Journal of Emerging Technologies in Web Intelligence Contents

118 JOURNAL OF EMERGING TECHNOLOGIES IN WEB INTELLIGENCE, VOL. 2, NO. 2, MAY 2010In an ideal software radio, Analog to Digitalconversion should occur near the RF end. This wouldrequire the following criteria’s to be met:• A very high or variable sampling rate to supportwide signals bandwidth.• Greater number of quantization bits to support ahigh dynamic range.• Operating bandwidth of several GHz to allowconversion of a signal over a wide range offrequencies.• Lesser distortion and wide dynamic range to allowrecovery of low level signals in the presence ofstrong interferers.• Economical components to meet the abovecriteria’s without consuming an excessive amountof power.Due to the limitations of the fabrication technology,few of the above mentioned criteria’s need to be tradedoff for an acceptable design solution for the softwareradio.D. Digital to Analog ConversionIn Software Radio, entire waveform and modulation isgenerated in digital domain and is then fed to Digital toAnalog Converter before final transmission. As shown in“Figure 6. , the translation of the signal from digital toanalog is performed via voltage mapping andreconstruction, where the digital signal is changed to acontinuous valued signal. The number of bits and thefrequency range of the DAC are the important factors asthey would determine the reconstruction of thecontinuous signal.In a traditional analog radio, the frequency synthesis isperformed via bulky devices like quartz crystal, inductor,capacitor, mechanical resonators etc. But differentfrequencies and waveforms can be generated in thedigital domain by Direct Digital Synthesis (DDS)techniques and it is fast replacing the analog devices asthey provide better accuracy, frequency can be changedwith software and is simple to implement.E. Digital Down ConversionA digital down converter (DDC) provides the linkbetween the analog RF front end and the digital basebandof a receiver. The received signal is usually sampled atmuch higher sampling rates than required to relax thespecifications of the anti-aliasing filter required. But toreduce the computational power required from DigitalSignal Processors (DSPs), the sampling rate isimmediately reduced by down converter. DSPrequirements are directly proportional to sampling rateand reducing sampling rates reduces CPU cycles requiredto perform the same digital signal processing algorithmsand thus saving significantly on cost and DSP powerconsumption.F. Digital Up ConversionA digital up converter (DUC) provides the linkbetween the digital base band and analog RF front endand is required on the transmitter end. The signal to betransmitted is generated by digital signal processing atlower sampling rates to reduce computations of DSP. Butbefore it can be fed to DAC for converting to analogsignal, it is up converted to higher sampling rate byDigital Up Converter to relax interpolation filterspecifications.G. Digital Signal ProcessingDSP is the brain behind all digital radio technology. ADSP core consists of an arithmetic logic unit (ALU),accumulator(s), multiply and accumulate MAC unit(s),data and the address buses. A DSP is designed to supporthigh performance, repetitive, numerically intensive tasksand very high I/O performance. Large accumulators inDSPs help reduce the precision problems. The digitalsignal processor performs all the functions of filtering,demodulating, generating the signal for transmissionusing various modulation techniques in software radio.DSPs also perform functions of data compression,encryption and special functions like speech recognition,image enhancement, neural networks for artificialintelligence etc. The three main categories of digitalhardware are ASICs (Application Specific IntegratedCircuit), FPGAs (Field Programmable Gate Array) andDSPs. A DSP represents the most generalized type ofhardware that can be programmed repeatedly using highlevel programming language to perform variousfunctions. An ASIC is the most specialized piece ofhardware and can be used only in the specific applicationfor which it has been designed. ASICs are generally usedwhen DSP has insufficient processing power and functionto be performed is fixed. The set-up of ASICs isimplemented on fixed silicon, which optimizes the speedand power consumption of the circuit.Figure 6. Digital to Analog Conversion© 2010 ACADEMY PUBLISHER