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

Future Mobile Communications Concepts and Field Trials 281Code(Asynchronous)TimeMulti-cell environment#7#6 #2#1#5#3#4user 2user 1Code(Synchronized)TimeFrequencyMC-DS-CDMASeamlessdeploymentusing the sameair interfaceFD-MC-DS-CDMAIsolated single-cellFrequencyuser 1 user 2 user 3Figure 5-47optionUplink transmission based on MC-DS-CDMA and with an FD-MC-DS-CDMAA code-multiplexed pilot structure is applied for channel estimation based on the principledescribed in the previous section. To combat the multiple access interference, a rakereceiver with an interference cancellation in conjunction with an adaptive array antennaat the base station is proposed. As shown in Figure 5-47, the capacity can be optimizedfor each cell configuration.In a multi-cell environment MC-DS-CDMA with complex interference cancellation atthe base station is used, where in a single-cell environment an orthogonal function inthe frequency (FD-MC-DS-CDMA) or time direction (TD-MC-DS-CDMA) is introducedinto MC-DS-CDMA. In addition, this approach allows a seamless deployment from amulti-cell to a single cell with the same air interface. The basic system parameters forthe uplink are summarized in Table 5-45.High order modulation like 16-QAM or 64-QAM is used even with no spreading in asingle cell with good reception conditions [10]. However, in a dense cellular system withhigh frequency selectivity and high interference the lowest order modulation QPSK withthe highest spreading factor will be deployed.In Figure 5-48, the throughput of an MC-DS-CDMA system in the uplink is shown [10].The throughput in Mbit/s versus the SNR per symbol in a Rayleigh fading channel isplotted. The system applies a spreading code length of L = 4, where three codes areused. Receive antenna diversity with two antennas is exploited. The channel estimationis realized with a code-multiplexed pilot channel within a frame. It can be observed fromFigure 5-48 that an average throughput over 20 Mbit/s can be achieved at an SNR ofabout 9 dB when using QPSK with rate 1/2 Turbo coding.