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Lecture 5
Types of signal modulation in wireless communication
Plan:
Manipulation in digital radio communication systems.
Binary Phase Shift Keying.
Quadrature phase and phase shift modulation.
FM-8 signals and quadrature amplitude modulation.
The central problem in the construction of any communication system is the choice and technical implementation of methods for introducing the transmitted information into a physical carrier at the point of transmission and extracting this information at the point of reception. This most difficult problem is known as the modulation and demodulation problem.
Modulation is the process of encoding information received from a source of information into a form that is most convenient for transmission over a communication channel. In general, this process involves shifting the baseband of the baseband ΔF to high frequencies. The resulting modulation radio signal s [t, u (t)] occupies a 2∆F band, the center frequency f0 of which is much higher than the upper cutoff frequency fb of the modulating signal spectrum. As a rule, the harmonic oscillation s (t) = A ∙ cos (2nf0t + φ) is used as a carrier of information, the main parameters of which, used for modulation, are amplitude A, frequency f0 and phase φ.
Virtually all modern communication systems use digital modulation techniques and digital signal processing for demodulation. Such systems are usually called digital transmission systems, in contrast to analog systems, in which analog modulation and analog demodulation are implemented. Modern advances in radio electronics provide the ability to implement in the transmitter and receiver of the communication system rather complex algorithms for digital processing of electrical signals. As a result, the quality of transmission of almost any message in digital systems is higher than the quality of transmission of these messages using analog communication systems.
Digital transmission systems (DSP) have two essential features:
- any messages are presented in digital form, i.e. in the form of bit sequences {ai, i =…, –1, 0, + 1,…}; for any value of the index i, the symbol ai takes values from the alphabet {0, 1};
- the transmitter of the system generates and transmits in turn to the transmission channel a finite number of signals {sm (t), m = 1, 2, ..., M}, differing in shape, which are called channel symbols;
- the duration of the channel symbol is designated as Tcs;
- one channel symbol "carries" one bit or more bits to be transmitted;
- if M = 2, then the transmission system is called binary;
- if the value M> 2, then the system is called M-ary.
Thus, the transmission of message sequences by digital systems is carried out by converting them into a sequence of bits, which are converted into a sequence of channel symbols.
The number of channel symbols M used and their shape are different in different DSPs, but they are known at the receiving point. Therefore, the main function of the receiver, or rather its demodulator, in a digital transmission system is to estimate which of the possible symbols (signals) was transmitted by the transmitter at a regular time interval of duration Tcs.
The problem of evaluating symbols (signals) arises because transmission systems must be designed in such a way that the transmitter emits and the receiver correctly receives the channel symbols with the lowest possible energy. The critical place in the transmission system, in which the energy of the channel symbols turns out to be the smallest, is the receiver input. The energy of the received symbol Ес here should be not less than a certain threshold value Епор, which is determined by a given threshold value of the signal-to-noise ratio (SNR) qthr.
Before the formation of the channel symbols, the bits to be transmitted are first converted into a sequence of positive and negative electrical pulses with a duration Tc of a rectangular shape, for which the notation v (t) is taken; the sequence of pulses obtained in this way is called a modulating signal. Conversion of a sequence of bits into a sequence of electrical pulses is carried out according to the following rule: 0 ≥ bv (t), 1 ≥ - bv (t), where b> 0 is the pulse amplitude. In this case, the modulating signal
(6.1)
In (6.1), the summation is carried out over all possible values of the index i, and the factor bi can take the values + b or - b.
Figure 6.1 shows examples of digital modulation signal realizations: a sequence of transmitted bits {аi}, a sequence of electrical pulses of a modulator of rectangular shape and different polarity {bi}, a sequence of electrical pulses of a demodulator {} restored in a receiver and a sequence of received bits {âi} ... Channel symbols are not shown here. The ˆ over the symbols indicates their ratings.
There are two important points to note:
- distortion of the signal shape;
- time delay when passing through the transmission channel.
Waveform distortion is caused by two factors:
• the presence of special devices in the transmitter and receiver
• formation and processing of electrical signals;
• the presence of interference in the transmission channel.
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