multiple acces and cellular structure
TRANSCRIPT
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Review Questions
What are the main differentiating features of 1G,2G and 3G?
What was the bandwidth used in one-direction in1G AMPS?
In DAMPS, how many users share the samebandwidth as AMPS?
What are the range of data rates for wideband and
broadband wireless systems? What is an unlicensed band? What are some of the
typical applications?
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Multiple Access Techniques
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Multiple Access (MA)
MA is setting up as many transmission
paths as required.
MA is the way in which the common
resources (power and bandwidth) are shared
by many users.
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Multiple Access TechniquesIn terms of circuit assignment
Pre-assignment
Demand assignment
Random assignment
In terms of modulation (waveform assignment)
Frequency division multiple access (FDMA)
Time division multiple access (TDMA)
Code division multiple access (CDMA)
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Consider that the following samples of 3users data to be multiple accessed
User 2 data
User 3 data
User 1 data
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Frequency Division Multiple Access
FDMA
f
bandwidth
BW
guard band
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Waveforms of Users 1,2 & 3 after FDMA
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Input to the amplifier after 3 FDMA
signals are added
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Nonlinear Effects in FDMA
Received signal is sum of multiple carriers.
Receiver power amplifiers are operated
nonlinearly (near saturation) for maximumefficiency.
The nonlinearities cause intermodulation(IM) frequencies to appear in the amplifier
output. IM components can interfere with other
channels in the FDMA system.
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Time Division Multiple Access
TDMA systems divide the radio spectrum
into time slots.
Only one user can transmit or receive
during one time slot.
Usually, each user may occupy the channel
once during a time frame, where one framecomprisesNtime slots.
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TDMA
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User 1,2&3 data before and after TDMA
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TDMA
t
Slot 1 Slot 2 Slot 3 SlotN
1 frame
sync. bits
t
trail bits info bits Guard time
Each slot requires overhead bits.
More overhead reduces efficiency.
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TDMA Systems
TDMA systems transmit data in a buffer
and burst method.
The transmission is noncontinuous.
Unlike FDMA systems which can transmit
analog signals, TDMA must transmit data
and digital modulation must be used.
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TDMA Features
Only one carrier. No IM.
Number of time slots per frame dependson bandwidth, desired date rate,modulation technique.
Receiver must synchronize to each time slot,thus more synchronization bits are required
in TDMA compared to FDMA. It is possible to allocate more than one time
slot per framebandwidth on demand.
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CDMA
In CDMA signals of different users overlap in time and
frequency. The separation is achieved by assigning
different codes to each user.
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Direct Sequence Spread
Spectrum
Modulator Channel Demodulator
PN sequencegenerator
PN sequencegenerator
18181818School of Inform at ion Technolog y and Engineer ingUniversi ty of Ottawa
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Data
PN-1
Data spread by PN-1
PN-2
Data despread by PN-2
Data despread by PN-1
1
1001010
1001010
1011100
1101001
1
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Power Spectrum of DS-SS Signals at Tx
t f
t
f
noise level
20202020School of Inform at ion Technolog y and Engineer ingUniversi ty of Ottawa
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Reuse Patterns
TDMA or FDMA CDMA
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X X XB1 B2 B3
d1
d2d3
u1
u2
Interference comes from non-adjacent channels. Suppose B1 tx at
power pa, B3 tx at power pb. Let the distances from B1 and B3 to u1 be
d1 and d3 respectively.
FDMA/TDMA Forward Link C/I at u1
C = pd
I pd
CI
pp
dd
a
n
b
n
a
b
n
n
1
3
3 3
3
1bg bg gbg
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X X XB1 B2 B3
d1
d2
d3u1
u2
Suppose u1 tx at power p1, u2 tx at power p2. Let their distance to B1
be d1 and d2 respectively.
FDMA/TDMA Return Link C/I at B1
C = pd
I pd
CI
pp
dd
n n
n
n
1
1
22
2 2
1
2
2
1bg b g gbg
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X X XB1 B2 B3
d1d4 d3d5 d2
u1u2 u3
C =
p
d I
p
d I
p
d
C
I I
p
d
p
d
p
d
b
n
a
n
c
n
b
n
a
n
b
n2
1
5
3
4 1 3
2
5 4
bg bg bgbg
bg bg, , ,
CDMA Forward Link C/I at u2
Suppose B1 tx at power pa, B2 tx at power pb and B3 tx at power pc
Let their distances to U2 be d5, d2 and d4 respectively.
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X X XB1 B2 B3
d1
d4d3
d5 d2u1 u2 u3
C = pd
I pd
I pd
CI I
p
dp
d
p
d
n n n
n
n n
2
2
1
1
5
3
3
4 1 3
2
2
1
5
3
4
bg bg bg bgbg bg
, , ,
CDMA Return Link C/I at B2
Suppose u1 tx at power p1, u2 tx at power p2 and u3 tx at power p3
Let their distances to B2 be d5, d2 and d4 respectively.
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Near-Far Effect
A major problem with DS-CDMA is the
dominance of a stronger user.
Consider 4th
power propagation loss. If both usersare transmitting at the same power, what is the
received power level difference (in dB) if one user
is 10 times closer than the other?
Near-far effect demonstrated in the above example
is not critical in frequency hopped systems.
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Intracell and Intercell Interference in CDMA
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A Summary on Multiple Access
FDMA:+ Flexible (modulation, transmission rate,etc).
Inefficient use of available power to avoidintermodulation (IM) products.
Many receivers needed to receive simultaneouslyfrom several transmitters.TDMA:+ No IM, efficient resource utilization.
Synchronization is critical.
Peak to average power ratio is high.CDMA:+ Flexible (for networking)
Generally self interference limited.
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Do systems fall into only one
of 3 categories?
f
t
f ,t
Answer: In practice NO.
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Cellular Concepts
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Clusters
Cluster size
N= 7
1
123
4
56
7
23
4
56
7
12
34
5 6 712
34
56
7
12
34
56
7
In TDMA and FDMAcells are grouped intoclusters.
The total number ofchannels are equallydistributed among allcells in a cluster.
The channelallocation pattern isrepeated in everycluster.
Cells that areallocated the samefrequencies causeinterference to oneanothercochannel
interference (CCI).
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Clusters (2)
12
34
1
2 3 4 12
34
12
3
4
43
21
4
3 2 1
Cluster size
N= 4
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Real Cellular Structure
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Clusters (3)
A cluster must be more or less symmetrical
and similar North/South and East/West
dimensions.
We cannot select cluster size arbitrarily.
Cluster sizes must satisfy the following:
N= i2
+ij+j2
where i andj are nonnegativeintegers.
Thus N= 1, 3, 4, 7, 9, 11, 12
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Frequency Re-use
The number of effective channels in a service area
can be increased by decreasing the cluster size.
However, we cannot decrease the cluster size to aslow as we like.
Decreasing cluster size decreases distance between
cells employing same frequencies which in turn
increases co-channel interference.
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Channel Assignment Strategies
Fixed assignment strategies.
Cells are allocated a predetermined set of voice
channels.
Dynamic assignment strategies.
Voice channels are not permanently allocated to
a cell.
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Fixed Channel Assignment
Strategies Cells are allocated a fixed set of voice channels.
Some channels are reserved for handoffs.
If all voice channels are in use, any new callrequests are denied access (blocked).
One variation allows channel borrowing. When acell has all of its channels in use, it may requestadditional channels from a neighboring cell.
The MSC supervises the borrowing procedure. TheMSC ensures that borrowing does not disrupt orinterfere with any calls in progress in the donor cell.
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Dynamic Channel Assignment
Each time a call request is made, the BS
requests a channel from the MSC.
The MSC then allocates a channel to thecell following an algorithm that takes many
factors into account.
MSC allocates a frequency that is not currentlyin use within the minimum distance.
MSC takes likelihood of future blocking in cell.
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Handoffs
Mobile unit periodically monitors, measures and ranks allpilots within its range
As a mobile moves from one cell (or sector) to another,connection is rerouted to the new cell.
Handoffs must be performed successfully, infrequently and
imperceptible to the users.(1) Idle hand-off(2) In-call hand-offIdle hand-offoccurs when the mobile unit is on but not
actively engaged in a call. In this state the mobile unit
monitors the paging channel. One of two conditions can trigger an idle handoff(1) Deterioration in reception quality(2) Another pilot is stronger by a certain level.
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Handoff (handover)
Hard handoff: Channel in source cell is releasedfirst and then channel in target cell is engaged.(break before make).This is the only possibility in
TDMA or FDMA.
Soft handoff: channel in source cell is retained andused for a while in parallel with channel in targetcell.(make before break). This is both a necessity
and a blessing for CDMA. Mobile communicates with multiple BSs during
handoff
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Prioritizing Handoffs
It is possible that many mobile users enter thesame cell, placing a heavy load on the new cell.
The rate of decrease in the received power (whichis usually a function of the velocity of the MS),determines the priority of a handoff.
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Softer Handoff
Softer handover refers to the handover
between the sectors of a cell.
Forward link is the same as soft handover.
In return link no need to communicate with
other Base stations.Thus it is faster.
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Advantages of Soft Handoff
Contact with a BS is made before the call is
switched, and this prevents the mobile
losing contact if handoff signal is not heard. Diversity reception provides additional
resistance to fading.
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Practical Handoff Considerations
With frequency reuse, a cellular provider canincrease system capacity by decreasing cell sizes.
With more cells in the same area, and by using thesame frequency reuse pattern, the number ofeffective channels over the same area is increased.
However, as the cells decrease in size, the numberof handoffs increase.
More channels must be reserved for handoffs.
As cells decrease in size, high speed users may requiremany handoffs per call, which increases probability ofcall being cut off.
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Practical Handoff Considerations
Low speed users, such as pedestrians, may not need a
handoff, even in very small cells.
How can we support both high speed and low
speed users while maintaining a high system
capacity and low number of handoffs?
Umbrella cell approach.
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Umbrella Cell Approach
Low speed users can be handled bymicrocells or picocells.
High speed users are handled by a largercell macrocell which is co-located withsmaller cells.
Speed of MS can be estimated by MSC byobserving the rate of change of signalstrength.
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Umbrella Cell Approach
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Cell Dragging
Low speed users in microcells often enjoy line of
sight communications with the BS even when
outside of the boundaries of the cell.
Thus handoffs may not be made even when the
user is well within the boundaries of a new cell.
Cochannel interference to other cells will be
increased.
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Interference and Its Effect on
Capacity Interference is the major limiting factor in the
performance of cellular radio systems.
The two major types of system-generatedinterference are co-channel interference (CCI) andadjacent channel interference (ACI)
CCI: several cells employ the same set of frequenciesin a service area. These are called co-channel cells. An
MS or the BS in a cell will receive signals from othercells which are on the same frequency.
ACI: sidelobes of signals in adjacent frequencies in thesame (or neigboring) cell can interfere with a userssignal.
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Co-channel Interference and
System Capacity Frequencies are reused everyNcells.
A user is assigned a pair of frequencies for a fullduplex call.
The user will receive the desired signal from theBS and it will receive from all other BSstransmitting on that frequency.
It is possible that the nearest neighbours are notcurrently transmitting on that frequency.
The BS receives the signal from the MS as well asfrom other MSs in other cells transmitting on thesame frequency.
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Co-channel Interference and
System Capacity 2 We cannot reduce the effect of CCI simply
by increasing the power with which we
transmit.This will create larger CCI in other cells who
will then increase their power to compensate.
The signal to CCI power ratio is determinedby the distance between co-channels and the
propagation exponent of the channel.
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Co-channel Interference and
System Capacity 3 Assuming:
Equal cell radii.
All MS transmit with same output power. All BS transmit with same output power.
The worst case Signal to Interference power ratio
for an MS located on the cell boundary can be
approximated by an equation that does not dependon cell radius, but on the cluster size and the path
loss exponent.
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Co-channel Interference and
System Capacity 41
2 4
56
7
1
2 3 4
56
7
12
3
4
56
7
R
(3N)1/2R
If both BSs are transmitting at same power
then signal from interfering BS is (3N)n/2
times weaker than desired signal
> (3N)1/2R
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Co-channel Interference and
System Capacity 5 Assuming hexagonal cell shapes, there are 6
nearest interfering cells.
Thus C/I > (3N)n/2
/6. We use this as worst caseC/I.
Example: We are setting up a cellular service in
an area where the path loss exponent is 3.88. We
want a minimum C/I of 18 dB. What is thesmallest cluster size which accommodates this?
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Co-channel Interference and
System Capacity 6 (3N)1.94/6 > 101.8.
(3N)1.94 > 63.16 = 378.6
N > 378.6(1/1.94)/3 = 7.1
N = 9
(N = 7 does not guarantee C/I > 18 dB and
N = 8 is not allowed).
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Co-channel Interference and
System Capacity 7 Thus we can see that the co-channel interference
places a constraint on the cluster size which has an
effect on the system capacity.
As cluster size decreases, the number of channels per
cell increases.
If we can accept more CCI, then we can reduce the
cluster size.
Also, if the pathloss exponent is higher, then the cluster
size can also be decreased.
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Adjacent Channel Interference
The BS receives signals from many userssimultaneously.
Although signals are filtered to reduce out-of-bandemissions, the filtering is not perfect andsidebands exist.
If all users transmit at the same power level, thenthe received power of signals originating fromvery close to the BS will be much higher than thereceived power of signals originating from nearthe cell boundary.
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Adjacent Channel Interference 2
Signal 2s sidelobe is as strong as signal 1s mainlobe.
Performance of signal 1 is severely degraded.
Spectrum of signal 1
Spectrum of signal 2
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Adjacent Channel Interference 3
Suppose a signal is received at the BS with
sidebands whose spectral density is 30 dB lower
than its main lobe.
If the adjacent signal is received 30 dB lower due
to distance, then the interfering sideband is
roughly the same strength as the desired signal.
Because of the near-far problem, a desired signalmay be lost in the sidebands of adjacent channel
signals.
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Adjacent Channel Interference 4
To reduce the effect of ACI, the received powerlevels are controlled.
This is also done to ensure that the MS istransmitting at the smallest power necessary tomaintain a reliable link.
All signals should be received by the BS withpower levels that are within a few dB of oneanother.
If a signal is received with a power level above theupper limit, a control message is sent on the FCC.This instructs MS to decrease transmitted power.
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Why Power Control?
1. To eliminate near-far effect2. To conserve transmit power.
What is power control?Power control ensures that each user receives and
transmits just enough energy to properly convey
information while interfering with other users no more
than necessary.Minimizing the transmitted power of a portable user
unit maximizes the interval between battery charges.
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Is Power Control Critical in Both
Directions? Critical in the uplink because of Multiple
Access Interference (MAI).
In the downlink not critical for MAI but is
needed to minimize the interference into
other cells and to compensate interference
from other cells.
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Power Control Limitations?
If power control can follow fading
perfectly, fading channel becomes an
AWGN channel. Hence the practical limitation is how
accurately and quickly can channel be
estimated. Then comes the step sizes,dynamic range etc.
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Types of Power Controlin CDMA
In the reverse direction:
1) Open loop for coarse power adjustment (at the start)
2) Closed loop for refined adjustments (every 1.25 msec)
3) Outer loop to reduce to min level (once per frame)
In the forward direction: adjust to maintain atarget quality by:messaging, or
bit signaling
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Open Loop Power Control
Adjust transmit power to compensate for distance dependentattenuation and shadowing.
Power correction based on total received power.
Mobile station transmit power defined by
tx power(dBm)+ rx power (dBm)= constant + tolerance
(typically) constant = -76 dB for 1900 MHz and -73 dB for
800MHztolerance = +/- 6dB
Tyipical time constant of open loop PC is 30 msec