EE302 Lesson 21: Transmission of Binary Data in Communication Systems

Transmission schemes Before launching into digital modulation we need to provide an overview of transmission methods. 6 combinations are discussed.

1. Analog channel (no modulation) Simplest type communications system. The analog intelligence signal not modulated, but is transmitted in the baseband channel. Baseband means the signal is transmitted at its original base frequencies (voice 300 Hz – 3 kHz) Example: intercom system. Analog input Analog baseband channel Analog output

2. Standard analog modulation AM and FM modulation systems previously studied previously Example: AM/FM radio stations Analog input Analog channel Analog output Modulator Demodulator

3. Digital transmission on digital channel. Direct computer/computer or computer to peripheral communication. Example: serial, USB, parallel connections Digital input Digital channel Digital output Coder Decoder

4. Digital transmission on analog channel. Digital signal is converted to analog for transmission on analog channel. Example: internet connection via phone line Digital input Analog channel Digital output Modem Modem

5. Analog transmission on digital channel Analog signal (voice, music) is converted to digital (PCM encoder). Example: optical connection between CD player and amplifier Analog input Digital channel Analog output A/D and coder Decoder and D/A

6. Digitized analog signal transmission on analog channel Analog signal (voice, music) is convert to digital (PCM encoder). Example: Digital cell phone (GSM, CDMA) Analog input Analog channel Analog output A/D and coder Modem Modem Decoder and D/A

Basic Modem Concepts Digital data are transmitted over the telephone and cable television networks by using broadband communication techniques involving modulation, which are implemented by a modem, a device containing both a modulator and a demodulator. Modems convert binary signals to analog signals capable of being transmitted over telephone and cable TV lines and by radio, and then demodulate such analog signals, reconstructing the equivalent binary output.

Basic Modem Concepts There are four widely used modem types: Conventional analog dial-up modems. Digital subscriber line (DSL) modems. Cable TV modems. Wireless modems.

Basic Modem Concepts Figure 11-12: How modems permit digital data transmission on the telephone network.

Modem Modulation Types The four main types of modulation used in modern modems are: Frequency-shift keying (FSK) Primarily used at lower speeds (<500 kbps) and in a noisy environment. Phase-shift keying (PSK) Operates in narrower bandwidths over a wide range of speeds. Quadrature amplitude modulation (QAM) Very high data rates in narrow bandwidths. Orthogonal frequency division multiplexing (OFDM) Covered in Section 11-5.

Frequency-shift keying (FSK) Frequency-shift keying (FSK) is the oldest and simplest form of modulation used in modems. In FSK, two sine-wave frequencies are used to represent binary 0s and 1s. A binary 0 is usually called a space. A binary 1 is referred to as a mark. For example, a space has a frequency of 1070 Hz and a mark has a frequency of 1270 Hz. These two frequencies are alternately transmitted to create the serial binary data.

Frequency-shift keying (FSK) Figure 11-13: Frequency-shift keying. (a) Binary signal. (b) FSK signal.

Phase-shift keying (PSK) In phase-shift keying (PSK), the binary signal to be transmitted changes the phase of a sine-wave character, depending upon whether a binary 0 or binary 1 is to be transmitted. Binary Phase-shift keying (BPSK) uses a phase shift of 180°. During the time that a binary 0 occurs, the carrier is transmitted with one phase; when a binary 1 occurs, the carrier is transmitted with a 180° phase shift.

Binary phase-shift keying (BPSK) Figure 11-18: Binary phase-shift keying.

Quadrature phase-shift keying (QPSK) One way to increase the binary data rate while not increasing the bandwidth required for the signal transmission is to encode more than 1 bit per phase change. In the system known as quadrature PSK (QPSK or 4-PSK), more bits per baud are encoded. The bit rate of data transfer can be higher than the baud rate, yet the signal will not take up additional bandwidth. In QPSK, each pair of successive digital bits in the transmitted word is assigned a particular phase. Each pair of serial bits, called a dibit, is represented by a specific phase.

Quadrature phase-shift keying (QPSK) Figure 11-24: Quadrature PSK modulation. (a) Phase angle of carrier for different pairs of bits. (b) Phasor representation of carrier sine wave. (c) Constellation diagram of QPSK.

Quadrature amplitude modulation (QAM). One of the most popular modulation techniques used in modems for increasing the number of bits per baud is quadrature amplitude modulation (QAM). QAM uses both amplitude and phase modulation of a carrier. In 8-QAM, there are four possible phase shifts and two different carrier amplitudes. Eight different states can be transmitted. With eight states, 3 bits can be encoded for each baud or symbol transmitted. Each 3-bit binary word transmitted uses a different phase-amplitude combination.

Quadrature amplitude modulation (QAM). Figure 11-29: A constellation diagram of a 8-QAM signal.

Quadrature amplitude modulation (QAM). 0000 0 90 180 270 0100 1100 1000 0001 0101 1101 1001 0011 0111 1011 0010 0110 1110 1010 A constellation diagram of a 16-QAM signal. 4 bits can be encoded.

Quadrature amplitude modulation (QAM). As each symbol gets closer together, the signal becomes more susceptible to noise. 64-QAM and 256-QAM are used in digital cable television and cable modems.

Spectral Efficiency and Noise Spectral efficiency is a measure of how fast data can be transmitted in a given bandwidth (bps/Hz). Different modulation methods give different efficiencies. Modulation Spectral efficiency, bps/Hz FSK <1 BPSK 1 QPSK 2 8-PSK 3 16-QAM 4

Spectral Efficiency and Noise The signal-to-noise (S/N) ratio clearly influences the spectral efficiency. The greater the noise, the greater the number of bit errors. The number of errors that occur in a given time is called the bit error rate (BER). The BER is the ratio of the number of errors that occur to the number of bits that are transmitted in a one second interval.