资源描述
Unit 7 Digital Modulation,Unit 7 Digital Modulation,Passage A Digital Modulation,Passage B Binary Modulated Bandpass Signaling,Passage C Multilevel Modulated Bandpass Signaling,Passage A Digital Modulation,1.Introduction,Modern communication systems use digital modulation techniques.Advancements in very large integration(VLSI)and digital signal processing(DSP)technology have made digital modulation less cost than analog transmission systems.Digital modulation offers many advantages over analog modulation.Some advantages include greater noise immunity and robustness to channel impairments,easier multiplexing of various forms of information(e.g.,voice,data,and video),and greater security.Furthermore,digital transmissions accommodate digital error-control codes that detect and/or correct transmission errors,and support complex signal conditioning and processing techniques such as source coding,encryption,and equalization to improve the performance of the overall communication link.,New multipurpose programmable digital signal processors have made it possible to implement digital modulators and demodulators completely in software.Instead of having a particular modern design permanently frozen as hardware,embedded software implementations now allow alterations and improvements without having to redesign or replace the modem.,In digital communication systems,the modulating signal(e.g.,message)may be represented as a time sequence of symbols or pulses,where each symbol has,m,finite states.Each symbol represents,n,bits of information,where,n,=log,2,m,bits/symbol.,1,Many digital modulation schemes are used in modern communication systems,and more and more are sure to be introduced.Some of these techniques have subtle differences between one another,and each technique belongs to a family of related modulation methods.,2.Factors That Influence the Choice of Digital Modulation,Several factors influence the choice of a digital modulation scheme.A desirable modulation scheme provides low bit error rates at low received signal-to-noise ratios,performs well in multipath and fading conditions,occupies a minimum of bandwidth,and is easy and cost-effective to implement.Existing modulation schemes do not simultaneously satisfy all of these requirements.Some modulation schemes are better in terms of the bit error rate performance,while others are better in terms of bandwidth efficiency.Depending on the demands of the particular application,tradeoffs are made when selection a digital modulation.,The performance of a modulation schemes is often measured in terms of its power efficiency and bandwidth efficiency.Power efficiency describes the ability of a modulation technique to preserve the fidelity of the digital message at low power levels.In a digital communication system,in order to increase noise immunity,it is necessary to increase the signal power.However,the amount by which the signal power should be increased to obtain a certain level of fidelity(i.e.,an acceptable bit error probability)depends on the particular type of modulation employed.The power efficiency,p,(sometimes called energy efficiency)of a digital modulation scheme is a measure of how favorably this tradeoff between fidelity and signal power is made,and is often expressed as the ratio of the signal energy per bit to noise power spectral density(,E,b,/,N,o,)required at the receiver input for a certain probability of error(say 10,-5,).,2,Bandwidth efficiency describes the ability of a modulation scheme to accommodate data within a limited bandwidth.In general,increasing the data rate implies decreasing the pulse-width of a digital symbol,which increases the bandwidth of the signal.Thus,there is an unavoidable relationship between data rate and bandwidth occupancy.However,some modulation schemes perform better than the others in making this trade-off.Bandwidth efficiency reflects how efficiency the allocated bandwidth is utilized and is defined as the ratio of the throughput data rate per hertz in a given bandwidth.If,R,is the data rate in bits per second,and,B,is the bandwidth occupied by the modulation RF signal,then bandwidth efficiency,B,is expressed as,(7.1),The system capacity of a digital communication system is directly related to the bandwidth efficiency of the modulation schemes,since a modulation with a greater value of,B,will transmit more data in a given spectrum allocation.,There is a fundamental upper bound on achievable bandwidth efficiency.Shannons channel coding theorem states that for an arbitrarily small probability of error,the maximum possible bandwidth efficiency is limited by the noise in the channel,and is given by the channel capacity formula.Note that Shannons bound applies for AWGN non-fading channels.,(7.2),Where,C,is the channel capacity(in bps),B,is the RF bandwidth,and,S,/,N,is the signal-to-noise ratio.,In the design of a digital communication system,very often there is a tradeoff between bandwidth efficiency and power efficiency
展开阅读全文