?e????? Spread spectrum (SS)

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Spread Spectrum systems

• ?e????? Spread spectrum (SS)
• ???t?? RAKE
• ????t???t?ta WCDMA
• ??e???? ?s???? (Power Control)
• ??af????? ?et?d?s? (Uplink ?a? downlink)
• ?etap?µp?? st? WCDMA
• ??µ? d??t??? UMTS

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?e????? 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
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?e????? Spread Spectrum
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
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?e????? Spread Spectrum
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?e????? Spread Spectrum
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?e????? Spread spectrum
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?e????? Spread Spectrum
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??a?e??e?? se µ?a d?ad??µ? multipath

• Fa???µe?? multipath.
• ??????a µe t?? f?se?? t?? s???st?s?? ?
  s???staµ??? e?te e??s??eta? e?te e?as?e?e?. ??t?
  p???a?e? t?? e????a t?? d?a?e??e?? st??
  pe??ß?????sa t?? ?aµßa??µ???? s?µat??.
• St?? d??t? RAKE ???e d?ad??µ? pe???aµß??e? p?????
  s???st?se? s?µat??. ???e d?ad??µ? ?e??e?ta? ?t?
  ?f?stata? d?a?e??e?? a?e???t?te? ap? t?? ???p??
  d?ad??µ??(typically Rayleigh fading).

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?a??st???s? ??ad??µ??

• ?? fa???µe?? multipath p???a?e? p????? ????f??
  st?? ???d? t?? p??sa?µ?sµ???? f??t??? (MF) t??
  d??t?
• ??t?st???e? µ?a d?ad??µ? se ???e ????f?

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S???????sµ??
???sa?µ?sµ??? f??t?? ???t? G?a ?a s?µpt???e? t?
?aµßa??µe?? s?µa µe t?? ?at?????? ??d??a p??pe?
?a ?p???e? s???????sµ??. ??t?? a????e?eta? µe t?
?at?????? ???sa?µ?sµ??? f??t??
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???t?? RAKE
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???t?? RAKE
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Multipath Channel
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????t???t?ta WCDMA
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????t???t?ta WCDMA
?s??? ????? (e??? ???st? st?? BS) s??a?t?se? t??
???st?? st? ??????
N-103 dBm ?5 dB R12.2 kbps W3.84 Mcps
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?et?d?s? WCDMA
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??e???? ?s???? ?? p??ß??µa Near-far
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??????? ???µ?s?? ?s????
? ??e???? ?s???? (PC) d?asfa???e? p?? ???e MS ?a?
BS e?? e?p?µp??? a??et? ?s?? ??a t?? µet?d?s? t??
p????f???a?, p???a???? t?? e????st? d??at?
pa?eµß??? st??? ???p??? ???ste?. ?p?????? 3
d?af??et???? ??e???? 1) ??e???? ?????t?? ß??????
??a a????? ???µ?s? ?s???? t?? MS 2) ??e????
??e?st?? ß?????? (TPC) (µe s????t?ta of 1.5 KHz
st? UMTS) ??t?sta?µ??e? t?? d?a?e??e?? (fast
and slow fading) ???s?µ?p??e?ta? t?s? st? UL
?s? ?a? st? DL T?te? µ?a (sta?e??) t?µ? st????
p???t?ta? (SIR)target sta MS / BS 3) ??e????
e??te????? ß?????? T?te? t? (SIR)target t?? PC
a????t?? ß?????? ß?se? t?? FER p?? a????e?eta?
st?? ??e??t? t?? ???t??? (RNC) ??t?sta?µ??e?
µetaß???? st?? s?????e? µet?d?s?? ???µ??e? t??
t?µ? t?? (SIR)target ??a t?? ep?te??? t?µ??
st???? ??a ta FER/BER/BLER a?????a µe t??
ta??t?ta t?? MS ?a? t?? d?a??s?µ?
p???d?ad??µ??? d?af???s?µ?t?ta ???s?µ?p??e?ta?
t?s? st? UL ?s? ?a? st? DL
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??e???? ??e?st?? ??????? Fast closed loop PC (TPC)
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??e???? ??e?st?? ??????? Fast closed loop PC (TPC)
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??e???? ???te????? ??????? (Uplink)

• ? ??e???? ??e?st?? ß?????? p??spa?e? ?a
  d?at???se? sta?e?? SIR. ??t? ?µ?? de?
  d?asfa???e?, t? apa?t??µe?? FER (p?? µp??e? ?a
  ?e????e? sa? t? ???t???? p???t?ta? t?? ?e????).
• ?p?µ???? ? p????f???a ??a t? FER µetad?deta?
  st?? ??e??? e??te????? ß?????? p?? ???µ??e? t?
  SIRtarget ? SIR setpoint s?µf??a µe a?t??.

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WCDMA Handover types
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 well
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Soft/softer handover
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.
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Why use soft / softer handovers ?
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 signalling
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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
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Soft handover
Softer handover probability about 20 - 40
Extra transmissions needed UL / DL soft HO
diversity processing very different MS MRC
RAKE combining RNC/BS frame selection
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)
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Intersystem Handover
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
networks
Load reason handover Coverage reason
handover Service reason handover
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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
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DL receiver diversity
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
diversity
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Coverage and capacity planning
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 WCDMA
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