Understanding Digital Radio and Modulation Techniques
1. Digital Radio and Transmission
Digital radio involves transmitting digitally modulated analog carriers between points in a communication system. This requires a physical facility between the transmitter and receiver.
2. Digital Radio Systems vs. Conventional Radio
In digital radio systems, the modulator input and output signals are digital pulses. This distinguishes them from conventional AM or FM radio, where the modulating signal is analog.
3. Digital Modulation Techniques
3.1 Amplitude Modulation
Digital amplitude modulation uses full carrier, double sideband modulation.
3.2 Frequency Shift Keying (FSK)
FSK is a form of angle modulation with constant amplitude. The modulating signal is a binary FSK signal, and the binary input signal drives the carrier frequency. With binary FSK, the output frequency changes each time the input signal’s logical condition changes.
3.2.1 Bit Rate and Baud Rate
In digital modulation, the rate of change at the modulator input is the bit rate or bit transfer rate. The rate of change at the modulator output is the baud rate, which is the reciprocal of the time of an element in the output signal. With FSK, the time of an element in the output signal equals the time of a single bit.
3.3 Binary Phase Shift Keying (BPSK)
BPSK is a form of square wave modulation with suppressed carrier of a continuous wave signal. In a BPSK transmitter, the balanced modulator acts as a phase reverse switch. A balanced modulator is a product modulator.
3.4 Quadrature Phase Shift Keying (QPSK)
QPSK is an M-ary coding technique where M = 4. It allows for four possible output phases for a single carrier frequency. In QPSK, the input binary data is grouped into two-bit units called dibits. In a QPSK transmitter, two bits are synchronized to the bit splitter, and the input data is divided into two channels.
3.5 Quadrature Amplitude Modulation (QAM)
QAM is a digital modulation technique where the digital information is contained in both the amplitude and phase of the signal. 8-QAM is an M-ary coding technique where M = 8.
4. Bandwidth Efficiency
Bandwidth efficiency is often used to compare the performance of different digital modulation techniques. It is normalized to a bandwidth of 1 Hz. In the case of BPSK, the minimum bandwidth and baud rate are equal.
5. Carrier Recovery
Carrier recovery is the process of extracting a carrier reference for coherent phase detection at the receiver. In PSK and QAM systems, the carrier is suppressed in the balanced modulators and is not transmitted. The square loop is a common approach for carrier recovery in BPSK. The received BPSK waveform is filtered and then squared.
6. Probability of Error
The probability of error is a function of the ratio of carrier power to noise power density. Noise power density is the noise power normalized to a bandwidth of 1 Hz.
7. Coherent and Non-Coherent FSK
In non-coherent FSK, the transmitter and receiver are not phase-synchronized. In coherent FSK, the receiver’s local reference signals are phase and frequency locked with the transmitted signals.
8. Digital vs. Analog Signal Processing
In digital transmission, it’s not necessary to evaluate the amplitude, frequency, and phase characteristics. Instead, the received pulses are evaluated at precise sampling intervals. Digital signals are better suited for processing and multiplexing than analog signals. Both digital and analog systems use signal processing.
9. Advantages of Digital Systems
Digital systems are better adapted to handle sampling errors. Flat topping introduces less distortion and requires a slower analog-to-digital converter sampling rate. The Nyquist sampling theorem establishes the minimum sampling frequency for a PCM (Pulse Code Modulation) system.
10. Pulse Code Modulation (PCM)
PCM samples the analog input signal and converts it into a serial binary code. Essentially, a sample and hold circuit functions as an AM modulator. The quantization error is equivalent to additive white noise.
11. Dynamic Range and Coding Efficiency
Dynamic range is the ratio between the maximum and minimum possible values that the DAC can decode. The number of bits used in a PCM code depends on the dynamic range. Coding efficiency is a measure of how effectively a PCM code is used. Nonuniform encoding is achieved using a nonlinear function.
12. Digital Compression-Expansion
In digital compression-expansion, the analog signal is first sampled and converted to a linear code.