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W-CDMA Principles

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The Spread Spectrum ConceptFig 1.

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Spreading – Direct SequenceFig 2.

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W-CDMA Key ParametersFig 3.

Chip rate = 3.84 Mcps

Bandwidth = 5Mhz

Frame Length = 10ms

Specified to utilise advanced receivers

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Access Methods for W-CDMAFig 4.

Harmonised Chip rates Frame lengths Modulation

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Spreading & Despreading

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Combining Data and Code Using “Exclusive OR”

Fig 5.

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Combining Data and Code by Multiplication

Fig 6.

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Spreading Multiple SignalsFig 7.

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Despreading Multiple SignalsFig 8.

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Processing Gain

1) Processing gain: Enhances wanted signal Spreads unwanted signal

2) Processing gain c/b or a/d

Fig 9.

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Processing Gain and ErrorsFig 10.

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Spreading Factors and CapacityFig 11.

Note: Noise level varies depending on which signal is being despread (As the wanted signal is despread, its contributions to the overall “system” noise level need not be considered with respect to the despread signal)

Capacity limited by Number of users Higher data rates

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Use of Codes in UMTS

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Use Of Codes in UMTSFig 12.

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Channelisation CodesFig 13.

Used to separate transmissions from a single source (from UE in uplink, from Node B in downlink)

Uplink code lengths: 4 to 256

Downlink code lengths: 4 to 512

Code lengths are 2N, derived using the OVSF scheme

Generate spreading

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Orthogonal Variable Spreading Factor (OVSF) Codes

Fig 14.

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Orthogonal Variable Spreading Factor (OVSF) Codes

Fig 14.

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Scrambling CodesFig 15.

Used to separate signals from different sources

Uplink: millions of codes

Downlink: limited to up to 512 codes

Spreading factor defines the number of channelisationcodes per scrambling code

Scrambling doesn’t change the chip rate

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Derivation of Scrambling CodesFig 16.

Pseudo Noise (PN) Sequences

Long codes are Gold Codes, truncated to 38400 chip lengths (in 10ms frames)– downlink– uplink

Short codes are S(2) Codes, length 256– uplink only

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Radio Issues

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Multipath DelayFig 17.

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Fast FadingFig 18.

Result is Fast fading 7.5cm = half wavelength

at 2GHz (equivalent to 0.00025µs)

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The Rake ReceiverFig 19.

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Near-Far EffectFig 20.

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Power ControlFig 21.

Uplink: counteract the near-far effect

Downlink: minimise interference with adjacent cells

Compensate for Fading

Two types: Open Loop & Closed Loop (with Outer Loop)

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Open Loop Power ControlFig 22.

Used on initial access to the Radio Access Network

Large separation of U/L and D/L frequencies

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Closed Loop and Outer Loop Power Control

Fig 23.

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Soft and Softer HandoverFig 24.

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Macro DiversityFig 25.

Also applies to softer handover(known as micro diversity)

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Cell BreathingFig 26.

More Users Higher Data Rates

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Improving Performance

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Multi-User DetectionFig 27.

* Includes rake reception and interference estimation

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Beam Forming AntennasFig 28.

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Use of Channels

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Channels