versus 11ac with 6.5 Mbps.
802.11ad MCS 6 uses QPSK and provides 1.54 Gbps. The highest QPSK rate for 11ac is MCS 2 at 19.5 Mbps in a 20 MHz channel. Because QPSK does not employ QAM (phase and amplitude) modulation it’s generally probable to achieve QPSK data rates in the real-world. It’s often not possible, in all parts of a coverage area, to achieve connection rates above 64-QAM due to environmental noise. It’s reasonable to assume that you’re going to get 39 Mbps (2 spatial streams, QPSK, 20 MHz channel) with 802.11ac but you’ll get 1.54 Gbps with 11ad. Remember that 11ad always has the advantage of a 2.16 GHz wide channel – but only 4 channels available. Because you need lots of channels to support a high user density the 11ad trade-off (compared to 11ac) is lower maximum user density with 11ad. A key differentiator is that 802.11ad provides greater throughput but to fewer overall users than 802.11ac. 802.11ad may be well-suited for use in a conference room (or home) but would not be a choice for a classroom.
Let’s compare 11ac and 11ad in an environment that may be typical: a carpeted office, school or hospital. 801.11ac with 2 spatial streams at MCS 5 (64-QAM) in an 80 MHz channel provides a 468 Mbps connection. 802.11ad in an environment supporting only 16 QAM (MCS 10) provides a 3.85 Gbps connection – 8 times more throughput than 11ac. If both 11ac and 11ad achieved a 64-QAM connection rate then the 2-stream 11ac link at 468 Mbps would compare to a 5.197 Gbps 802.11ad link (MCS 22).
The bottom line is that 802.11ad can provide dramatically greater throughput than 802.11ac in the real world. As you compare 11ac and 11ad starting at the lowest rates and working up, 11ad immediately has greater throughput. Although the maximum theoretical 11ac and 11ad throughput is similar the expectations in the real world are not. Although 11ad has greater practical throughput than 11ac there’s “no free lunch in the Physics department” - there's always a trade-off.. The throughput of 11ad is available only over a very limited range while 11ac provides its throughput in a range that’s typical of conventional WiFi. 11ac and 11ad provide a wonderful example of making trade-offs while obeying the Laws of Physics.