Mobile Cellular Systems - Alak Roy

SLIDE #2.1
Mobile Cellular Systems
MOBILE COMPUTING
NIT Agartala, Dept of CSE
Jan-May,2012
ALAK ROY.
Assistant Professor
Dept. of CSE
NIT Agartala
Email-alakroy.nerist@gmail.com

What we will learn in this topic:
• Cellular Concept
– Cellular Architecture
– Frequency Reuse
• Multiple Access Methods
– FDMA, TDMA, and CDMA
• In particular, we focus on CDMA.
2

Cellular Implementations (Gs)
• 1G: Analog cellular systems (450-900 MHz)
– Frequency shift keying for signaling
– FDMA for spectrum sharing
• NMT- Nordic Mobile Telephone (Europe),
• AMPS – Advanced Mobile Phone System (US)
• 2G: Digital cellular systems (900, 1800 MHz), < 14.4 kbps
– TDMA/CDMA for spectrum sharing
– Circuit Switching
• GSM-Global System for Mobile communications (Europe),
• IS-136 – Interim Standard 136 (US),
• PDC – Pacific Digital Cellular (Japan),
• CDMAOne – US, Korea, Asia.
3

Cellular Implementations (Gs)
• 2.5G: Packet switching extensions , 14.4 – 144 kbps
– Digital: GSM to GPRS (General Packet Radio Service)
– Analog: AMPS to CDPD (Cellular Digital Packet Data)
– CDMA2000 1X
• 3G:
– High speed (384 kbps to 2 Mbps), data and Internet services
– IMT-2000 (International Mobile Telecommunications)
– CDMA2000 1X EV-DO, EV-DV, 3X, W-CDMA etc.
4

Cellular Implementations(Gs)
• 4G:
– High speed (20 – 100 Mbps in mobile mode),
Conversed data and voice over IP, Seamless
and smooth handoff.
– OFDM (Orthogonal FDM) and MC-CDMA
(Multi Carrier CDMA)
– Hybrid Architecture: integration of Wireless
LAN (WiFi, Bluetooth) and wide area
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1G Cellular Systems
• Many Different Standards:
– AMPS (US)
– NMT (Northern Europe)
– TACS (Europe)
– NTT (Japan)
– many others...
• Spectrum
– around 800 and 900 MHz
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Frequency Division Duplex (FDD)
mobile
Forward Link
Reverse Link
Two separate frequency bands are used for
forward and reverse links.
Typically, 25 MHz in each direction.
base
station
AMPS: 824-849 MHz (forward or downlink)
869-894 MHz (reverse or uplink)
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Frequency Division Multiple Access
(FDMA)
• The spectrum of each link (forward or reverse) is
further divided into frequency bands
• Each station assigned fixed frequency band
frequency bands
idle
idle
idle
8

Number of User Channels in AMPS
• Bandwidth allocated to each user in each link
(forward or reverse) is 30 KHz.
• No. of user channels
= Total bandwidth / user bandwidth
= 25 MHz / 30 kHz
= 833
• Is it enough?
9

f
Frequency Reuse
Radio coverage,
called a cell.
The same frequency can be
reused in different cells, if they
are far away from each other
f
10

Cellular Architecture
MSC PSTN
MS
BSC
MS – Mobile Station
BSC – Base Station Controller
MSC – Mobile Switching Center
PSTN – Public Switched Telephone
Network
segmentation
of the area
into cells
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Geometric Representation
• Cells are commonly represented by hexagons.
• Why hexagon?
• How about circle?
• How about square, or triangle?
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Hexagonal
Cells
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Channel Reuse
• The total number of channels are divided into
K groups.
– K is called reuse factor or cluster size.
• Each cell is assigned one of the groups.
• The same group can be reused by two different
cells provided that they are sufficiently far
apart.
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Example:
K = 7
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Reuse Distance
• How far apart can two users share the same
channel?
– It depends on whether signal quality is acceptable
or not.
– The larger the distance between the two users, the
better the signal quality.
• How to measure signal quality?
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Nyquist Bandwidth
(for noiseless channel)
• For binary signals (two voltage levels)
– C = 2B
• With multilevel signaling
– C = 2B log 2 M
• M = number of discrete signal or voltage levels
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Signal Quality
• The signal quality depends on the ratio between
signal power and interference (noise) power.
S
I
i
S
Ii
Interference from the i-th
interfering BS.
• This is called signal-to- noise (interference) ratio
(SNR or SIR).
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Signal-to-Noise Ratio
• Ratio of the power in a signal to the power contained
in the noise that’s present at a particular point in the
transmission
• Typically measured at a receiver
• Signal-to-noise ratio (SNR, or S/N)
( SNR)
dB 10log10
signal power
noise power
• A high SNR means a high-quality signal, low
number of required intermediate repeaters
• SNR sets upper bound on achievable data rate
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Shannon Capacity Formula
• Equation:
C
B log
2 1
SNR
• Represents the theoretical maximum data rate that can
be achieved for noisy channel
• In practice, only much lower rates are achieved
– Formula assumes white noise (thermal noise)
– Impulse noise is not accounted for
– Attenuation distortion or delay distortion not accounted for
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Example of Nyquist and Shannon
Formulations
• Spectrum of a channel between 3 MHz and 4
MHz ; SNR dB = 24 dB
B
SNR
SNR
4 MHz 3 MHz 1MHz
dB
24 dB 10
log
251
• Using Shannon’s formula
C
10
6
log
2
10
SNR 6
1
251 10 8
8Mbps
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Example of Nyquist and Shannon
Formulations
• How many signaling levels are required?
C
810
4
M
2B
6
log
2
16
log
2
2
M
M
6
10
log
2
M
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24
HANDOFF IN CELLULAR
SYSTEMS

Handoff in Cellular Systems
• Handoffs occur when a mobile host starts
communicating with a new base station.
• Continuation of an active call when the mobile is
crossing cell boundaries Parameters:
Minimize Forced
Termination Prob
Minimize Call Blocking
Prob.
More handoffs means
more FTP
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Handoff Decisions
• Usually based on received signal strengths (RSS)
from Current BS and neighboring BSs.
– Handoff occurs when the signal from the serving BS
is weak while the signal from a neighboring BS is
strong.
– The received signal strength is averaged over time
using an averaging window to remove momentary
fading due to geographical and environmental factors
• Ping Pong Effect
– Signal strength fluctuates randomly
– Several handoffs occur back and forth between two
BSs.
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Sample RSS
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Traditional Handoff Algorithms
Choose B new (i.e. handoff from B old to B new) if
1. P new > P old ,
Relative Signal Strength: The RSSs are measured over time and
the BS with strongest signal is chosen to handoff. Due to signal
fluctuations, several handoffs can be requested while B old’s
RSS is still sufficient to serve the Mobile Station (MS). So,
more Ping-Pong effect and more FTP.
2. P new > P old and P old < T,
Relative Signal Strength with Threshold: Introduces a threshold
value to overcome the ping-pong effect. Handoff process is
initiated if B new’s RSS is stronger than B old’s RSS and B old’s RSS
is lower than the threshold (T).
** Here, P new : B new’s RSS by MS and P old : B old’s RSS by MS
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Traditional Handoff Algorithms
3. P new > P old + H,
Relative signal strength with Hysteresis: Handoff process is
initiated when the B new’s RSS exceeds the B old’s RSS by the
hysteresis value H.
4. P new > P old + H and P old < T,
Relative Signal Strength with Hysteresis and Threshold:
Combines both the threshold and hysteresis values concepts to
come with a technique with minimum number of handoffs. The
handoff is requested when the B old’s RSS is below the threshold
(T) and B new’s RSS is stronger than B old’s RSS by the hysteresis
value H.
All the above techniques should ensure initiation of handoff
before the B old’s RSS falls below Receiver Threshold (minimum
acceptable RSS for call continuation). Otherwise ongoing call is
dropped.
A multi-level thresholds based algo which assigns diff thresholds to the
users according to their speed.
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Example
Receiver Threshold
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Handoff
• What happens when a user is mobile?
- Especially when crossing a cell boundary while continuing the
call.
• Handoff strategy is invoked.
– Find a new base station, allocating new voice and control
channel of the new BS
– Process handoff
– higher priority over new call invocation, when allocating unused
channels in a cell
– Optimal received signal level to initiate a handoff.
– Delta =Phandoff – Pusable , should not be too small or large
– Large -> unnecessary handoff, small->insufficient time to
complete handoff before a call is lost due to weak signal
– Dropped call also due to excessive delay by MSC (loads, no
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channels free in nearby cell)

Who and When
• Who initiates handoff
– Network Controlled Handoff (NCHO): The network
determines RSSI based on the locator receiver signal strength
info from all BSs. MSC decides whether handoff is required or
not. Used in AMPS. Load on network is high. Typically needs 5-
10 secs.
– Mobile Assisted Handoff (MAHO) : Mobile helps the tower,
mobile periodically measures the received power from
surrounding BSs and report to serving BS. Handoff initiated
when the recv power from a neighboring BS exceeds the power
recv from the current BS (by a certain amt or period), MSC
decides when to handoff. Used in GSM. Typically needs ~ 1sec.
– Mobile Controlled Handoff (MCHO): MS determines handoff.
MS make necessary measurements and if the RSS of a
surrounding BS exceeds a threshold, it initiates handoff. Used in
DECT. Typically needs ~ 100ms.
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Who and When
• When to initiate handoff (the reasons for handoff)
– When the mean signal (over some predetermined time)
from the current BS is below some threshold (or by
using any one of the previous 4 methods). That is to
avoid termination of calls.
– To release some channels in the current cell, so that
new calls can be accommodated
– channel used by a mobile affected by another phone
using the same channel in a different cell, then the call
is transferred to a different channel of the same cell or
on a different channel in another cell to avoid
interference.
– To reduce potential interference to other cells/users in
Micro cell/ Macro cell based system
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Types of Handoff
• Hard handoff
– Mobile user is passed between disjoint towers that assign different
frequency or adapt different air-interface technology.
– Communication channel is released first and the new channel is acquired
later from the neighboring cell.
– Service disruption, reduction in QoS
– Used by systems which use TDMA and FDMA such as GSM and GPRS
• Soft handoff
– Mobile user communicates to two or more towers simultaneously and the
signal is treated as a multipath signal
– Can establish multiple connections with neighboring cells.
– Used in CDMA systems, where the cells use same frequency band using
diff code words.
– Each MS maintains an active set of BSs, where BSs are added and removed
based on 2 RSS thresholds. So addition or removal of a BS to the active set
is called Soft handoff.
– Used by IS-95 and WCDMA
• Without handoff
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• Horizontal handoff
Types of Handoff
– Mobile user is passed between towers of
homogeneous networks
– Example: handoff involving two GSM cells.
• Vertical handoff
– Handoff between towers of different types of
networks.
– Handoff in heterogeneous networks. More complex
compared to Horizontal handoff.
– Example: handoff from one GSM based cell to one
WCDMA-based cell .
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High priority for Handoff
• In non-prioritization schemes, new calls and
handoff calls are treated the same way. Idle
channels of BS are assigned due to FCFS basis.
• Does not provide lower forced termination prob
• In order to provide lower forced termination prob,
prioritization schemes assigns more channels to
the handoff calls.
• Two prioritization schemes are: Guard Channels
(GC) and Queuing Handoff calls (QHC).
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Guard Channels(GC)
• Reserves some fixed or adaptively changing number of
channels for handoff calls only.
• The rest of the channels are used by new and handoff
calls.
• Forced termination prob is decreased.
• Cost is an increase in call blocking prob, decrease in
total carried traffic.
• BSs can get number of MSs in pre handover zone (PHZ)
from neighboring BSs and accordingly reserve that many
number of guard channels.
• Assign number of channels adaptively. When FTP
exceeds a predefined limit, the guard channel no is
increased. The no of guard channels is decreased in case
BS does not use reserved guard channels significantly.
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Queuing Handoff Calls (QHC)
• Queues the handoff calls when all of the channels are
occupied in a target BS. (Queuing does not guarantee a zero
FTP, since large delays will cause RSS to drop below recvr thres)
• When a channel is released, it is assigned to one of the
handoff calls in the queue.
• A new call request is assigned a channel if the queue is
empty and if there is at least one free channel in the BS.
• Time interval between handoff initiation and receiver
threshold makes it possible to use queuing handoff calls.
• QHC can be used with or without guard channel scheme.
• QHC can be timer based: when a channel is released at
BS, a timer is started. If a handoff request is done before
the timer expires, channel is assigned to it. Otherwise,
channel can be assigned to new or handoff calls
depending on their arrival order.
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Queuing Handoff Calls
• QHC can be Measurement based (MBPS):
Handoff calls are assigned priority dynamically
based on the power level they have. Calls with
power level close to receiver threshold have the
highest priority. Provided better results from
FCFS basis.
• QHC can be Most Critical First based (MCF):
determines the first handoff call that will be cut
off and assigns the first released channel to that
call. Use simple radio measurements to predict
the first cut off call.
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Queuing Handoff Calls
• QHC can be with guard channels: Both new calls
and Handoff calls are queued. A number of guard
channels are reserved for handoff calls. When
new calls are congested, a channel from guard
channels is used if it is available.
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Other practical problems with handoff
High speed vehicles can cross many “small”
cells in a short time (due to wide range of mobile
velocities).
Umbrella cell which is co-located with some smaller microcells.
Large cell with a powerful tower to handle high speed vehicles
(Macrocell/Microcell concepts).
No of handoff is minimized for high speed devices and provide
additional microcell channels for pedestrian users.
Ping-Pong Effect
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Other practical problems with handoff
• Another problem is called cell dragging.
– Happens when the user (in LOS) moves slowly away from the
cell and the tower didn’t recognize it due to strong average
signal. (RSS at B old may be above handoff threshold, so no HO)
– Creates interference and traffic management problem because
users has meanwhile traveled deep within the neighboring cell.
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