RF Basic Background: 802.11 Physical Layer (Advanced Modulation)

WiFi provides mobility and coverage. But early versions (e.g. 802.11a/b/g) of WiFi family didn't achieve data rates to meet the needs of today's application. Recent antenna technologies made a significant breakthrough, providing access to the users with increased throughput.

This article will discuss the main technologies used in physical layer of 802.11 protocol. The below diagram shows general functions of OFDM-MIMO transmission system which is widely used in today's wireless products (802.11n/ac).
 Radio Carriers (CCK and OFDM)

Radio Carriers

802.11 protocols use two radio carrier techniques, CCK/DSSS and OFDM. The following diagram shows carrier technologies used in different 802.11 physical layer.

DSSS/CCK: DSSS/CCK uses the entire available bandwidth as one single channel. DSSS specified in 802.11 standard works in conjunction with CCK. CCK applies sophisticated mathematical formulas to the DSSS codes, permitting the codes to represent  large information by increasing the data clock rate. CCK works with DSSS to increase the peak data rate from 2 to 11 Mbps.

OFDM: OFDM (Orthogonal Frequency Division Multiplexing) is a subset of frequency division multiplexing in which a signal channel utilizes multiple sub-carriers on adjacent frequencies. Sub-carriers in an OFDM system are overlapping to maximize spectral efficiency. The overlapping sub-carriers in an OFDM system are precisely orthogonal to one another to avoid interference.
  In above diagram, there are seven sub-carriers for each individual channel. 

Modulation

The RF carriers need to be modulated. More complex schemes give faster bit rates for the data, but requires better SNR to the full potential. In 802.11n the highest order modulation is 64-QAM (Quadrature Amplitude Modulation). 802.11ac increased the constellation configuration to 256-QAM. Going from 64-QAM to 256-QAM allows for a 33% speed burst at shorter, yet still usable, 256-QAM doesn't’t require more spectrum or more antennas than 64-QAM. 
 The constellation diagram for 256-QAM signal in different SNR environment.

MIMO

802.11n introduced single-user multiple-input multiple-output. It allows four spatial streams. 802.11ac goes all the way increasing the number of MIMO streams from four to eight. Single-user MIMO means an AP can transmit multiple spatial streams at the same time, but only directed to a single device/user. From 802.11ac Wave2, a new technology is defined, called multi-user MIMO (MU-MIMO). For multi-user MIMO, an AP is able to use its antennas to transmit multiple frames to different devices/user, all at the same time and over the same frequency spectrum.

Therefore, 802.11n acts like an Ethernet hub that can only transfer a single frame at a time to all its ports. 802.11ac Wave2 AP can work as a switch with MU-MIMO, allowing an AP to send multiple frames to multiple clients at the same time over the same frequency spectrum, and with multiple antennas.

Data Rate

The data rate depends on Bandwidth, Symbol duration, modulation type (Bits per Symbol and Coding rate), Number of sub-channels and spacial streams number. 

• Symbol duration (symbols per second) (SDur) : Symbol duration depends on "Guard Interval" between symbols.
• Bits per symbol (NBits)
• Coding rate (CRate): a proportion K/N of the data-stream. K is the useful information. N is the total bits of the data-stream. 
• Number of sub-channels (NChan): 802.11n/802.11ac have more sub-channels in the same bandwidth compared to 802.11a/11g.