Understanding modulation and coding schemes
Beginning with the introduction of IEEE 802.11n, modulation and coding schemes (MCS) are used to determine the data rate of a wireless connection using high-throughput orthogonal frequency division multiplexing (HT-OFDM). Pre-802.11n systems using OFDM had defined data rates between 6 and 54 Mbps based on what type of modulation and coding were used. HT-OFDM uses other parameters, such as channel size, number of spatial streams, coding method, modulation technique, and guard interval. Each MCS is based on a combination of these parameters. There are 77 different types of MCS for both 20 and 40 MHz channels, eight of which are mandatory for 20 MHz channels, and correspond to mandatory basic data rates.
The concept of multiple spatial streams was covered in a previous segment. A new concept is that of the guard interval (GI). When data is encoded by any of the various methods described, bits are converted to symbols, which are then modulated onto the carrier signal for transmission. In a legacy OFDM system, a symbol contains 288 bits, of which 216 are data and 72 are used for error correction. A typical symbol has a duration of 4 µs. To avoid the problem of intersymbol interference (ISI), a guard interval of 800 nanoseconds is inserted into each symbol (the symbol is actually 3.2 µs, the GI is .8µs). ISI occurs because of the multipath phenomenon; symbols that reach the receiver at different times, called the delay spread, can interfere with and corrupt one another, resulting in frequent retransmissions. Maximum delay spread is typically about 200 nanoseconds, so the guard interval is set at four times the maximum.
802.11n HT-OFDM systems also use an 800 nanosecond GI, but there is an option to use a 400 nanosecond GI. Using a smaller GI will increase throughput, but could also compromise system reliability. A shorter GI corresponds to a shorter symbol time and ISI can occur. In solid, well-planned wireless systems, the shorter GI will work fine and an increase in throughput will be achieved; however, if the radio frequency (RF) environment is poor, use of the shorter GI will adversely affect system throughput and reliability.
802.11n systems determine the proper MCS to use based on channel conditions as discerned from feedback from the receiver. The MCS is negotiated during communication and serves to strike a balance between maximum possible data rate and maximum acceptable error rate. Figure 1 provides a sample of modulation and coding schemes for 802.11n HT-OFDM:
As you can see, there is about a 10% increase in throughput using the shorter guard interval. Note the increase in throughput as more spatial streams are brought into play.
A word about coding, or information rate: Recall our previous discussion about convolutional coding. In communication theory, convolutional coding is used as a forward error correcting code. Given the ratio k/n as the code rate, the code generates a total of n bits to every k bits of useful information, which results in n-k bits being redundant. For a convolutional code of 1/2, 3/4, or 5/6, one redundant bit is inserted after each single, third, or fifth bit, respectively; the data is then decoded at the receiving end by a specialized algorithm. The purpose of this operation is to introduce redundancy, increase the reliability of the information, and avoid retransmissions by correcting the errors, if any, at the receiving end. A full discussion of this technique is well beyond this tutorial, but much deeper explanations can be found on the web.
– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA). He can be reached at email@example.com. Edited by Chris Vavra, production editor, CFE Media, Control Engineering, firstname.lastname@example.org.
This blog also appears in the December 2014 print edition of Control Engineering.
The formal name for the standard is IEEE 802.11n-2009 – IEEE Standard for Information technology — Local and metropolitan area networks — Specific requirements — Part 11: Wireless LAN Medium Access Control (MAC)and Physical Layer (PHY) Specifications Amendment 5: Enhancements for Higher Throughput.
www.controleng.com/blogs has other wireless tutorials from Capano on the following topics:
OFDM: Orthogonal frequency division multiplexing
MIMO and spatial multiplexing
Propagation revisited: Multipath
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.