Modulation Techniques for Mobile Radio

Modulation Techniques for Mobile Radio

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  Modulation Techniques forMobile Radio UNIT-3
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  What is modulationModulation is the process of encoding information from a message source in a manner suitable for transmission It involves translating a baseband message signal to a bandpass signal at frequencies that are very high compared to the baseband frequency. Baseband signal is called modulating signal Bandpass signal is called modulated signal
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  Modulation Techniques Modulationcan be done by varying the Amplitude Phase , or Frequency of a high frequency carrier in accordance with the amplitude of the message signal. Demodulation is the inverse operation: extracting the baseband message from the carrier so that it may be processed at the receiver.
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  Analog/Digital Modulation AnalogModulation The input is continues signal Used in first generation mobile radio systems such as AMPS in USA. Digital Modulation The input is time sequence of symbols or pulses. Are used in current and future mobile radio systems
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  Goal of ModulationTechniques Modulation is difficult task given the hostile mobile radio channels Small-scale fading and multipath conditions. The goal of a modulation scheme is: Transport the message signal through the radio channel with best possible quality Occupy least amount of radio (RF) spectrum .
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  Digital Modulation Theinput is discrete signals Time sequence of pulses or symbols Offers many advantages Robustness to channel impairments Easier multiplexing of variuous sources of information: voice, data, video. Can accommodate digital error-control codes Enables encryption of the transferred signals More secure link
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  Digital Modulation Themodulating signal is respresented as a time-sequence of symbols or pulses. Each symbol has m finite states: That means each symbol carries n bits of information where n = log 2 m bits/symbol. ... 0 1 2 3 T One symbol (has m states – voltage levels ) ( represents n = log 2 m bits of information ) Modulator
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  Factors that InfluenceChoice of Digital Modulation Techniques A desired modulation scheme Provides low bit-error rates at low SNRs Power efficiecny Performs well in multipath and fading conditions Occupies minimum RF channel bandwidth Bandwidth efficieny Is easy and cost-effective to implement Depending on the demands of a particular system or application, tradeoffs are made when selecting a digital modulation scheme.
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  Power Efficiency ofModulation Power efficiency is the ability of the modulation technique to preserve fidelity of the message at low power levels . Usually in order to obtain good fidelity, the signal power needs to be increased. Tradeoff between fidelity and signal power Power efficiency describes how efficient this tradeoff is made E b : signal energy per bit N 0 : noise power spectral density PER : probability of error
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  Bandwidth Efficiency ofModulation Ability of a modulation scheme to accommodate data within a limited bandwidth. Bandwidth efficiency reflect how efficiently the allocated bandwidth is utilized R: the data rate (bps) B: bandwidth occupied by the modulated RF signal
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  Linear Modulation TechniquesClassify digital modulation techniques as: Linear The amplitude of the transmitted signal varies linearly with the modulating digital signal , m(t). They usually do not have constant envelope. More spectral efficient. Poor power efficiency Example: QPSK. Non-linear
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  Binary Phase ShiftKeying Use alternative sine wave phase to encode bits Phases are separated by 180 degrees. Simple to implement, inefficient use of bandwidth. Very robust, used extensively in satellite communication . Q 0 State 1 State
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  BPSK Example DataCarrier Carrier+  BPSK waveform 1 1 0 1 0 1
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  Quadrature Phase ShiftKeying Multilevel Modulation Technique: 2 bits per symbol More spectrally efficient, more complex receiver. Two times more bandwidth efficient than BPSK Q 11 State 00 State 10 State 01 State Phase of Carrier:  /4, 2  /4, 5  /4, 7  /4
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  4 different waveforms11 01 00 10 cos+sin -cos+sin cos-sin -cos-sin
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  Constant Envelope ModulationAmplitude of the carrier is constant, regardless of the variation in the modulating signal Better immunity to fluctuations due to fading. Better random noise immunity Power efficient They occupy larger bandwidth
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  Frequency Shift Keying(FSK) The frequency of the carrier is changed according to the message state (high (1) or low (0)). Continues FSK Integral of m(x) is continues.
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  FSK Example 1 1 0 1 Data FSK Signal
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  SPREAD SPECTRUM MODULATIONTECHNIQUES