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(b) Figure 10.13. OFDMA timing offset requirement with respect to ISI: (a) timing offset causing ISI (max(m1, m2) > Ncyclic - Lp); (b) timing offsets with no ISI (max(m1, m2) Ncyclic - Lp).
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nal spreading sequence to ensure good performance. On the other hand, for OFDMA-based systems, there is a exible tradeoff between the timing synchronization requirement of the K uplink users and the guard period allocated per OFDM symbols Ncyclic. Hence, we can always relax the timing synchronization requirement by increasing Ncyclic at the expense of higher overhead. 10.5.4 Effect of Frequency Offsets
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To consider the effect of the residual frequency offsets on system performance, we expand Equation (10.54) as follows:
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Hence, we can easily see that the frequency offsets from the K users 1, . . . , K will contribute to the multiuser interference in the DFT outputs. In the special case when 2 = 3 = . . . = K = 0, we have G(0) = I and F1D(k) (mk) = 0|B1| Lp. m Hence, the DFT outputs Y(1) for user 1 is free from the multiuser interference m terms and can be expressed as j 2p 1 mNT ( Ym1) = exp A(m 1 , nf where A(m 1 ,
1 1 ) = F1G ( 1 )D(m) (m1 ) 1
)h 1 + F1w m
In practice, the major contribution of the frequency offset is due to the local oscillators at the mobile transmitters and the base station receiver. Since the uplink and downlink offsets of the local oscillators are the same,4 the uplink frequency offset synchronization problem can be simpli ed by utilizing the downlink pilot. All the K mobiles estimate the combined frequency offsets with reference to a common pilot channel from the base station. Each mobile compensates for the frequency offsets in the uplink transmission on the basis of the downlink estimate. Typical residual frequency offset (normalized with respect to the subcarrier spacing fs) after compensation is of the order of 10-3. Hence, the frequency offset effect can be mitigated. Besides, from Equation (10.56), we can deduce that assigning a group of subcarriers to a user or introducing some empty subcarriers between band groups can help to enhance the robustness of the system with respect to multiuser interference due to frequency offsets. For example, the subcarrier allocation for user 1 and user 2 can be B1 = {1, 2, . . . , 32} and B2 = {34, 35, . . . , 40}. 10.6 SUMMARY
In this chapter,we have elaborated on the cross-layer design for wideband transmissions over frequency-selective fading channels. When the transmit bandwidth of the signal is much larger than the channel coherence bandwidth, there will be multiple distinct echos (frequency selective fading) in the receiver input, and this will result in ISI.We discuss two common and effective physical layer designs the DS-CDMA/MISO and OFDMA/MISO systems to combat frequency-selective fading channels. We consider a base
In typical transceiver design, the transmit and receive paths are clocked by the same oscillator.
station with nT transmit antennas and K mobiles (each with a single receive antenna). For DS-CDMA/MISO systems, information from the users is spread across both the temporal and spatial domains by a spreading matrix. As a result of the spreading action, the signals from a user occupies the entire wide transmission bandwidth and therefore achieves high-frequency diversity order (given by the number of resolvable multipaths Lp). It is shown that for suf ciently large spreading factor NSF and suf ciently large transmission bandwidth, the maximum data rate (in terms of Shannon s capacity) can be decomposed into nT independent scalar channels, and the capacity value approaches the ergodic capacity. In other words, the frequency diversity suppresses uctuations of user data rate and therefore does not allow multiuser diversity. On the other hand, OFDMA is the multiaccess extension of the regular OFDM systems where a user can be assigned a subset of the subcarriers depending on the instantaneous CSI. Together with the nT transmit antennas, there are nT spatial channels and nf frequency channels as system resources that can be assigned to some of the K mobile users. In other words, together with the cross-layer scheduling algorithm, both the nT spatial channels and the nf frequency channels contribute to the multiuser selection diversity gain. The optimal cross-layer scheduling algorithm for OFDMA/MISO is formulated, and the optimal solution is obtained. Because of the high computational complexity involved, we use some suboptimal search algorithms (such as the greedy algorithm and the genetic algorithm used in 6) to obtain the multiuser performance. The performance is compared with the DSCDMA/MISO systems, and it is found that signi cant gains of the OFDMA/MISO systems can be obtained because of the expanded dimension of multiuser diversity. From this, we conclude that combatting the fading from a point-to-point level is much less effective than that based on multiuser selection. In the context of multiuser performance, the physical layer should not suppress the uctuation of user data rate. Finally, we elaborate on the practical implementation issues of OFDMA systems with respect to the time and frequency synchronization requirements. Timing and frequency offsets of OFDMA-based systems in the uplink direction can be quite complicated because different users may have different residual timing and frequency offsets due to the variation in the propagation delay, transmitter frequency offset and mobile speed. The timing offsets among users may contribute to ISI between consecutive OFDM symbols if the maximum residual timing offsets among the K active users exceed the guard period NcyclicT0. On the other hand, if the maximum timing offset is within the guard period, the ISI can be completely eliminated because the timing offset is equivalent to a linear phase shift at the DFT outputs. On the other hand, frequency offsets among the K users will contribute to multiuser interference. Since different users may have different timing offsets and frequency offsets, it is not possible to apply correction at the base station receiver. Instead, the base station needs to estimate the frequency offsets and timing offsets for all the K users and instruct the individual mobile transmitters to compensate for these offsets.