Analog and Digital Communications
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The analog signal can be one of infinite number of values, while the digital signal can only be one of finite number of values. The analog signal transmission from one part to other parts is called analog communications, while the digital signal transmission from one part to other parts is called digital communications.
In practice, most signals that exist in the nature are analog signals, such as voice, image, video, temperature, pressure, and so on. There are infinite numbers of possible values for analog signal, which is continuous with time in most cases. According to the Nyquist theorem, the low-pass and band-pass analog signals can be converted into digital signals through sampling, quantization, and coding processes, which is known as analog-to-digital converting (ADC). During the ADC process, the error called quantization error is caused in the quantization process. After ADC process, the analog signal is converted into digital signal as “0” and “1” binary sequence (Hui and Yeung 2003).
The modulation modes adopted in analog communications include amplitude modulation, frequency modulation, and phase modulation. Digital communication system uses digital amplitude modulation, digital phase modulation, digital frequency modulation, and APK modulation. Since the digital signal can only be one of finite number of values, the main difference of demodulations between analog and digital communications is that a sampling and decision module is used in digital communications. The sampling and decision module maps the demodulated signals into one of the finite number of values. If the noise caused in the digital communication systems is less than the noise tolerance threshold, it can be removed by the sampling and decision unit. However, the quantization noise is irreversible in digital communications.
Analog and digital communications are adopted in mobile communications. The emergence of mobile communications began in the early twentieth century. The mobile communication schemes have experienced from analog communications to digital communications. In 1G to 4G communications, 1G cellular network adopts analog communications, and the others are digital communications (Kumar et al. 2010).
Analog Communication Systems
The analog communication systems transmit analog signals, which have infinite number of possible values. Therefore, the receiver cannot completely eliminate the interference noise caused by the transmission channel.
Digital Communication Systems
Different from Fig. 1, the source coding and channel coding are adopted at the transmitter, and the corresponding source decoding and channel decoding are adopted at the receiver. Due to the overwhelming bias toward digital communications, there is no effective analog channel coding designed for analog communications.
In digital communications, the irreversible quantization error caused by ADC process is introduced into digital transmission. The source coding leads to a serious dependence among the transmission stream and has the drawback of error propagation during decoding. In the transmission process on time-varying fading channel, the channel quality changes dynamically. When the channel quality is higher, the digital modulation cannot obtain extra transmission quality after correctly demodulating the digital signal.
In analog communications, due to lack of effective analog channel coding scheme, the analog communications are not good at overcoming channel noise. However, analog communications can be well adapted to the dynamics of channel quality, and better signal-to-noise ratio can be obtained when the channel quality is higher. Hence, effective analog channel coding and power allocation schemes are encouraged in future research works.
- Kumar A, Liu Y, Sengupta J, Divya JS (2010) Evolution of mobile wireless communication networks 1G to 4G. Int J Electr Commun Technol 1(1):68–72Google Scholar