IEEE 802.11 modulation and encoding techniques

This section details the techniques of modulation and encoding in IEEE 802.11 protocol. These techniques are not unique to 802.11; they also apply to 802.15 protocols and, as we will see, to cellular protocols as well. The methods of frequency hopping, modulation, and phase-shift keying are fundamental methods that should be understood by an architect as different techniques balance range, interference, and throughput.

Digital data transmitted by an RF signal must be transformed into analog. This occurs at the PHY no matter what RF signal is being described (Bluetooth, Zigbee, 802.11, and so on). An analog carrier signal will be modulated by a discrete digital signal. This forms what is called a symbol or a modulation alphabet. A simple way to think about symbol modulation is a piano with four keys. Each key represents two bits (00, 01, 10, 11). If you can play 100 keys per second, that implies you can transmit 100 symbols per second. If each symbol (tone from the piano) represents two bits, then it is equivalent to a 200 bps modulator. While there are many forms of symbol encoding to study, the three basic forms include:

  • Amplitude Shift Keying (ASK): This is a form of amplitude modulation. Binary 0 is represented by one form of modulation amplitude and 1 a different amplitude. A simple form is shown in the following figure, but more advanced forms can represent data in groups using additional amplitude levels.
  • Frequency Shift Keying (FSK): This modulation technique modulates a carrier frequency to represent 0 or 1. The simplest form shown in the following figure is Binary Frequency Shift Keying (BPSK), which is the form used in 802.11 and other protocols. In the last chapter, we talked about Bluetooth and Z-Wave, those protocols use a form of FSK called Gaussian Frequency Shift Keying (GFSK) that filters the data through a Gaussian filter, which smooths the digital pulse (-1 or +1) and shapes it to limit spectral width.
  • Phase Shift Keying (PSK): Modulates the phase of a reference signal (carrier signal). Used primarily in 802.11b, Bluetooth, and RFID tags. PSK uses a finite number of symbols represented as different phase changes. Each phase encodes an equal number of bits. A pattern of bits will form a symbol. The receiver will need a contrasting reference signal and calculate the difference to extract the symbols and then demodulate the data. An alternative method requires no reference signal for the receiver. The receiver will inspect the signal and determine if there is a phase change without referencing a secondary signal. This is called Differential Phase Shift Keying (DPSK) and is used in 802.11b. 

The following figure pictographically depicts the various encoding methods:

Different forms of symbol encoding using keying techniques: amplitude keying, frequency keying, and phase keying. Note how phase keying changes phase for every "1" encountered.

The next form of modulation technique is hierarchical modulation, specifically Quadrature Amplitude Modulation (QAM). The following constellation diagram represents the encoding in a 2D Cartesian system. The length of any one vector represents the amplitude, and the angle to a constellation point represents the phase. Generally speaking, there are more phases that can be encoded than amplitudes, as shown in the following 16-QAM constellation diagram. The 16-QAM has three amplitude levels and 12 total phases angles. This allows for 16 bits to be encoded. 802.11a and 802.11g can use 16-QAM and even higher density 64-QAM. Obviously, the denser the constellation, the more encoding can be represented and the higher the throughput. 

The figure shown illustrates a QAM encoding process pictographically. On the left is a representation of a 16 point (16-QAM) constellation diagram. There are 3 amplitude levels indicated by the length of the vectors and 3 phases per quadrant indicated by the angle of the vector. This allows for 16 symbols to be generated. These symbols are reflected in varying the phase and amplitude of the signal generated. On the right is an 8-QAM example waveform diagram showing the varying phases and amplitudes that represent a 3 bit (8 value) modulation alphabet. 

Quadrature Amplitude Modulation (QAM). Left: 16-QAM Constellation. Right: 8-QAM Waveform Encoding.
QAM has practical limits. Later, we will see very dense constellations that radically increase throughput, one can only add a certain amount of phase angles and amplitudes. Noise generated by Analog to Digital Converters (ADC) and Digital to Analog Converters (DAC) will introduce quantization errors and noise and will need to sample signals at very high speeds. Additionally, the Signal to Noise Ratio (SNR) must exceed a certain value to achieve a good Bit Error Rate (BER).

The 802.11 standards employ different interference mitigation techniques that essentially spread a signal across a band:

  • Frequency Hopping Spread Spectrum (FHSS): Spreads signal over 79 non-overlapping channels that are 1 MHz wide in the 2.4 GHz ISM band. Uses a pseudo-random number generator to start the hopping process. Dwell Time refers to the minimum time a channel is used before hopping (400 ms). Frequency hopping was also described in the last chapter and is a typical scheme for spreading signals.
  • Direct sequence spread spectrum: First used in 802.11b protocols and has 22 MHz-wide channels. Each bit is represented by multiple bits in the signal transmitted. The data being transmitted is multiplied by a noise generator. This will effectively spread the signal over the entire spectrum evenly using a pseudo-random number sequence (called the Pseudo-Noise PN code). Each bit is transmitted with an 11-bit chipping sequence (phase shift keying). The resulting signal is an XOR of the bit and the 11-bit random sequence. DSSS delivers about 11 million symbols per second when we consider the chipping rate.
  • OFDM: Used in IEEE 802.11a and the newer protocols. This technique divides a single 20 MHz channel into 52 sub-channels (48 for data and four for synchronization and monitoring) to encode data using QAM and PSM. A Fast Fourier Transform (FFT) is used to generate each OFDM symbol. A set of redundant data surrounds each sub-channel. This redundant band of data is called the Guard Interval (GI) and is used to prevent Inter-symbol Interference (ISI) between neighbor subcarriers. Notice the subcarriers are very narrow and have no guard bands for signal protection. This was intentional because each subcarrier is spaced equally to the reciprocal of the symbol time. That is, all the sub-carriers transport a complete number of sine-wave cycles that when demodulated will sum to zero. Because of this, the design is simple and doesn't need the extra cost of bandpass filters. IEEE 802.11a uses 250,000 symbols per second. OFDM is generally more efficient and dense (hence more bandwidth) than DSSS and is used in the newer protocols.
Fewer symbols per second have an advantage in situations where there are reflections of signals on walls and windows. Since the reflections will cause what is called multi-path distortion (copies of the symbol hit the receiver at different times), a slower symbol rate allows more time to transmit a symbol and there is more resilience to delay spread. However, if a device is moving, there can be Doppler effects that affect OFDM more than DSSS. Other protocols such as Bluetooth use one million symbols per second.

The following figure depicts an OFDM system with 52 subcarriers in two 20-MHz channels:

An example of OFDM. Here a channel is subdivided into 52 smaller slots or subcarriers (each carrying a symbol).

The set of different modulations available for each standard is called the Modulation and Coding Scheme (MCS). The MCS is a table of available modulation types, guard intervals, and coding rates. One references this table with an index. 

Since 802.11b entered the market, before 802.11a they used different encoding schemes. Each is incompatible with each other. There is some confusion in the market as to the difference as protocols were released nearly simultaneously.
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