Download - presentation in CDMA
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Transmission Techniques
Traffic channels: different users are assigned unique code and transmitted over the same frequency band, for example, WCDMA and CDMA2000
Traffic channels: different frequency bands are allocated to different users, for example, AMPS
Traffic channels: different time slots are allocated to different users, for example,
GSM
FrequencyTime
Power
FrequencyTime
Power
FrequencyTime
Power
FDMA
TDMA
CDMA
User
User
User User
User User
Introduction
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Time slots and frame
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Capacity of cellular CDMA:
N: the number of usersS: the signal power of each userR: baseband information bit rateW: total RF bandwidthη: background thermal noise in the spread bandwidthAssume perfect power control
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The number of users that can access the system is thus given as
Where W/R is called the processing gain
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The principle behind the spreading of a signal is explained by the
Shannon channel capacity formula:
Bω = bandwidth in HertzC = channel capacity in bits/secondS = signal powerN = noise power
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The received energy per bit is the received signal power divided by
the data rate R (bits/s) ,
Eb = Ps/R
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1-Direct sequence Spread spectrum (DSSS). 2- Frequency hopping spread spectrum (FHSS). 3-Time hopping spread spectrum (THSS). 4-Hybrids Spread spectrum (HSS).
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6.17
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W - Spread bandwidth in HzR = 1/Tb = Date RateS - Received power of the desired signal in WJ - Received power for undesired signals like multiple access users, multipath, jammers etc in WEb - received energy per bit for the desired signal in W N0 - equivalent noise spectral density in W/Hz
00
0
NERW
NEWT
TEWN
SJ
bb
b
bb
min0max NE
RWSJ
b
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)()((db)margin Jammingmin0max
dbNEdb
RW
SJ b
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Are portion of a physical communications channel that is used to for a particular communications purpose. There are two groups of channels used in the CDMA system; control channels and traffic channels. Control channels are used to setup, manage, and terminate communication sessions. Traffic channels are primarily used to transfer user data but can also transfer some control information.
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SyncPilot
FW Traffic(for user #1)
Paging
FW Traffic(for user #2)
FW Traffic(for user #3)
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Figure 2.9: CDMA Forward and Reverse Channels
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`
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Cells Different Frequencies or
Codes
Base Station Fixed transceiver
Mobile Station Distributed transceivers
Downlink
UplinkHandoff
Multiple Access
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CDMA:”make before break”---soft handoffOther systems: “make after break”---hard handoff
Use soft handoff, decrease drop-call rate
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Definition of Coverage Areas
Location area
MSC area
PLMN area
Service area
Sectorarea
CDMA Number Planning
Cell area
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Rate of reception level variation is known as the fading rate or the
Doppler rate, fd, and depends on the velocity of movement V and carrier wavelength. It can be expressed as:
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W2n=Wn Wn
Wn Wn
W1 =0
W2=0 00 1
W4=
0 00 1
0 00 1
0 00 1
Walsh code
Walsh Code is one kind of orthogonal code.
1 11 0
Orthogonal codes are easily generated by starting with a seed of 0, repeating the 0
horizontally and vertically, and then complementing the 1 diagonally
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suppose we have four stations each has a sequence of chips which we designate as (A,B,C) and (D)(see the figure). Each station is assigned a code which is sequence of number called chips.
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