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

336 Additional Techniques for Capacity and Flexibility Enhancement(PDUs) from the DLC layer are submitted to the baseband processing unit, consistingmainly of FEC encoder, mapper, spreader, and multi-carrier (i.e. OFDM) modulator. Afterdigital I/Q generation (digital IF unit), the signal can be directly up-converted to the RFanalogue signal, or it may have an analogue IF stage. Note that the digital I/Q generationhas the advantage that only one converter is needed. In addition, this avoids problems ofI and Q sampling mismatch. Finally, the transmitted analogue signal is amplified, filtered,and tuned by the local oscillator to the radio frequency and submitted to the Tx antenna.An RF decoupler is used to separate the Tx and Rx signals.Similarly, the receiver functions, being the inverse of the transmitter functions (but morecomplex), are performed. In case of an analogue IF unit, it is shown in Reference [27]that the filter dimensioning and sampling rate are crucial to support several standards.The sampling rate is related to the selected wideband analogue signal, e.g. in the case ofdirect down-conversion [26]. However, the A/D resolution depends on many parameters:(a) the ratio between the narrowest and the largest selected channel bandwidths, (b) theused modulation, (c) the needed dynamic for different power levels, and (d) the receiverdegradation tolerance.As an example, the set of parameters that might be configured by the controller givenin Figure 6-37 could be:– higher layer connection parameters (e.g. port, services);– DLC, MAC, multiple access parameters (QoS, framing, pilot/reference, burst formattingand radio link parameters);– ARQ/FEC (CRC, convolutional, block, Turbo, STC, SFC);– modulation (M-QAM, M-PSK, MSK) and constellation mapping (Gray, set partitioning,pragmatic approach);– spreading codes (one- or two-dimensional spreading codes, spreading factors);– multi-carrier transmission, i.e. OFDM (FFT size, guard time, guard band);– A/D, sampling rate, and resolution;– channel selection;– detection scheme (single- or multi-user detection);– diversity configuration;– duplex scheme (FDD, TDD).Hence, SDR offers elegant solutions to accommodate various modulation constellations,coding, and multi-access schemes. Besides its flexibility, it also has the potential ofreducing the cost of introducing new technologies supporting sophisticated future signalprocessing functions.However, the main limitations of the current technologies employed in SDR are:– A/D and D/A conversion (dynamic and sampling rate);– power consumption and power dissipation;– speed of programmable components; and– cost.The future progress in A/D conversion will have an important impact on the furtherdevelopment of SDR architectures. A high A/D sampling rate and resolution, i.e. high