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

290 Applicationssignal while the time synchronization results from the use of MAC management packetsconveyed through the broadcast channel. Furthermore, the so-called periodic rangingsignals are transmitted from the base station to individual terminal stations for timingmisalignment adjustment and power control purposes.The DVB-RCT OFDMA based system employs either 1024 (1 k) or 2048 (2 k) subcarriersand operates as follows:– Each terminal station transmits one or several low bit rate modulated sub-carrierstoward the base station.– The sub-carriers are frequency locked and power ranged and the timing of the modulationis synchronized by the base station. In other words, the terminal stations derivetheir system clock from the DVB-T downstream. Accordingly, the transmission modeparameters are fixed in a strict relationship with the DVB-T downstream.– On the reception side, the uplink signal is demodulated, using an FFT process, likethe one performed in a DVB-T receiver.5.6.2 Channel CharacteristicsAs in the downlink terrestrial channel, the return channels suffer especially from highmulti-path propagation delays.In the DVB-RCT system, the downlink interaction data and the uplink interactive dataare transmitted in the same radio frequency bands, i.e. VHF/UHF bands III, IV, and V.Hence, the DVB-T and DVB-RCT systems may form a bi-directional FDD communicationsystem that shares the same frequency bands with sufficient duplex spacing. Thus, it ispossible to benefit from common features in regard to the RF devices and parameters (e.g.antenna, combiner, propagation conditions). The return channel (RCT) can also be locatedin any free segment of an RF channel, taking into account existing national and regionalanalogue television assignments, interference risks, and future allocations for DVB-T.5.6.3 Multi-Carrier Uplink TransmissionThe method used to organize the DVB-RCT channel is inspired by the DVB-T standard.The DVB-RCT RF channel provides a grid of time–frequency slots, each slot usable byany terminal station. Hence, the concept of the DVB uplink channel allocation is basedon a combination of OFDMA with TDMA. Thus, the uplink is divided into a number oftime slots. Each time slot is divided in the frequency domain into groups of sub-carriers,referred to as sub-channels. The MAC layer controls the assignment of sub-channels andtime slots by resource requests and grant messages.The DVB-RCT standard provides two types of sub-carrier shaping, of which only onewill be used at a certain time. The shaping functions are:– Nyquist shaping in the time domain on each sub-carrier to provide immunity againstboth ICI and ISI. A square root raised cosine pulse with a roll-off factor α = 0.25is employed. The total symbol duration is 1.25 times the inverse of the sub-carrierspacing.– Rectangular shaping with guard interval T g that has a possible value of T s /4,T s /8,T s /16,T s /32, where T s is the useful symbol duration (without guard time).