Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

136 Implementation IssuesHowever, in order to provide a better channel selectivity in the receiver regarding adjacentchannel interference, a higher sampling rate than the channel bandwidth might be used,i.e. f samp >N c /T s .4.1.4.2 I/Q GenerationAt least two methods exist for modulating and demodulating a carrier (I and Q generation)with a complex OFDM time signal. These are described below.Analogue quadrature methodThis is a conventional solution in which the in-phase carrier component I is fed by thereal part of the modulating signal and the quadrature component Q is fed by the imaginarypart of the modulating signal [70].The receiver applies the inverse operations using an I/Q demodulator (see Figure 4-6).This method has two drawbacks for an OFDM transmission, especially for large numbersof sub-carriers and high order modulation (e.g. 64-QAM): (a) due to imperfections inthe RF components, it is difficult at moderate complexity to avoid cross-talk betweenthe I and Q signals and, hence, to maintain an accurate amplitude and phase matchingbetween the I and Q components of the modulated carrier across the signal bandwidth; thisimperfection may result in high received baseband signal degradation, i.e. interference;and (b) it requires two A/D converters.A low cost front-end may result in I/Q mismatching, emanating from the gain mismatchbetween the I and Q signals and from nonperfect quadrature generation. These problemscan be solved in the digital domain.Digital FIR filtering methodThe second approach is based on employing digital techniques in order to shift the complextime domain signal up in frequency and produce a signal with no imaginary componentsthat is fed to a single modulator. Similarly, the receiver requires a single demodulator.However, the A/D converter has to work at double sampling frequency (see Figure 4-7).The received analogue signal can be written asr(t) = I(t) cos(πt/T samp ) + Q(t) sin(πt/T samp ), (4.12)Low pass filter A/D convertercos(.)LocaloscillatorSampling rate 1/T sampf csin(.)Low pass filterA/D converterIQN c -pointFFT(complexdomain)Figure 4-6Conventional I/Q generation with two analogue demodulators