All Hail OFDM

Oct. 8, 2009
Virtually all major new wireless technologies, including WiMAX and LTE, are based on orthogonal frequency-division multiplexing (OFDM), which has emerged as the wireless spectral efficiency leader.

Virtually all major new wireless technologies, including WiMAX and LTE, are based on orthogonal frequency-division multiplexing (OFDM), which has emerged as the wireless spectral efficiency leader.

As long as you have the spectrum, you can get more bits per hertz than almost any other technology. It also has an inherent way to implement an access scheme for multiple users. On top of that, it’s a great fit with multiple-input, multiple-output (MIMO), which can add speed and link reliability where needed.

With DSP, OFDM is a relatively standard technology today, as proven with Wi-Fi (802.11a/g/n). As a result, the physical-layer (PHY) side of OFDM has become the easy part. All of the complexity lies with the baseband protocols.

OFDM takes a fast serial data stream and divides it into many parallel slower data paths, modulating each on a separate carrier. The transmission channel is divided into many narrow-band subchannels or subcarriers. The channel spacing and data rates are “tuned” or selected to provide orthogonality, meaning one adjacent channel won’t interfere with the next, and there are no guard bands between them. With a sinx/x or sinc function spectrum for each channel, the nulls in the spectrum of one subcarrier line up with the peaks of the adjacent subcarriers, so no interference occurs.

The only practical way to generate such a complex signal is to use the inverse fast Fourier transform (iFFT) at the transmitter. The standard FFT is used to recover the signals at the receiver. Both require significant processing power to do so. But with current DSPs, ASICs, and FPGAs, it is practical and affordable.

OFDM lengthens the symbols for the data and makes them less susceptible to multipath reflections and fading. This provides a more solid data link. Also, the data rate that can be achieved in relatively narrow bandwidths is very high compared to what can be achieved by other techniques. The spectral efficiency is a very high 3 to 6 bits/ Hz, when advanced modulation like 16QAM or 64QAM is used, and better than four times that achieved with HSPA. Even higher spectral efficiencies are achieved by adding MIMO antenna technology to OFDM.

WiMAX and Long-Term Evolution both use orthogonal frequency-division multiple access (OFDMA) for access. OFDMA enables many subscribers to use the same main channel. Each user is assigned one or more subchannels for transmission. And, don’t forget that LTE and WiMAX are IPpacket- based (Internet Protocol) rather than circuit-switched, as is the current cellular system. Though they’re embarrassingly similar, key differences do exist.

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