4. Basic Operation of Mobile Communication System

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• 4. Basic Operation of Mobile Communication System
• 4.1 Basic principle of CDMA
• A spread-spectrum modulation technique must
  fulfill two criteria
• The transmission bandwidth must be much larger
  than the information bandwidth.
• The resulting radio-frequency bandwidth is
  determined by a function other than the
  information being sent (so the bandwidth is
  statistically independent of the information
  signal). This excludes modulation techniques like
  frequency modulation (FM) and phase modulation
  (PM).

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• The ratio of transmitted bandwidth to information
  bandwidth is called processing gain Gp of the
  spread-spectrum system,
• Gp Bt /
  Bi (1)
• Where Bt is the transmission bandwidth and Bi is
  the bandwidth of the information-bearing signal.
• Because of the coding and the resulting enlarged
  bandwidth, SS signals have a number of properties
  that differ from the properties of narrowband
  signals. The most interesting from the
  communication systems point of view are discussed
  below

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• Multiple access capability
• If multiple users transmit a spread-spectrum
  signal at the same time, the receiver will still
  able to distinguish between the users provided
  each user has a unique code that has a
  sufficiently low cross-correlation with the other
  codes.

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1
2
Figure 4.a1 Principle of spread-spectrum
multiple access
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1. Protection against multipath interference
2. Privacy
3. Interference rejection
4. Anti-jamming capability

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• Low probability of interception (LPI)
• Because of its low power density, the
  spread-spectrum signal is difficult to detect and
  intercept by a hostile listener.

4.2 Spread-Spectrum Multiple Access 4.2.1 Direct
Sequence
Figure 4.a3 Block diagram of a DS-SS transmitter
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Figure 4.a4 Generation of a BPSK-modulated SS
signal
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Figure 4.a5 Receiver of a DS-SS signal
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The binary data signal modulates a RF carrier.
The modulated carrier is then modulated by the
code signal. This code signal consists of a
number of code bits called chips that can be
either 1 or 1. To obtain the desired spreading
of the signal, the chip rate of the code signal
must be much higher than the chip rate of the
information signal. The rate of the code signal
is called the chip rate one chip denotes one
symbol when referring to spreading code signals.
In figure 4a4, 10 code chips per information
symbol are transmitted (the code chip rate is 10
times the data rate) so the processing gain is
equal to 10.
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4.2.2. Frequency hopping
Figure 4.a6 Block diagram of an FH-CDMA
transmitter and receiver
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Example Spreading of user data A, B C
CH code for A
In Air
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Example Despreading of user data A, B C
Same for user B C
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4.2 Spreading Codes
Spreading codes can be divided into pseudo-noise
(PN) codes and orthogonal codes. PN codes are
pseudo-random codes generated by a feedback shift
register. The most commonly considered PN codes
for DS-CDMA systems are generated using linear
shift registers. The cross-correlation between
orthogonal codes is zero for a synchronous
transmission. Orthogonal codes such as Walsh
sequences are typically used for channel
separation in DS-CDMA systems.
4.2.1 Basic Properties of Spreading Codes
In a DS-CDMA transmitter, the information signal
is modulated by a spreading code and in the
receiver it is correlated with a replica of the
same code. Thus, low cross-correlation between
the desired and interfering users is important to
suppress the multiple access interference. Good
autocorrelation properties are required for
reliable initial synchronization, since large
sidelobes of the autocorrelation function might
lead to erroneous code synchronization decisions.
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Furthermore, good autocorrelation properties are
important to reliably separate the multipath
components. Since the autocorrelation function of
a spreading code should resemble, as much as
possible, the autocorrelation function of white
Gaussian noise, the DS code sequences are also
called pseudo-noise (PN) sequences. The
autocorrelation and cross-correlation functions
are connected in such a way that it is not
possible to achieve good autocorrelation and
cross-correlation values simultaneously. Figure
4.b1 illustrates auto and cross-correlation
functions for a 31 chip length M-sequence.
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31 bit (5,3) autocorrelation
31 bit (5,3) and 31 bit (5,4,3,2)
crosscorrelation
Figure 4.b1 Auto and cross-correlation functions
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Typically, PN sequences are generated with a
feedback shift register generator, depicted in
Figure 4.b2. The output of the shift register
cells are connected through a linear function
formed by exclusive-or (XOR)-type logic gates
into the input of the shift register.
Linear Feedback
1
2
3
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Figure 4.b2 Feedback Shift Register
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4.2.2 Case Study
Let us begin with a particular example shown in
Figure 4b.3. The shift register is initiated with
contents 111.
Figure 4b.3 Shift generator for m3
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We start from clock timing 1 and proceed. The
status at different timing is summarized in Table
4.2.1.
Contents of shift register states
 
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Note that at clock pulse 7, the state contents
are the same as that for clock pulse 0, implying
that a new period begins.
Remark 1 1. The stage contents include all
possible three-tuple except the all-zero
vector. 2. The last column 1 1 1 0 0 1 0
represents a PN sequence. 3. The length of
produced PN sequence is equal 23 17 4. All the
column contain the same elements and furthermore,
one is a cyclic- shifted version of another.
Each linear feedback shift register has its
special feedback connections, which can be
represented by a polynomial. For the relation
shown in the shift register above can be
expressed as x3 x 1 (1)
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Or equivalently we can define a polynomial
f(x) x3 x 1 (2) This is the
generator polynomial. In this course, we adopt
the convention that the highest order is on the
leftmost. Note here, all additions are modulo-2,
a negative sign has the same effect as a positive
sign. A general polynomial can be written as
xm am-1 zm-1a1z1 (3) Its
implementation is illustrated in Figure 4b.4. The
coefficients as can take on the value of one or
zero.
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Figure 4b.4 Implementation of a polynomial
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4.2.2 Three Types of Codes used in 3G system a)
Scrambling Code Ways to separate cells and
users b) Channelisation Code Ways to separate
different channels c) Spreading Code Ways to
separate transmissions
Network Common
Information Dedicated
Information (Spreading Code
( Spreading Code
Scrambling Code )
Scrambling Code x CH code)
Terminal (UE)
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Vocational Training Council - IVE (Tsing Yi)
TN3431 Mobile Networks Department of Information
Communications Technology
4.3 Power Control
In the uplink of a DS-CDMA system, the
requirement for power control is the most serious
negative point. The power control problem arises
because of the multiple access interference. All
users in a DS-CDMA system transmit the messages
by using the same bandwidth at the same time and
therefore users interfere with one another. Due
to the propagation mechanism, the signal received
by the base station from a user terminal close to
the base station will be stronger than the signal
received from another terminal located at the
cell boundary. Hence, the distant users will be
dominated by the close user. This is called the
near-far effect. To achieve a considerable
capacity, all signals, irrespective of distance,
should arrive at the base station with the same
mean power. A solution to this problem is power
control, which attempts to achieve a constant
received mean power for each user. Therefore, the
performance of the transmitter power control
(TPC) is one of the several dependent factors when
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deciding on the capacity of a DS-CDMA system.
In contrast to the uplink, in the downlink all
signals propagate through the same channel and
thus are received by a mobile station with equal
power. Therefore, no power control is required to
eliminate near-far problem. The power control is,
however, required to minimize the interference to
other cells and to compensate against the
interference from other cells. The worst case
situation for a mobile station occurs when the
mobile station is at the cell edge, equidistant
from three base station. However, the
interference from other cells does not very very
abruptly. In addition to being useful against
interfering users, power control improves the
performance of DS-CDMA against fading channel by
compensating the fading dips. If it followed the
channel fading perfectly, power control would
turn a fading channel into AWGN channel by
eliminating the fading dips completely. There
exists two types of power control principles
open loop and closed loop. The open loop power
control measures the interference conditions for
the channel and adjusts the transmission
accordingly to meet the desired frame error rate
(FER) target.
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channel and adjusts the transmission accordingly
to meet the desired frame error rate (FER)
target. However, since the fast fading does not
correlate between uplink and downlink, open loop
power control will achieve the right power target
only on average. Therefore, closed loop power
control is required. The closed loop power
control measures the signal-to-interference ratio
(SIR) and sends commands to the transmitter on
the other end to adjust the transmission power.
IS-95 has three different power control
mechanisms. In the uplink, both open loop and
fast closed loop power control are employed. In
the downlink, a relatively slow power control
loop controls the transmission power
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4.3.1 Open Loop Power Control
Figure 4.b5 Uplink Open Loop Power Control
Principle
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The open loop power control has two main
functions it adjusts the initial access channel
transmission power of the mobile station and
compensates large abrupt variations in the
pathloss attenuation. The mobile station
determines an estimate of the pathloss between
the base station and mobile station by measuring
the received signal strength at the mobile using
an automatic gain control (AGC) circuitry, which
gives a rough estimate of the propagation loss
for each user. The smaller the received power,
the larger the propagation loss, and vice-versa.
The transmit power of mobile station is
determined from the equation Mean output power
(dBm) -mean input power (dBm) offset power
parameters (4) The offset power for
the 800-MHz band mobiles (band class 0) is 73dB
and for the 1900-MHz band mobiles (band class 1)
76dB. The parameters are used to adjust the
open-loop power control for different cell sizes
and different cell effective radiated power (ERP)
and receiver sensitivities. These parameters are
initially transmitted on the synchronization
channel.
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4.3.2 Closed Loop Power Control
Since the IS-95 uplink and downlink have a
frequency separation of 20 MHz, their fading
process are not strongly correlated. Even though
the average power is approximately the same, the
short term power is different, and therefore, the
open loop power control cannot compensate for the
uplink fading. To account for the independent of
the Rayleigh Fading in the uplink and downlink,
the base station also controls the mobile station
transmission power. Figure 4.b6 illustrate the
closed loop power control.
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Figure 4.b6 Closed Loop Power Control Principle
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4.3.3 Downlink Slow Power Control
The base station controls its transmission power
to a given mobile station according to the
pathloss and interference situation. The main
purpose of the slow downlink power control is to
improve the performance of mobile stations at a
cell edge where the signal is weak and the
interfering base station are strong. The
downlink power control mechanism is as follows.
The base station periodically reduces the
transmitted power to the mobile station. The
mobile station measures the frame error ratio
(FER). When the FER exceeds a predefined limit,
typically 1, the mobile station request
additional power from the base station. This
adjustment occurs every 15 to 20 ms. The dynamic
range of the downlink power control is only 6
dB. Both periodic and threshold reporting may be
enabled simultaneously, either one of them may be
enabled, or both forms of reporting may be
disabled at any given time.
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• 4.3.4 Power Control Criteria
• As discussed earlier, all wideband CDMA proposals
  use open and fast closed loop power control
  methods. Depending on the power control criteria,
  several different algorithms can be derived. Most
  typical criteria are
• Path loss based power control
• Quality-based power control.
• Normally, the power control algorithm is a
  combination of these two basic criteria. Quality
  can be measured through Signal to Interference
  Ratio (SIR). Since different SIRs correspond to
  the same Frame Error Rate (FER) in different
  radio environments, we need to have a function
  that maps the desired FER into the required SIR
  target. This is performed by continuously
  measuring the FER and SIR, and then adjusting the
  SIR target to produce the desired FER.

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4.3.5 Power Control Step Size Power control step
size defines how much a power control command
changes the transmission power. Either a simple
up/down adjustment or several power adjustment
levels can be used. Typical step size are between
0.5 and 1 dB. It should be noted that power
control adjustment is relative to the previous
power setting, since an absolute power setting
would require extremely accurate, and thus
expensive, power control circuitry.
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4.4 Soft Handover In soft handover a mobile
station is connected to more than one base
station simultaneously. Soft handover is used in
CDMA to reduce the interference into other cells
and to improve performance through macro
diversity. Softer handover is a soft handover
between two sectors of a cell. A separate pilot
channel is usually used for the signal strength
measurements for handover purposes. Figure 4.b7
illustrate the soft handoff principle with two
base stations involved. In the uplink the mobile
station signal is received by the two base
stations, which, after demodulation and
combining, pass the signal forward to the
combining point, typically to the base station
controller (BSC). In the downlink the same
information is transmitted via both base station,
and the mobile station receives the information
from two base stations as separate multipath
signals and can therefore combine them.
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Figure 4.b7 Principle of soft handover with two
base station transceivers
(BTS).
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Summary

• Make Before Break
• Soft Handoff between neighbouring cells
  controlled by same
• switch
• Softer handover between sectors of the same cell

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• In IS-95, the handover decision is based on the
  pilot strength measurements of the downlink only.
  In wideband CDMA for third generation systems
  with asymmetric traffic, more decision parameters
  are needed. At least the following parameters can
  be identified
• Distance attenuation
• Uplink interference
• Downlink interference.