|Series:||The Springer International Series in Engineering and Computer Science , #487|
|Edition description:||Softcover reprint of the original 1st ed. 1999|
|Product dimensions:||6.10(w) x 9.25(h) x 0.03(d)|
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Chapter 7: Detection Strategies and Cancellation Schemes In A MC-CDMA SystemWe have to notice that in MC-CDMA systems it is most important to have frequency non-selective fading over each subcarrier and this implies that each modulated subcarrier does not experience significant dispersion. In some other MCschemes it is therefore sometimes necessary to convert the original high data rate signal from serial to parallel before spreading over the frequency domain, in order to prevent frequency selective fading. This method is called MC-DS-CDMA but it is not examined here. In our MC-CDMA system a proper choice of the number of subcarriers and guard interval is important in order to increase the robustness against frequency selective fading. There exists an optimal value in the number of subcarriers and the duration of the guard interval to minimise the bit error probability . If we apply perfectly orthogonal Walsh-Hadamard code (WH-code) sequences over a linear, time invariant, frequency non-selective channel with perfect chip-synchronised transmission, the performance of DS-CDMA and MC-CDMA is equivalent, as the orthogonal multi-user interference completely vanishes . In practice, multipath channels are less ideal and channel dispersion erodes the orthogonality of CDMA signals.
7.3 Multiuser Interface In CDMA Systems
In contrast to FDMA and TDMA techniques which are frequency bandwidth limited, CDMA systems are interference limited. In CDMA systems, each user's data is spread by a unique pseudorandom code. All users then transmit in the same frequency band and are distinguished at the receiver by the user specific spreading code. All other signals are notdespread because they use different codes. These signals appear as interference to the desired user because of non-zero cross-correlation values between the spreading codes. As the number of users increases, the signal to interference ratio (SIR) decreases until the resulting performance is no longer acceptable: Thus, this multi-user interference must be reduced to achieve higher capacities and that is strictly connected to the cross-correlation factor between different users. We can reduce cross-correlation in spread spectrum systems by:
- Spreading the signal by orthogonal codes which have zero cross-correlation. This technique is very efficient in downlink transmission, because a base station can transmit to all users simultaneously and the signals are spread synchronously at chip level. Transmitting asynchronously in the uplink, to restore the orthogonality of the codes, the mobile users can be time-aligned by a synchronisation method.
- Cancellation schemes that usually work subtracting the interference caused by other users and require a significant processing power; they are very useful especially to solve near-far problems.
More in general other important techniques like power control schemes, voice activity detection or antenna sectorization are used to minimise the performance degradation caused by the total co-channel interference (intra and inter-cell) from other users.
7.4 Synchronisation In a System
Synchronisation is one of the most determinant parameters to reduce multi-user interference. The main issue of synchronous MC-CDMA is timing. For the downlink, timing is not a problem because there is only one station that generates the codes . The uplink is much more complicated, because different transmitters have different clock offsets and propagation delays, so uplink synchronisation seems to be difficult to establish when we face a fading multipath mobile radio channel. In an ideal radio channel the perfectly orthogonal spreading codes maintain orthogonality only if the codes remain strictly synchronised. Due to multipath propagation the orthogonality is partly lost; the delayed multipath signals are clearly not synchronised to the first arriving path and besides the dominant components are time-shifted for the different propagation delays. Because some synchronicity error exists, special orthogonal codes with small cross-correlation must be used. A great advantage of MC-CDMA is its relatively long symbol duration that could allow to establish a quasi-synchronous channel state.
The synchronisation offsets should be a small fraction of the chip period if we want to minimise multi-user interference by orthogonal signals. Increasing the chip period would make it easier to synchronise the signals. However, for a given processing gain, an increase in the chip period results in an increase in the symbol period, hence a decrease in the data rate; the solution for such a problem is the adoption of a multicarrier signalling scheme able to maintain the original data rate. Increasing the chip period by a factor M, the number of subcarriers, reduces the bandwidth of each of the subband signals by a factor M relative to the bandwidth of the original spread spectrum signal so that the overall bandwidth is approximately the same as that of the single-carrier signal...
Table of ContentsPreface. 1. Spreading Techniques, a Far-Reaching Technology; P.W. Baier, et al. 2. A Linear Model for CDMA Signals Received with Multiple Antennas over Multipath Fading Channels; L.K. Rasmussen, et al. 3. Antenna Arrays for Cellular CDMA Systems; P.M. Grant, et al. 4. Spatial Filtering and CDMA; M.P. Lötter, et al. 5. Topics in CDMA Multiuser Signal Separation; Y. Bar-Ness. 6. LMMSE Receivers for DS-CDMA Systems in Frequency-Selective Fading Channels; M. Latva-aho. 7. Detection Strategies and Cancellation Schemes in a MC-CDMA System; F. Kleer, et al. 8. Coding vs. Spreading over Block Fading Channels; E. Biglieri, et al. 9. Turbo-Codes for Future Mobile Radio Applications; P. Jung, et al. 10. Software Radio Receivers; T. Hentschel, G. Fettweis. 11. Blind Space-Time Receivers for CDMA Communications; D. Gesbert, et al. References. Index.