Book contents
- Frontmatter
- Contents
- Preface
- 1 Introduction
- 2 Network architecture and protocols
- 3 Downlink access
- 4 Single-carrier FDMA
- 5 Reducing uplink signal peakiness
- 6 Transmit diversity
- 7 MIMO spatial multiplexing
- 8 Channel structure and bandwidths
- 9 Cell search and reference signals
- 10 Random access
- 11 Channel coding
- 12 Scheduling, link adaptation and hybrid ARQ
- 13 Power control
- 14 Uplink control signaling
- 15 Downlink control signaling
- 16 Inter-cell interference control
- 17 Single frequency network broadcast
- 18 Spatial channel model
- 19 LTE performance verification
- Index
3 - Downlink access
Published online by Cambridge University Press: 28 February 2011
- Frontmatter
- Contents
- Preface
- 1 Introduction
- 2 Network architecture and protocols
- 3 Downlink access
- 4 Single-carrier FDMA
- 5 Reducing uplink signal peakiness
- 6 Transmit diversity
- 7 MIMO spatial multiplexing
- 8 Channel structure and bandwidths
- 9 Cell search and reference signals
- 10 Random access
- 11 Channel coding
- 12 Scheduling, link adaptation and hybrid ARQ
- 13 Power control
- 14 Uplink control signaling
- 15 Downlink control signaling
- 16 Inter-cell interference control
- 17 Single frequency network broadcast
- 18 Spatial channel model
- 19 LTE performance verification
- Index
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 BroadbandAir Interface Technologies and Performance, pp. 20 - 69Publisher: Cambridge University PressPrint publication year: 2009
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