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8/8/2019 CDMA Cellular System
1/13
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 128 National University of Singapore
CDMA Cellular System
Earlier lecture on cellular system mainly for
FDMA/TDMA systems, i.e., reuse factor > 1.
- We determine the cochannel cell interference based on
the desired SIR for a given modulation technique.
- Path loss model come into picture when determining the
received signal power and the interference powers from
from co-channel cell. User in the same cell does not
interfere the referenced user because they use different
time slots or frequencies to transmit.
- For example, if only first tier and no sectorization,
==
6
1i
ni
n
D
R
SIR
- Conversely, if a minimum SIR is given, the cluster size
Ncan be determined. The total number of available
voice channels then split among these cells within a
cluster, and this is the total number of handover/newly
generated calls that each cell can handle.
- We demostrated the use of Erlang B table for newlygenerated traffic. A more complete scenerio is to
include the handover traffic.
- Resource management newly generated vs handover
traffic (dropped call probabilities), maintaining GoS
(blocking probability) and qaulity-of-service QoS (such
as, packet loss probabilities, delay constraints etc)
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 129 National University of Singapore
CDMA systems are different in terms of
- Reuse factor is 1.
- The capacity of CDMA systems is interference limited,
whilst it is bandwidth limited in FDMA and TDMA.
Hence, any reduction in the interference will cause an
increase in the capacity. Alternatively, this means that
if the number of users decreases, the link performance
will be improvedfor the same transmit power.
(a) Asynchronous short code
(bit asynchronous but chip synchronous)
Alternative way of representing the evaluation process
1231[]3113111[
1111[]1111111[2
13313113111
]1111111[]111-1111[3
]1111111[]1111111[2
]1111111[]1111111[1
+++++++
+++++
++++++
++++++
+++++++++++++++
Despread
Rx
Tx
7+ will decode as bit +1
8/8/2019 CDMA Cellular System
2/13
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 130 National University of Singapore
(b) Synchronous short code
Alternative way of representing the evaluation process
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 131 National University of Singapore
In the downlink, BS is capable of synchronizing the
transmission of all the users signals, such that the symbol
durations are aligned with each other. Such a scenario is
known as symbol (bit) synchronous transmission.
Designing for perfectly orthogonal code in this case is
simple since the use of short code is sufficient. However,
there will still be interference from users in neighboring
cell BSs. Further, the presence of multiple paths could
distort the orthogonality.
Short code cannot be used in uplink since orthogonality
cannot be achieved when transmission is asynchronous
(no coordination in the transmission of users signals).
Long codes such as PN sequences perform much better
but perfect orthogonality cannot achieve for such codes.
Example to show why short code cannot be used in
symbol asynchronous transmission.
Consider two orthogonal Walsh codes
]1111[ and ]1111[
Data transmit is A=[1 1 ... ] B=[-1 1 1...]
If bit synchronous transmission,
=
02
20
02
20
11
11
1
1
1
1
1
1
1
1
8/8/2019 CDMA Cellular System
3/13
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 132 National University of Singapore
At the receiver,
=
44
44
02
20
02
20
1111
1111
If transmission is bit asynchronous but chip
synchronous, offset by a chip period for user B
1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1
2 2 2 0 0 0 0 0
1 1 1 1 1 1 1 1
( ) 6 0)N
A
B
Tx
A
+ + + +
+ + + + + + +
+ + + +
At the receiver, an error may occur due to loss in
orthogonality of the short code.
Multipath interference is always present in both the
forward and reverse link. Multipath interference is due
to the different arrival times of the same signal via the
different paths at the receiver (also known as self-interference). This is analogous to the signals
transmitted from other users and generates interference.
Hence, multipath interference is normally analyzed in
the same way as MAI.
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 133 National University of Singapore
Consider Walsh codes ]1111[
Data transmit is [1 1 ... ]
At the receiver, due to multipath (assume no phase
change and only power split in 9:1)
++
++++
++++
++++
3.67.3
0.18.00.18.00.18.00.19.0
1.01.01.01.01.01.01.01.0
9.09.09.09.09.09.09.09.0
Multipath reduces the received SNR if conventional
single user detector is used.
A chip ofprevious
bit
Channel
Multiple0.9
0.1
8/8/2019 CDMA Cellular System
4/13
8/8/2019 CDMA Cellular System
5/13
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 136 National University of Singapore
- Now examine only thek
W term defined in (1). Since all
the a s take only 1 or 1 equally, kW only takes on
discrete values. Performing some manupulation to the
expression of kW , and using the following results:
[ ]2
1
0
c
T
c
k
Td
TE
c
==
[ ]3
12
0
22 c
T
c
k
Td
T
Ec
==
It can be proved that 1
[ ]3
22 GWE k = , or
6
22 GPT kc
Ik=
- Here the expectation is taken over all values of kk , and b (bits).
-1
Reference for the proof can be found in Textbook,Appendix C and Smart antennas for Wireless
Communications by Joseph C Liberti and T.S
Rappaport, pp.305).
- Now go back to the MAI component when detecting the
information of user 0.
=
=
=
=
=
=
=
=
==
1
1
1
1
21
1
21
1
21
1
21
1
222
][][
])[(][
K
k
K
llk
K
kk
K
kk
K
k
k
K
k
k
IIEIEIE
IEIEEE
Assume that the terms kI are independent, i.e.,
0][][][ == jkjk IEIEIIE , this leads to
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 137 National University of Singapore
=
===
1
1
21
1
22
6][
K
kkc
K
kk PGTIE
- The desired detected signal has mean given by
bTbP
s 0,00
02
=
- Can show that in general
Variance of interference =
= +
1
10
2
42
K
kbkc
TNPa
GT
- where a is constant determine by
a =3 (chip and phase asynchronous) previous derived
a =1 (chip and phase synchronous)
a =2 (chip synchronous and phase asynchronous)
adopted
a =1.5 (phase synchronous and chip asynchronous)
- For adopted case, i.e. chip synchronous and phase
asynchronous
Variance of interference =
=
+1
1
0
2
44
K
k
bk
c TNPGT
The bit error rate is therefore given by
+=
+
=
=
=
WNP
GPQ
TNP
GT
TP
QPK
kk
bK
kk
c
b
e
0
1
1
0
01
1
2
20
2
44
2
8/8/2019 CDMA Cellular System
6/13
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 138 National University of Singapore
where BWTTGcb
// == andbb
RTB == /1 is the
(unspread) signal bandwidth, bR is the data rate, Wis
the spread signal bandwidth.
- If the number of usersKis large, then thermal noise is
negligibly small.
)2(2
0
1
1
0 GQ
WNP
PGQP
K
kk
e =
+
=
where
WNP
PK
pkk
k
p
p
01
+=
=
is defined as the signal-to-
interference plus noise ratio (SINR) for userp.
- For BPSK, another way of viewing this is to define
WN
PG
WNP
PGG
N
E
IK
kk
p
I
b 0
0
1
1
0 =+
==
=
Ib NE / is defined at the output of matched filter whilst
SIR at the front end of the receiver.
(Note: in AWGN, 0/ NEb is defined as SNR and BER isgiven by )2( SNRQ , but here we just called it Ib NE / to
distinguish from SIR or SINR defined in CDMA system)
+=
=
1
10
K
kkI PWNWN
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 139 National University of Singapore
Power Control
Unless otherwise stated, we will assume that
- Single user detector is used. Power control is necessary
to avoid near far problem each mobile must achieve
the required SNR to achieve its pre-specified
performance (normally specified as minimum BER).
- Omni-directional antennas are used in both mobiles andBS. This affects how the amount of interference imposed
on a referenced user.
- Thermal noise is less significant if number of MSs is large.
Power control is needed
- CDMA allows all MSs to access the common bandwidth
without setting time slots or frequency bands.
- Power control is to maintain the required link QoS by
adapting to MS movements and also the statistical
variations in radio propagation paths. This helps to
mitigating interference to increase the system capacity.
- Without power control and simply let all MSs transmit
at same power, their signals will arrive at BS with
vastly different power levels near-far effect.- When an user transmits at excessive power level, its
performance gets improve but will degrade the
performance of other MSs. Others will try to increase
their transmit power to restore their link reliability, and
hence results in unhealthy competition among all MSs
are not properly co-ordinated.
8/8/2019 CDMA Cellular System
7/13
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 140 National University of Singapore
- In the uplink, power control is to minimize power
consumption and prolong the battery operating time.
An example : centralized power control algorithm
- Requires the knowledge of all the radio link gains.
- If feasible solution exists, it gives the optimal solution
for the power control problem. This solution can become
a performance measurement criterion.- We consider a cellular system with N BSs and M MSs.
The transmitted power of MS k is denoted byk
P and theM dimensional vector P is the transmitted power
vector to be determined.
- We consider the ith MS (referenced) communicates with
BS nearest to it. The BS is then also labeled as i. The
link gain between the i th BS and the j th MS is denoted
by ijg . Then iig corresponds to the desired radio links,whereas ijg corresponds to the unwanted interference
links.
- The link gain ijg depends on the path loss model,
lognormal fading and fast fading component.
- Assuming that MS i knows which BS it is connected to
(an example is by connecting each MS to the nearest
BS). Its channel gain to that BS is defined as iig . If MS2 also connects to BS i, then we have 222 ggi = , 211 ggi = ,
jij gg 2= ,
(i.e., in the example, the BS has two identities, 2 and i)
Note: sometimesgis used to represent path loss, but sometimes is used as voltage gain. In
the following, we use it as voltage gain.
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 141 National University of Singapore
BS
User i
User 1
1ig
2ig
User j
ijg
User 2iig
MS
- The CIR of mobile i at its base station is given by2
2
0
1
,i ii
i M
j ij
jj i
P g
P g N W
=
=+
Mi 1
or can be rewritten as
WNPA
PM
ijj
jij
ii
01
+=
=
, Mi 1 ,
where
2
;
0;
ij ii
ij
g g i j
Ai j
= =
- There exists a SNR threshold *i that must be achieved
by each individual MS to ensure certain level of receiver
performance. For two different MSs, this threshold
needs not be the same.
8/8/2019 CDMA Cellular System
8/13
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 142 National University of Singapore
*ii
for Mi 1
- Repeat writing down the relationship for all MSs. We
can solve for all powers using the Msimultaneous
equations.
Power control is less critical in the forward link if power
consumption is not the main issue. This is because
orthogonal short codes are used and transmission is
symbol synchronous. However, power control still helpsin reducing interference picked up by MSs in
neighboring co-channel cells and the effect of resolvable
multipath.
From the implementation point of view, power control
can be classified into two classes
Open loop: The forward and reverse links are assumedto be symmetric. For example, in this ideal case, MSs
can determine its uplink transmited power according
to its downlink received power. This method only
suitable for the case where path loss and shadowing
which affect the signal propagation change relatively
slow and exhibit reciprocity in the forward and reverse
link.
Closed loop: BS provides continuous feedback to each
MS so that the MS varies transmit power accordingly.
Depending on the speed at which power level control is
performed, closed loop control is used to compensate
either for fast fading + lognormal shadowing or
lognormal fading.
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 143 National University of Singapore
- In practical system, a combination of open- and closed
loop control can be used.
Non-ideal power control
- For perfect power control, the transmitter power should
be identical to the inverse channel variation. However,
due to fast signal fading variations in the channel and
the control loop delay, the transmit signal power does
not follow the inverse channel variations. As a result,there is a significant residual fading in the received
signal after power control.
- In practical system, the received signal power variation
in the BS after power control are approximated by a
lognormally distributed. Through experimental data,
the standard deviation of the lognormal distribution isabout 1 to 2 dB.
8/8/2019 CDMA Cellular System
9/13
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 144 National University of Singapore
Reverse link Capacity
First we shall consider only a single cell system, Nusers
(MSs) are transmitting and the desired signal arrived at
the BS with received powerP(i.e. ideal power control to
overcome near-far problem, assuming single user
detector is used). The received power isPand the
interference power is (N-1)P. At receiver front end,
1
1
)1( =
=
NPN
PSIR
After the dispreading and the matched filter T
dt0 )( , bit
energy-to-noise ratio is given by
1/)1(
/
=
=
N
G
WPN
RP
N
E b
I
b
where W is the RF bandwidth, bR is the bit rate, G is
known as the processing gain. If the number of MSs
increases, the interference signal increases linearly.
- For a required ( )min
0/ NEb , total number of users
min01
=
N
E
N
G
N
E b
I
b
( )
+=
min0max 1
NE
GN
b
- Increase the number of MSs beyond this threshold only
result in the slight degrading in performance soft
capacity.
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 145 National University of Singapore
- More accurate analysis should consider co-channel
interference, AWGN, imperfect power control,.
Voice activity factor and AWGN
- Silent periods in a voice conversation occupy more time
than nonsilent periods, the ratio is about 60/40.
- Introduce the term voice activity factor . Similarly
for data transmission, data activity factor is defined.
Inter-cell and intra-cell interference
- Each BS in a cellular system receives interference
coming from MSs in the same cell, called intracellinterference. Interference coming from MSs in
neighboring cells is called intercell interference.
- How to evaluate intercell interference?
(1) Consider the MS located at ),( r with respect to
the BS it is communicating with. The desired signal
power received at that BS is given by PPr = , the
transmitted power tp can be obtained using the pathloss model.
(2) The received interference power IP imposed on
the referenced BS can be found from tP and the path
loss model, with the distance between the referenced
BS and MS, obtained from Cosines Rule:
8/8/2019 CDMA Cellular System
10/13
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 146 National University of Singapore
)cos2(
22
DrDrrI += D is the distance between the two BSs.
(3) The interferenceI
P the MS at the interference
cell to the reference cell BS can be obtained again
from the path loss model.
Irr
Interference user
D
For example, using simple path loss model, the exact
expression fort
P cannot be obtained directly because
0r= is undefined. However, it is not difficult tofigure out that
( , )
n n
I
I I I
P r r P r P
P r r
= =
Assuming circular cell with radius R, the average
interference is roughly given by
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 147 National University of Singapore
- If all MSs perform ideal power control with respect to its
own BS, and assume that all cells have the same sizeand user distribution density. Under these
circumstances, it can be shown that the average
intercell interference normalized to the total average
intra-cell interference can be characterized by a
constant, , known as the interference factor.2
inter
0 0
R
I I P d dr
=
intraI P= (why?)
2
2 20 0 ( 2 cos )
nR
IP r
d drP r D Dr
= = +
- The total intercell interference from all first tier
interfering cells without sectorization is given by 1P ,where
16 = .
Exercise: Write down a general expression for the
interference factor.
Underlaying cell (space)
8/8/2019 CDMA Cellular System
11/13
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 148 National University of Singapore
More accurate model
- The interference plus noise power spectral 0I ( IN= ) foruserjth communicates to BS0 has to modify
WNPPWIK
kkiki
n
i
K
jkk
kk 0
ceinterferenintercell
11
ceinterferenintracell
1000 ++=
===
)0(1)1( ==== kikiki PP is the voice activity factorfor the ith user in the kth BS. j is the binomial random
variable that model the voice activity. All cells are
havingKMSs.
- When just consider average interference, then the above
expression can be simplified:
WNPPWIK
k kiki
n
i
K
jkk kk 0
ceinterferenintercell
11
ceinterferenintracell
1 000
++= ==
=
Exercise: Assume all cells have uniform user
distribution and can support the same number of users,
N. What is the maximum number of users N each cell
can support to guarantee a Ib NE / of* . Taking into
consideration the voice activity factor, intercell
interfence and assumed perfect power control and
lognormal shadowing..
Exercise : What happen if cluster size greater than 1 is
used? The bandwidth in this case is partition into a
few carrier channels.
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 149 National University of Singapore
Rake Receiver
Multipath channel
1
2
L )(tn
)(2
t
)(tL
)(1 t
)(tr)(ts
+==
L
lil tntsttr
1
)()()()( - for only one user.
where )()()( tctbPts = and )(tl is the time-variantcomplex channel gain of the ith path.
Spread spectrum techniques can improve reception
through using a receiver structure named RAKE
receiver. It inherited the name from the analogy of a
garden rake, whereby the fingers constitute the
resolvable paths. The point where the handle and
fingers meet is where diversity combining takes place.Possible methods of diversity combining are:
Selection combining (SC)
Maximal ratio combining (MRC)
Equal gain combining (EGC)
Combining of the n best signals (SC-n)
8/8/2019 CDMA Cellular System
12/13
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 150 National University of Singapore
++
+1
1
)1(
b
b
TiiT
)(*
2 t
)(*
tM
)(*
1 t
b)(tr
)( 2*
tc
)(*
Mtc
)( 1*
tc
++
+2
2
)1(
b
b
TiiT
++
+Lb
Lb
TiiT
)1(
( 1)
* *
1
( 1) 2 *
1
( 1)
* *
1
sgn ( ) ( ) ( )
sgn ( ) ( ) ( ) ( )
( ) ( ) ( ) ( )
b l
b l
b l
b l
b l
b l
i TM
i l ll iT
i TM
l l l l l iT
i TL
l i l ii iTi l
b t r t c t dt
P t b t c t c t dt
t t c t c t
+ +
= +
+ +
= +
+ +
= +
=
=
+
2
1
sgn ( ) inter-finger interferenceM
l il
dt
t Pb=
= +
We assume there is Mrake fingers ( LM ) is used,
( ) ( ) 0i j
c t c t dt if ji and =1 ifi j= .
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 151 National University of Singapore
1
time
- For optimum performance of the RAKE receiver usingcoherent demodulation, the path attenuation and phase
must be accurately estimated. This estimation is
performed by another process known as channel
estimation.
Soft handover
- CDMA allows each cell to use the same broadbandfrequency channel, eliminating the need for a mobile to
change its freuqency when moving into another cell.
This is the pre-requirement for soft handover.
- In establishing the connection, the mobile detects and
tracks the BS with the highest power. At the same time
the mobile monitors the powers of neighoring BSs. The
second BS is introduced when its power becomes
significant relative to the first BS power. The first BSwill be abondoned only when the first BSs signal
becomes weak relative to second BS.
8/8/2019 CDMA Cellular System
13/13
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 152 National University of Singapore
A
AB
B
EE5401 Cellular Mobile Communications
Institute for Infocomm Research 153 National University of Singapore
Spectral Overlay Systems
DS spread spectrum systems can share common
spectrum with the currently operating cellular or fixed
microwave systems in order to achieve efficient
bandwidth utilization.
The signals of coexisting users would appear as
narrowband interference in the spectrum of a wideband
signal. This is still possible because spread spectrum
modulation has an inherent ability to reject narrowband
interference. The effectiveness of narrowband
interference rejection depends on the processing gain.
f