Digital Modulation: Pulse Shaping and Efficiency, Study notes of Electrical and Electronics Engineering

Download Digital Modulation: Pulse Shaping and Efficiency and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! Pulse shaping notes Digital modulation most often looks like either discrete phase or frequency shifts in a signal. Frequency shifts keying (FSK) compares fairly well to FM, except that the data is not an analog signal imposed on a carrier. Instead, it consists of a limited number of discrete frequency shifts. Section 6.4 Digital modulation offers significant benefits over analog modulation (AM & FM). It offers 1 greater noise immunity (you just have to get close to one of a limited set of discrete states), 2 greater security (public key encryption) 3 compression 4 FEC 5 equalization Digital modulation quality is generally measured in terms of two efficiencies that are readily compared in terms of higher order modulation. Higher order modulation involves subdividing the average available amplifier power into more than just two choices (+/-1). As an example, one could send three bits of information at a time with 23 energy levels, but they’d have to be closer together than with just two for the same average power. Thus, they’re more susceptible to noise. 1 power efficiency (Eb/N0) We’ve seen this a lot with our BER curves a. Higher-order mod is predictably more susceptible to noise and less power efficient than lower-order modulation b. We’ve seen power efficiency a lot and will return to it again later 2 bandwidth efficiency Rb/B a. Higher order modulation passes multiple bits per symbol, say 3 bits at a time when there are 23 available levels per symbol. Thus, the symbol rate goes down by a factor of three and the modulation uses less (3x less) bandwidth. But bandwidth is an elusive term. If you use the null-to-null definition (p 281), the bandwidth of BPSK is 2Rb. But adjacent side-lobes are only down 13 dB and leaves little room for margin in cell systems. The FCC uses 99% power confinement. To get that low requires 0.5% on either side of bandwidth edges or -23 dB. This is near -13 -6 - 6 or about three nulls on either side of the center frequency. Thus, the bandwidth could also be called 6Rb or more. That really eats into the number of users that you can support in a bandwidth.