With DSSS we spread the chips of a single data stream into one wide 22 MHz channel, and because of the constant chip rate of 11 MHz we are restricted to 11 Mbps of data throughput. On the other hand, Orthogonal Frequency-Division Multiplexing (OFDM) sends data bits in parallel over multiple frequencies , all contained within a single 20 MHz channel. Each channel is divided into 64 sub-carriers (hence the Frequency-Division) which are spaced 312.5 kHz apart. There are 3 different types of sub-carriers:
- Guard – 12 sub-carriers which are used to separate each channel and help receivers lock onto a channel. These actually aren’t transmitted but stay silent as spacing.
- Pilot – 4 sub-carriers which are equally spaced and always transmitted to help receivers determine the noise state of the channel.
- Data – 48 sub-carriers which are devoted to carrying data.
Since OFDM is transmitting data in parallel it is able to get high aggregate throughput through its relatively low throughput sub-carriers. Since the data is also sent in parallel we can also modify how much of it needs to be unique or repeated data for error prevention. The coding schemes in OFDM are named using fractions to identify the ratio of new bits to repeated bits (coder ratio); BPSK 1/2 indicates that one half of the bits are new and the other half repeated. BPSK 3/4 therefore indicates that three-fourths of the bits are new and only one fourth are repeated.
At the lower speeds, BPSK modulation can be used with two different coder ratios. OFDM and BPSK 1/2 results in a 6 Mbps throughput and with BPSK 3/4 achieves 9 Mbps throughput. If we combine OFDM with QPSK 1/2 we can achieve 12 Mbps and QPSK 3/4 can achieve 18 Mbps. If you recall the DSSS post we introduced QPSK and the fact that it uses 2 binary bits to give four possible phase shifts. Therefore for us to break through 18 Mbps we would need a additional modulations options.
Quadrature Amplitude Modulation (QAM) combines QPSK phase shifting with multiple amplitude level to give an even greater number of modulation options. As an example 16-QAM uses 2 bits for the QPSK modulation and an additional 2 bits for the amplitude for a total of 4 bits used for modulation changes. 4 binary bits would give use 16 unique modulation options (hence the name). The coder ratios still apply when we move to QAM so the names still carry a fractional prefix indicating the ratio. The current OFDM supposed modulation options include:
- OFDM QPSK 1/2 – 12 Mbps
- OFDM QPSK 3/4 – 18 Mbps
- OFDM 16-QAM 1/2 – 24 Mbps
- OFDM 16-QAM 3/4 – 36 Mbps
- OFDM 64-QAM 2/3 – 48 Mbps
- OFDM 64-QAM 3/4 – 54 Mbps
- OFDM 256-QAM 3/4 – 78 Mbps
- OFDM 256-QAM 5/6 – 86 Mbps
The IEEE 802.11g amendment was introduced in 2003 and is also commonly called Extended Rate PHY (ERP) or ERP-OFDM. ERP is just another name for 802.11g in the 2.4 GHz band. Since 802.11g was based on OFDM as opposed to DSSS in the previous standard devices cannot directly understand each other’s RF signals. 802.11g was intended to be backwards compatible with legacy 802.11b devices by downgrading and using DSSS, however the reverse is not true. To allow both OFDM and DSSS devices to coexist a protection mechanism was included. When using 802.11g Protection Mode, before a device transmits it will send a warning message with DSSS before transmitting its data with OFDM. Protection mode is enforced automatically if an 802.11b devices is detected on the WLAN, and once it leave the network it is lifted. Since the protection mode adds the additional DSSS warning messages it greatly reduces network throughput.
So we’ve already covered 802.11b and 802.11g why are we just now circling around to 802.11a (yes they do go in order). Actually almost as soon as 802.11 was ratified the need was recognized to limit interference. 802.11a was introduced in 1999, earlier in the same year at 802.11b however since migrating from 2.4 GHz to 5 GHz required new hardware it was never widely adopted. IEEE 802.11a restricts devices to use OFDM only and is based on channels that are 20 MHz wide. Since it only supported OFDM it was not backwards compatible with any devices and depending on the modulation scheme any of the supported data rates were available.