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Published online by Cambridge University Press:  28 February 2011

Farooq Khan
Affiliation:
Samsung Telecommunications America, Richardson, Texas
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Summary

The current 3G systems use a wideband code division multiple access (WCDMA) scheme within a 5 MHz bandwidth in both the downlink and the uplink. In WCDMA, multiple users potentially using different orthogonal Walsh codes are multiplexed on to the same carrier. In a WCDMA downlink (Node-B to UE link), the transmissions on different Walsh codes are orthogonal when they are received at the UE. This is due to the fact that the signal is transmitted from a fixed location (base station) on the downlink and all the Walsh codes are received synchronized. Therefore, in the absence of multi-paths, transmissions on different codes do not interfere with each other. However, in the presence of multi-path propagation, which is typical in cellular environments, the Walsh codes are no longer orthogonal and interfere with each other resulting in inter-user and/or inter-symbol interference (ISI). The multi-path interference can possibly be eliminated by using an advanced receiver such as linear minimum mean square error (LMMSE) receiver. However, this comes at the expense of significant increase in receiver complexity.

The multi-path interference problem of WCDMA escalates for larger bandwidths such as 10 and 20 MHz required by LTE for support of higher data rates. This is because chip rate increases for larger bandwidths and hence more multi-paths can be resolved due to shorter chip times. Note that LMMSE receiver complexity increases further for larger bandwidths due to increase of multi-path intensity. Another possibility is to employ multiple 5 MHz WCDMA carriers to support 10 and 20 MHz bandwidths.

Type
Chapter
Information
LTE for 4G Mobile Broadband
Air Interface Technologies and Performance
, pp. 20 - 69
Publisher: Cambridge University Press
Print publication year: 2009

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  • Downlink access
  • Farooq Khan
  • Book: LTE for 4G Mobile Broadband
  • Online publication: 28 February 2011
  • Chapter DOI: https://doi.org/10.1017/CBO9780511810336.004
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  • Downlink access
  • Farooq Khan
  • Book: LTE for 4G Mobile Broadband
  • Online publication: 28 February 2011
  • Chapter DOI: https://doi.org/10.1017/CBO9780511810336.004
Available formats
×

Save book to Google Drive

To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Google Drive.

  • Downlink access
  • Farooq Khan
  • Book: LTE for 4G Mobile Broadband
  • Online publication: 28 February 2011
  • Chapter DOI: https://doi.org/10.1017/CBO9780511810336.004
Available formats
×