QPSK Versus QAM

Rather than attempting to be all things to all subscribers, WiMAX delivers a gradation of QoS dependent on distance of the SS from the BS: The greater the distance, the lower the guarantee of QoS. WiMAX utilizes three mechanisms for QoS; from highest to lowest, these mechanisms are 64-QAM, 16-QAM, and QPSK. Figure 1 illustrates modulation schemes.

Figure 1: Modulation schemes focus the signal over distance.

By using a robust modulation scheme, WiMAX delivers high throughput at long ranges with a high level of spectral efficiency that is also tolerant of signal reflections. Dynamic adaptive modulation allows the BS to trade throughput for range. For example, if the BS cannot establish a robust link to a distant subscriber using the highest order modulation scheme, 64-QAM, the modulation order is reduced to 16-QAM or QPSK, which reduces throughput and increases effective range. Figure 2 demonstrates how modulation schemes ensure throughput over distance.

Figure 2: Modulation schemes ensure a quality signal is delivered over distance by decreasing throughput.

QPSK and QAM are the two leading modulation schemes for WiMAX. In general the greater the number of bits transmitted per symbol, the higher the data rate is for a given bandwidth. Thus, when very high data rates are required for a given bandwidth, higher-order QAM systems, such as 16-QAM and 64-QAM, are used. 64-QAM can support up to 28 Mbps peak data transfer rates over a single 6 MHz channel. However, the higher the number of bits per symbol, the more susceptible the scheme is to intersymbol interference (ISI) and noise. Generally the signal-to-noise ratio (SNR) requirements of an environment determine the modulation method to be used in the environment. QPSK is more tolerant of interference than either 16-QAM or 64-QAM. For this reason, where signals are expected to be resistant to noise and other impairments over long transmission distances, QPSK is the normal choice.

Multiplexing in OFDM

As shown in Figure 3, an efficient OFDM implementation converts a serial symbol stream of QPSK or QAM data into a size M parallel stream. These M streams are then modulated onto M subcarriers via the use of size N (N ≤ M) inverse FFT. The N outputs of the inverse FFT are then serialized to form a data stream that can then be modulated by a single carrier. Note that the N-point inverse FFT could modulate up to N subcarriers. When M is less than N, the remaining N−M subcarriers are not in the output stream. Essentially, these have been modulated with amplitude of zero.

Figure 3: Block diagram of a simple OFDM transmitter

Although it would seem that combining the inverse FFT outputs at the transmitter would create interference between subcarriers, the orthogonal spacing allows the receiver to perfectly separate out each subcarrier. Figure 4 illustrates the process at the receiver. The received data is split into N parallel streams that are processed with a size N FFT. The size N FFT efficiently implements a bank of filters, each matched to N possible subcarriers. The FFT output is then serialized into a single stream of data for decoding. Note that when M is less than N, in other words fewer than N subcarriers are used at the transmitter, the receiver only serializes the M subcarriers with data.

Figure 4: Block diagram of a simple OFDM receiver