Praise for Fundamentals of WiMAX

68 Chapter 3 • The Challenge of Broadband Wireless Channels3.1 Communication System Building BlocksAll wireless digital communication systems must possess a few key building blocks, as shown inFigure 3.1. Even in a reasonably complicated wireless network, the entire system can be brokendown 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 aselectromagnetic waves toward the receiver. The key steps in the digital domain are encoding andmodulation. The encoder generally adds redundancy that will allow error correction at thereceiver. The modulator prepares the digital signal for the wireless channel and may comprise anumber of operations. The modulated digital signal is converted into a representative analogwaveform by a digital-to-analog convertor (DAC) and then upconverted to one of the desiredWiMAX radio frequency (RF) bands. This RF signal is then radiated as electromagnetic wavesby a suitable antenna.The receiver performs essentially the reverse of these operations. After downconverting thereceived RF signal and filtering out signals at other frequencies, the resulting baseband signal isconverted to a digital signal by an analog-to-digital convertor (ADC). This digital signal canthen 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 communicationsystem has an endless number of choices. It is important to note that the IEEE 802.16 standardand WiMAX focus almost exclusively on the digital aspects of wireless communication, in particularat the transmitter side. The receiver implementation is unspecified; each equipment manufactureris welcome to develop efficient proprietary receiver algorithms. Aside from agreeingon 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 understandwhat the transmitter did in order to make sense of the received signal—but not vice versa.Next, we describe the large-scale characteristics of broadband wireless channels and seewhy they present such a large design challenge.DigitalAnalogBitsEncoderDigitalModulatorDigital/AnalogRFModuleWirelessChannelBitsDecoderDemodulatorAnalog/DigitalRFModuleFigure 3.1 Wireless digital communication system