Communication System Building Blocks

All wireless digital communication systems must possess a few key building blocks. Even in a reasonably complicated wireless network, the entire system can be broken

down into a collection of links, each consisting of a transmitter, a channel, and a receiver.

The transmitter receives packets of bits from a higher protocol layer and sends those bits as

electromagnetic waves toward the receiver. The key steps in the digital domain are encoding and

modulation. The encoder generally adds redundancy that will allow error correction at the

receiver. The modulator prepares the digital signal for the wireless channel and may comprise a

number of operations. The modulated digital signal is converted into a representative analog

waveform by a digital-to-analog convertor (DAC) and then upconverted to one of the desired

WiMAX radio frequency (RF) bands. This RF signal is then radiated as electromagnetic waves

by a suitable antenna.

The receiver performs essentially the reverse of these operations. After downconverting the

received RF signal and filtering out signals at other frequencies, the resulting baseband signal is

converted to a digital signal by an analog-to-digital convertor (ADC). This digital signal can

then be demodulated and decoded with energy and space-efficient integrated circuits to, ideally,

reproduce the original bit stream.

Naturally, the devil is in the details. As we will see, the designer of a digital communication

system has an endless number of choices. It is important to note that the IEEE 802.16 standard

and WiMAX focus almost exclusively on the digital aspects of wireless communication, in particular

at the transmitter side. The receiver implementation is unspecified; each equipment manufacturer

is welcome to develop efficient proprietary receiver algorithms. Aside from agreeing

on a carrier frequency and transmit spectrum mask, few requirements are placed on the RF units.

The standard is interested primarily in the digital transmitter because the receiver must understand what the transmitter did in order to make sense of the received signal—but not vice versa.