1 Elements of a Transmission System • Input signal coming from transducer (source) • Transmitted signal suitable to the characteristics of the transmission channel (involves encoding and modulation) • Channel introduces some amount of attenuation and signal distorsion • In the channel noise is also generated which produce alterations of the signal shape • Receiver include filtering to take away out-of-band noise, amplification to compensate for channel attenuation, equalizing to compensate for distortion, demodulation and decoding Basic concept of transmission system. 118 2 Modulation Methods • Modulation: generate a signal suited to the characteristics of a transmission channel • The message, s(t), alters the amplitude, phase or frequency of the high-frequency carrier: – Amplitude: AM (amplitude modulation), A = A [s(t)] – Frequency: FM (frequency modulation), f = f [s(t)] – Phase: PM (phase modulation), φ = φ [s(t)] s(t) A cos (2 π f0 t + φ) 119 3 Amplitude modulation The original carrier wave has a constant peak value (amplitude) and it has a much higher frequency than the modulating signal, the message 120 4 Frequency modulation 121 5 Phase modulation In PM the instantaneous phase is varied linearly according to the message 122 Digital PM 6 In digital binary PM, called binary phase shift keying (BPSK), the phase of the carrier is varied according to whether the digital signal is “1” or “0”. We need only two carrier phases, which are chosen to be 0° for binary 0 and 180° for binary 1: When four carrier phases are used, each phase transmits the value of two binary bits and we talk about quadrature phase shift keying (QPSK). A pair of bits is taken from the incoming bit stream (110001101111...) of the modulator and the carrier phase is shifted according to the value of these two bits until the next two bits are received: 123 7 Constallation diagrams In the constellation diagram, the I axis refers to the in-phase carrier wave and Q stands for the quadrature carrier (90° phase shift). Each point represents one possible transmitted “symbol”, i.e. elementary waveform: for example, the bit combination “01” is sent as a carrier with a +90° phase shift in QPSK (also called 4-PSK). Using eight different phases as in 8-PSK (below, left), bit rate is three times that of BPSK (it is two times as BPSK in QPSK). The distance of points from origin is the same: envelope is constant and distance between points is lower. Lower noise levels cause errors in the receiver. We can combine AM and PM to get 16-QAM ( quadrature amplitude modulation ). Each of 16 different carrier waveforms (below, right), represent one combination of four bits. If the symbol rate is the same in 16 -QAM as in BPSK, the spectral width of the radio signal remains the same but the bit rate of 16-QAM is four times that of BPSK. 124 Constellation diagram of 8-PSK Constellation diagram of 16-QAM.