Digital Modulation: Transmission and Reception of Digital Information - Prof. Mary Ingram, Study notes of Mass Communication

Download Digital Modulation: Transmission and Reception of Digital Information - Prof. Mary Ingram and more Study notes Mass Communication in PDF only on Docsity! 1 Introduction to Digital Modulation Instructor: M.A. Ingram ECE4823 Digital Communication Transmitter User Interface Encoder Modulator RF Front End antenna bits 0110 more bits 011001 waveforms (symbols) Frequency up-conversion and amplification A simplified block diagram: interleaver Digital Communications Receiver A simplified block diagram: User InterfaceDecoderDemodulator RF Front End antenna bits 0110 “soft detections” of coded bits 011001 waveforms (faded symbols plus noise) Low-noise amplification and frequency down-conversion deinterleaver Symbols In each symbol period, Ts, a digital modulator maps N coded bits word to a transmitted waveform from a set of M=2N possible waveforms Each waveform corresponds to an information symbol, xn For Binary symbols, N=1 Detection The job of the receiver is to determine which symbols were sent and to reconstruct the bit stream that created them Definitions Bit Rate (bits per sec or bps) Bandwidth Efficiency (bps/Hz) where B is the bandwidth occupied by the signal STNR /= BRB /=η 2 Shannon Theorem In a non-fading channel, the maximum bandwidth efficiency, or Shannon Capacity is SNR = signal-to-noise ratio ( )SNR MAXB += 1log 2η Pulses A symbol period, Ts, suggests a localization in time Localization in frequency is also necessary to enable frequency division multiplexing Regulatory agencies provide spectral masks to limit the distribution of power in the frequency domain Example: Linear Modulation s(t) is the output of the modulator g(t) is the complex envelope p(t) is the basic pulse xn is the nth symbol A is the amplification in the transmitter { } )()( )(Re)( 2 S n n tfj nTtpxAtg etgts c −= = ∑ π Rectangular Pulses Suppose p(t) is a rectangular pulse This pulse is not used in practice, but is OK for illustration ST0 t ST 2 )(tp Binary Phase Shift Keying (BPSK) For BPSK, each symbol carries one bit of information { }1,1−∈nx 0 ST t ST A 2 )(tg L ST2 ST A 2− 1 1 1 1000 BPSK-Modulated Carrier The information is in the phase of the carrier t ST A 2 )(ts L ST A 2− 1 1 1 1000