1
INTRODUCTION

This chapter briefly describes the digital mobile radio communication system and the significance of digitization in mobile radio communications.

1.1 DIGITAL MOBILE RADIO COMMUNICATION SYSTEM

A schematic block diagram of a digital mobile radio communication system is shown in Figure 1.1. A voice signal is converted into a digital signal via a voice coder. The digitized voice signal is transmitted through a digital mobile radio channel and is converted into a reconstructed analog voice signal at the receiver. The target of voice coding technology is to achieve a lower coding rate while keeping an acceptable voice quality. The digital signals are processed by logic circuits for several system requirements: typically, channel coding, scrambling, and framing. Channel coding is a process which inserts additional bits in order to correct or to detect errors in received signals. Scrambling is designed to hide a transmitted signal from a third party by performing a complicated transformation which is known only to an authorized recipient. Framing is a process where information signals are grouped into blocks with other signals as shown in Figure 1.2. The purpose of framing is to multiplex different signals in the time domain (time division multiplexing (TDM)) to introduce channel coding and to make it possible to adopt a synchronized scrambling technique.

FIGURE 1.1 Digital mobile radio communication system.

FIGURE 1.2 Framed signal.

The next stage of a transmitter is digital modulation. There is no difference in principle between digital modulation and analog modulation—modulation is a process where the amplitude and/or phase of a coherently oscillating carrier signal (although noncoherent carrier as noise is used for some systems) are varied in proportion to a modulating signal. A narrow spectrum bandwidth of the modulated signal is desired. For this purpose, a low-pass filter or an equivalent IF bandpass filter is employed to limit the bandwidth of the modulating signal. Transfer functions, or impulse response of the low-pass filters, are specially designed for digital transmission as discussed in Chapter 2. After the modulation stage, frequency conversion up to a final RF frequency band, power amplification, and transmission via an antenna follows in subsequent stages.

The signal that is received through an antenna is amplified, frequency down-converted, and then band-limited with a bandpass filter. A demodulator outputs a transmitted baseband signal, which is corrupted to some extent by noise and imperfections in the function of the transmitter, channel, and/or receiver. The principle for the demodulation of a digitally modulated signal is the same as for an analog signal, that is, coherent or noncoherent demodulation can be used. In some digital transmission systems, a carrier signal, which is necessary for coherent demodulation, can be extracted from a received signal by using the knowledge that the digitally modulated signals take specific phases.

The demodulated signal is fed into a decision circuit, where the received signal is sampled and discretized to one of the allowable values of the transmitted signals. A timing recovery circuit generates a timing signal for sampling. The timing signal is extracted from the received signal by detecting the change in digital signal levels at clock frequency duration. The decision error rate is the main concern at the receiving side. Decision is a process particular to a digital transmission system. The principle of the decision process is based on the fact that digital signals take one of several discrete states. If there is no decision error, then noise, interference, or distortion will have no effect on the signal transmission. This fact becomes especially important for a multihop transmission system, where the signal is regenerated and transmitted at the repeaters: no accumulation of noise and interference occurs in the digital system in contrast to an analog system.

Output of the decision circuit takes the form of a logic signal. Channel decoding, descrambling, and deassembling of the framed signal, which are inverse operations to respectively channel coding, scrambling, and framing, are carried out with logic circuits.

The digital signal is converted into an analog voice signal with a voice decoder. As long as a voice signal is involved, the phase of the transfer function is not important, because human ears are not much sensitive to the phase of the voice signal. On the other hand, decision error in the digital transmission is sensitive to both the phase and the amplitude of the transfer function. This is the reason why pulse waveforms should be transmitted without distortion.

The transmission of data signals differs from voice signal transmission in several aspects. We should assure a very low error rate, because great value may depend on accurate data transmission, for example, in a banking service. In contrast to this we can tolerate a rather higher bit error rate for speech communication, because the voice signal has a lot of redundancy, and humans are very intelligent communication terminals: we can ask for repetition and confirm the meaning, if received voice signal is not correct due to transmission error.

The intelligence of the data communication system is given by protocols imbedded in the data terminal. The protocol affects the efficiency of data transmission even in the same transmission channel. In a mobile radio communication channel, in which error rate performance is not generally good, an automatic repeat request with error detection seems indispensable for data transmission. Real-time communication is not particularly important for data transmission, in contrast to spoken conversation.

A mobile radio communication channel is characterized as a channel without direct propagation. When mobile equipment moves quickly, a transmitted signal encounters rapid fading phenomena. The depth of the fading can reach as deep as several tens of dB. The fading speed is proportional to a carrier frequency and the speed of motion. For example, the maximum Doppler frequency is as high as 90 Hz for a carrier frequency of 900 MHz and a car speed of 100 km/h. A receiver has to cope with this fast fading phenomena. On the other hand, in the case of hand-carried equipment, the fading speed is quite slow because of the low speed of motion. In this case another problem occurs, the fading duration becomes too long: in this case we cannot communicate for a long time. As the digital transmission rate increases, the channel exhibits frequency-selective fading, where the fading is different at different frequencies and, as a result, the transfer function of the channel loses its distortion free characteristic. The distortion in the frequency-selective fading channel degrades bit error performance due to intersymbol interference.

Noise is a general problem in communications. Carrier frequencies for mobile radio communications are predominately in the VHF and UHF bands. At these frequencies, the man-made noise and atmospheric noise are rather high compared with the microwave communication band. For the purpose of obtaining higher receiving sensitivity, it is ineffective to use a low noise receiver which has a noise level lower than the channel noise. Digital transmission offers a solution, that is, an error control scheme, to make communication secure in a noisy channel.

Eavesdropping on a conversation by a third party can occur in mobile radio communications if the transmitted signal is not protected: the transmitted signal propagates everywhere around the transmitter and one can easily get eavesdropping equipment (i.e., a scanning receiver). Voice scramblers for analog transmission are not sufficient to assure the desired degree of protection or voice quality. Digital transmission offers a high security and high voice quality scrambling to combat eavesdropping.

Mobile radio communication equipment is, in some sense, comparable to other consumer products: the number of mobile terminals in the market is large in contrast to other communication systems, such as satellite communication or microwave communication equipment. A mobile terminal is so slim that it slips into pocket and is so inexpensive that consumers can easily afford it. For a digital mobile radio terminal, an increase in the price or size will not be accepted in the market, as long as the service is equivalent.

The biggest difference between mobile radio communications and other consumer products is that the former is supported with a huge system, while the latter is not. Consider a mobile telephone system; for example, the mobile switching centers and a number of base stations, as well as the mobile terminals, are incorporated into the system. Digitization of the mobile telephone system operates on all of these facilities. The switching center and the wire line communications were digitized prior to the digitization of the base stations and the mobile terminals. Communications between the base station and the mobile terminals (i.e., through the air interface) is the major concern of this book. This interface includes digital modulation/demodulation, channel access methods for use of the channels by many users on demand, and other topics related to the signal transmission between the base station and the subscriber stations.

Digital mobile communication service became widespread all over the world. And businesses associating with the digital mobile communications have expanded to contribute to increasing...