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 22.214.171.124.2, 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)|
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.
ETR for MCS_1 = (NbrProtocolDursPerTxOP*MSDU_Octets*NbrOfBitsPerOctet)/(SIFS+AIFS*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|
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.