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WCDMA/UMTS Network

Advanced Mobile Communication

Course

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UMTS Evolution

DL: 384 kbps

UL: 384 kbps

DL: 14 Mbps

UL: 0.4 Mbps

DL: 14 MbpsUL: 5.7 Mbps

DL: 28 MbpsUL: 11 Mbps

16QAM UL64QAM DLMIMO 2x2

DL: 42 MbpsUL: 11 Mbps

MIMODual Carrier

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UMTS Network Architecture

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Allocated Bands

Main Band (in Europe and most of Asian Countries)

1920 - 1980MHz / 2110 - 2170MHz

Other allocations for 3G

1850 - 1910MHz / 1930 MHz - 1990MHz (USA)1710 - 1785MHz / 1805 1880MHz (Japan)

890 - 915MHz / 935 - 960MHz (Australia)

UTRA Absolute Frequency Channel Number (UARFCN) Central Frequency 5

UL frequency channel number 9612-9888DL frequency channel number : 10562-10838

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DS CDMA Multiple Access and Duplexing

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Spreading and De-spreading

Direct Sequence Spread Spectrum (DS-CDMA) i.e. bandwidth of user data

increased after being multiplied by the spreading sequence.

Spreading sequence bit rate = 3.84 Mchips/sec. 5 MHz carrier.

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Spreading and De-spreading

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Basic Aspects of DS-CDMA

= Chip Rate/Bit Rate

After the signal is spread and modulated, all the users occupy the whole 5 MHz band andthus contribute to the overall noise at the receiver.

A scenario can arise that may cause a UE with low pathloss jamming the overall cell. This isknown as near far effect. The solutions to this problem are Fast Power Control andSoft/Softer Handover.

Coverage and Capacity are related.

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Wideband CDMA Advantages

Increased multiple MS capability.

Good wideband interference resistance.

Good narrowband interference resistance.

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In Downlink, all connections share the same power amplifier. In a lightly loaded system, a UE relatively farfrom BTS may be able to connect. However in a heavily loaded system, the UE at the cell fringe may not beable to connect due to unavailability of DL power.

In the uplink, more the users, more will the noise floor will be raised, thus limiting the uplink coverage.

Therefore the cell is planned for certain system loading.

Cell breathing

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Power Control

In order to avoid scenarios such as Near-Far effect and optimizing the capacity, received power at the BSfrom all the UEs should ideally be equal.

In uplink, UEs power is controlled by the BTS. BTS continuously estimates the received Signal toInterference Ratio (SIR) and compares it to the target SIR to increase/decrease the UE output power. Fastpower control (with power control commands 1500 times per sec) is implemented in WCDMA.

In downlink, there is no near-far problem. However power control is required in downlink to;Conserve DL Tx power.More power required for low speed mobiles as coding does not work effectively.

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Power Control Outer Loop

Due to varying radio conditions at the UE side the SIR Target needs to be adjusted, so as to keep theinterference level at the BTS under desired limits. This requires further optimization of the UE Tx power.

Outer loop PC adjusts the target SIR at the BTS for an individual radio link aiming at constant quality(defined in terms of BER and BLER).

BTS reports the results of frame decoding to the RNC, which based on these measurements adjusts thetarget SIR at the BTS.

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Soft Handover

Without Soft HO there would be near far scenarios of a UE penetrating from one cell

deeply into the other, without being power controlled.

Very fast and frequent handovers could largely avoid this problem. But there will bedelays incurred causing the near-far problem could be developed.

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Soft Handover along with Power Control are powerful interference mitigating tools.

Soft Handover

Uplink Power Control in Soft Handover state Downlink Power Control in Soft Handover state

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Multipath and RAKE receiver

Arrival of signal energy at distinct points of time due to multipath components. Energy is smeared intomultipath profile.

Fast fading due to destructive interference of multipath components.

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To counter the effects of multipath, RAKE receiver isemployed.

o Delay Dispersive Energy is combined usingmultiple fingers of the RAKE.

o Fast power control and the diversity reception areused to mitigate fast fading problem.

o Strong coding helps signal recovery in deep fades.

Assign individual RAKE finger to a multipath componentwith significant energy.

Within each finger, track the fast-changing phase andamplitude values originating from fast fading.

Combine the demodulated and phase-adjusted symbolsacross all fingers.

RAKE receiver

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Spreading and De- spreading Orthogonal Codes

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Spreading and De- spreading Orthogonal Codes

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Spreading and De- spreading Orthogonal Codes

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Orthogonal Variable Spreading Factor (OVSF) Codes

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OVSF Code Tree

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OVSF Code Tree Usage

Channelization code is used to distinguishdifferent physical channels of one transmitter

For downlink, channelization code( OVSF code ) is used to separate

different physical channels of one cell.

For uplink, channelization code ( OVSFcode ) is used to separate differentphysical channels of one UE.

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Similar OVSF code tree is used by base stations and UEs for channelization in the respectivetransmitters. This makes them unsuitable for multiple access (i.e. for identification oftransmitting entities).

Additional step known as scrambling is added in overall spreading process.

Every 10msec, radio frame containing 15 x 2560 = 38,400 chips are XORed with scramblingcode of length 38,400. Thus, scrambling does not alter the signal bandwidth.

Psuedo-Random Scrambling Codes

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Scrambling Codes

Scrambling code is used to distinguish different transmitters;

For downlink, scrambling code is used to separate differentcells. Out of available 8192 scrambling codes, 512 are usedin practical implementations.

For uplink, scrambling code is used to separate different UEs.A total of 2 24 scrambling codes are available in the uplink.Scrambling code in the uplink are assigned by RNC.

Scrambling codes are required to have lowautocorrelation as well as low cross correlation.

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Downlink Transmission in a Cell

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Uplink Transmission on a Cell