The Modulation and Coding Scheme (MCS) Index is a shorthand notation for the single carrier(SC) PHY modes of operation as originally specified in the IEEE 802.11ad-2012 Standard (page 472, Clause, Table 21-18).  These MCS indices, which range in value from MCS_1 to MCS_12, are an index into the type of modulation (e.g. BPSK, QPSK, 16QAM), Code Rate and “raw” PHY rate that are available for each SC_PHY mode of operation.  Table 1 lists the Modulations, Code Rates, and PHY Rates for each MCS_Index.

TABLE 1: SC_PHY Modulation and Coding Schemes

MCS Index Modulation Code Rate PHY Rate (Mbps)
1 pi/2-BPSK 1/2 385
2 pi/2-BPSK 1/2 770
3 pi/2-BPSK 5/8 962.5
4 pi/2-BPSK 3/4 1,155
5 pi/2-BPSK 13/16 1,251.2
6 pi/2-QPSK 1/2 1,540
7 pi/2-QPSK 5/8 1,925
8 pi/2-QPSK 3/4 2,310
9 pi/2-QPSK 13/16 2,502.5
10 pi/2-16-QAM 1/2 3,080
11 pi/2-16-QAM 5/8 3,850
12 pi/2-16-QAM 3/4 4,620

One of the assumptions that engineers and consumers tend to make when reading TABLE 1 is to assume the PHY Rate is the actual transfer rate at which data frames will be delivered wirelessly between two or more 802.11ad compliant devices. There are a number of factors that effect the “effective throughput rate” (ETR) for each MCS_index. For instance factors such as: the number of bits that are used to define the various fields of the PHY and MAC headers of each data frame, the length of each MAC-service-data-unit (MSDU), the time it takes for a receiving device to switch between the reception of a data packet and the transmission of an ACK packet acknowledging the successful receipt of a data packet, the number of transmissions allowed per Transmit Opportunity (TxOP) and the Packet Error Rate (PER) caused by extraneous factors such as other SC_PHY devices, multipath, and the distance between transceivers.

Equation 1 provides an example calculation for the Effective Throughput Rate as a function of Protocol Durations per TxOP, MSDU length and various protocol timings such as SIFS, AIFS, Average Backoff and TxOP Duration.

Equation 1:
ETR for MCS_1 = (NbrProtocolDursPerTxOP*MSDU_Octets*NbrOfBitsPerOctet)/(SIFS+AIFS[3]*aSlotTime)+AvgBkOff+TxOP_Dur)
= ( 7 * 7920 octets * 8bits/octet)/(3usec+(3*5)usec+37usec+1280usec)
= 332.2 Mbps

Table 2 captures the results of multiple such calculations to summarize the relationship between the PHY rate and Effective-Throughput-Rate (ETR) for best case(i.e. PER=0%) over-the-air data transfers using MAC-service-data-units(MSDU)s with a fixed length of 7920 octets, which is the maximum allowed MSDU length specified by the IEEE 802.11ad-2012 Standard.

TABLE 2: SC_PHY rates and their corresponding MSDU Effective-Throughput-Rates(ETR)

MCS Index PHY Rate (Mbps) MSDU ETR (Mbps) Protocol OvrHd
1 385 332.2 13.7%
2 770 617 19.9%
3 962.5 759.4 21.1%
4 1,155 901.8 21.9%
5 1,251.2 949.2 24.1%
6 1,540 1,091.6 29.1%
7 1,925 1,281.4 33.4%
8 2,310 1,518.7 34.3%
9 2,502.5 1,613.7 35.5%
10 3,080 1,851 39.9%
11 3,850 2,135.7 44.5%
12 4,620 2,373.0 48.6%

As you can see from perusing Table 2, the maximum allowed MSDU length of 7920 octets in conjunction with the MAC Protocol Overhead has a significant negative impact upon the MSDU ETR as the SC_PHY rate is increased from 385 Mbps to 4,620 Mbps (i.e. almost half the MCS_12 PHY_Rate is lost due to Protocol Overhead). A clever solution to this problem, which is defined in the IEEE 802.11ad-2012 Standard, is to transmit Aggregated MPDUs (A-MPDU)s instead of MPDUs over the air.  I will provide, in another BlogPosting, more details regarding  how the transmission of A-MPDUs between two WiGig devices has a significant positive effect upon the  MSDU ETR as the PHY rate is increased to 4,620Mbps.