Διαφάνειες CDMA

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  • Spread spectrum (SS) RAKE WCDMA (Power Control) (Uplink downlink) WCDMA UMTSSpread Spectrum systems

  • Spread Spectrum The spread spectrum techniques: Direct sequence, DS (WCDMA, IS-95) Frequency Hopping (GSM) In direct sequence user bits are coded by a unique binary sequence (=Code).The bits of code are called chips. The chip rate (W) is typically much higher than the bit-rate (R). The signal spreading is achieved by modulating the data modulated signal a second time by wideband spreading signal The used code in WCDMA is pseudorandom sequence which is constructed by combining two codes: Orthogonal Variable Spreading Factor, OSVD-code Cell specific scrambling code

  • If Tx and Rx are using the same code which are synchronized, the received narrowband user data is amplified with the factor of W/R = Processing Gain. Other power component (=interference) coming to the receiver (other users, background noise) won't have PG Processing gain includes spreading gain and channel coding gain The processing gain is different for different services over 3G mobile network (voice, www-browsing, videophone) due to different R This means that the coverage area and capacity is different for different services Spread Spectrum

  • Spread Spectrum

  • Spread Spectrum

  • Spread spectrum

  • Spread Spectrum

  • multipath multipath. . . RAKE . (typically Rayleigh fading).

  • multipath (MF)

  • .

  • RAKE

  • RAKE

  • Multipath Channel

  • WCDMA

  • ( BS) # WCDMAN=-103 dBm=5 dBR=12.2 kbpsW=3.84 Mcps

  • WCDMA

  • Near-far

  • (PC) MS BS , . 3 :1) MS2) (TPC) ( of 1.5 KHz UMTS) (fast and slow fading) UL DL () (SIR)target MS / BS3) (SIR)target PC FER (RNC) (SIR)target FER/BER/BLER : MS UL DL

  • Fast closed loop PC (TPC)

  • Fast closed loop PC (TPC)

  • SIR. , FER ( ). FER SIRtarget SIR setpoint . (Uplink)

  • Intra-system handovers Intra-frequency handovers softer (MS is connected to two sectors of the same BS simultaneously) soft (MS is connected to two sectors of different BSs simultaneously) hard (MS is connected to only one sector at time. Causes short dealys) Inter-frequency handovers hard (MS is connected to only one sector at time working at different frequencies. Causes short dealys) between cell layers (from large macrocells to small microcells, for example) Inter-system handovers Handover WCDMA - GSM900/1800 Handover WCDMA - some other system as wellWCDMA Handover types

  • In Soft handover the mobile is connected to two or more base stations (or sectors in the case of softer HO) at the same time. This means that the same information flows through many BSs. The receiver combines these signals. The MS enters to soft handover state if the difference between measured pilot signals (or meas. pilot Ec/I0 to be exact) from several BSs are within the threshold value See figure: when the mobile is close to the cell border between BS1 and BS2 and transmits with a very high power it might interfere the neighboring cell (BS2). However, with the soft handover this mobile is connected also to BS 2 which allocates RAKE fingers for that mobile and collects all its signal energy. The mobile is also power controlled by both of BS.Soft/softer handover

  • + Decreases interference from one sector/BS to another Increases capacity Without soft/softer handover the interference power would be very high because the same frequency is used in adjacent cells+ Coverage gain due to diversity The signal to other BS can be temporarily very weak. Because of the soft/sofer handover branch the resulting signal can be relatively good Seamless handover No delays in the handover Requires additional resouces at the BS + additional signallingWhy use soft / softer handovers ?

  • Softer Handover softer handover is handled by BS internally softer handover probability about 5 - 15 % no extra transmissions between BSs needed basically same RAKE MRC processing as for multipath/antenna diversity (BS / S). More RAKE fingers needed. provides additional diversity gain softer handover does create additional interference and needs BS LPA resources

  • Softer handover probability about 20 - 40 % Extra transmissions needed UL / DL soft HO diversity processing very different MS: MRC RAKE combining RNC/BS: frame selectionSoft handover Except for the TPC symbol exactly the same information (symbols) is sent via air. Differential delay in order of fraction of symbol duration The combination point (UL) depends on the network architecture (RNC or BS)

  • Seamless coverage extension for CDMA with existing GSM network Capacity extension for GSM with load sharing between WCDMA and GSM Service control - different services to different networksIntersystem HandoverLoad reason handoverCoverage reason handoverService reason handover

  • UL receiver diversity Two antennas receive multipaths with different phases sum field (receive envelope) of the antennas are uncorrelated With diversity combining (maximum ratio combiningm MRC, in WCDMA) the total received SIR is maximised UL diversity decreses the received Eb/N0 and transmitted Eb/N0 better radio performance Additional antenna is just like additional multipath, so it needs additional RAKE finger

  • Space diversity is not applicable in MS because the separation between two MS antenna would be too small branches would be correlated The DL diversity has been implemented as Tx diversity Two methods: Open loop diveristy: two BS antennas have been coded differently and the decoder can combine the signals optimally closed loop diversity: the mobile asks BS to change the phase difference between Tx antennas to obtain optimum performance Gain against fast fading can be achieved, almost as much as in UL diversityDL receiver diversity

  • In CDMA coverage and capacity are tight together: When the number of users increases, the interference levels increases and therefore the needed powers in order to keep constant quality. Due to infinite power resources this means that the coverage decreases. This leads to Cell Breathing: the coverage area changes as the load of the cell changes Therefore, the coverage and the capacity has to plan simultaneously Effective control of cell breathing by radio resource management (RRM) is needed in WCDMACoverage and capacity planning