cellular-communications

1. 1
Introduction to
Cellular Mobile Communications
Dr. Essam Sourour

2. Section Overview
• Historical Development
• Cellular System Components
• Cellular Concept & Frequency Reuse
• Co-Channel Interference
• Cell Sectoring & Beam Tilting
• Channel Assignment Techniques
• Cell Splitting
• Handoff and Handoff Types
• Power Control, open loop and closed loop
• System Channels & Typical Call

3. Historical Development
• Started with isolated wireless service
areas
– Stand alone areas
– No connection to public telephones
• Added communication among service
areas belonging to same system
• Cellular Concepts developed
– Interconnection
– Connection to Public Telephony
– Handoff for uninterrupted service

4. Historical Development …
Switch
SwitchPSTN/ISDNPSTN/ISDN
Isolated Mobile Systems
Basic Cellular System
Historical Background

5. • First generation cellular mobile
communications: (1980 )
– Technology: FDMA and Analog Technology.
– Systems: AMPS(USA), NMT-900(Sweden),
HCMTS(Japan)
– Shortages: Low capacity, poor Security, Low
quality, voice only (no data)
– Advantages: enough for the time
Historical Development

6. Historical Development …
• Second generation, 2G : (1992)
– Technology: TDMA, TDMA hybrid FDMA
– Systems: DAMPS(USA, IS-54), GSM
– Advantages: Higher Capacity, good Security, good
speech quality
– Technology: CDMA (Qualcomm)
– Systems: CDMA(IS-95)
– Advantages: Good Security, Higher & Soft Capacity,
Speech Activity, Multipath Diversity Rx., good speech
quality
– Shortages: Mainly Voice Service & low data rate
Services

7. Historical Development …
• 2.5 Generation, 2.5 G: (1996-2000)
– GPRS in Europe: Higher data rate (up ~ 150
kbps), Packet Switched Data, compatible
with GSM
– IS-95 B in US: Higher Data rate (up to ~114
kbps), Packet or Switched Data, compatible
with IS-95

8. Historical Development …
• The third generation 3G: (2001-2005)
– Support Multimedia Service, especially Internet
Service, 144kb/s(Outdoor and higher velocity ),
384kb/s(from Outdoor to indoor, lower velocity),
2Mb/s(indoor)
– Better Speech Quality and other services
– New Technologies like Tx Diversity, Turbo coding,
Multiuser Detection and Interference Cancellation,
Beam forming and Smart Antennas

9. System Components
• Mobile Stations
– Transceiver
– Antenna
– Control circuitry
– Moves at pedestrian or vehicle speeds
• Base Transceiver Station (BTS)
– Several transmitters and receivers
– Tower that supports several transmitting and receiving antennas
• Base Station Controller (BSC)
– Control one or more BTS
– Bridge between all mobile users of the BTSs and a Mobile switching
center
• Mobile Switching Center (MSC)
– Connects MSs to the PSTN (public-switched telephone network)
– Coordinates activities of all BSCs
– Controls all billing and system maintenance functions
– Several MSCs in large cities

10. System Components …

11. Cellular Concept
• Idea: replace high power transmitter
with several lower power transmitters to
create small “cells”
• Multiple cells cover a geographic area
• Each cell assigned a set of frequencies
• Neighboring cells assigned different
group of frequencies to reduce adjacent-
cell interference

12. Cellular Concept…
 Enables spatial frequency reuse
 Increase capacity by increasing number of
transmitters and decreasing transmit power
 Enables fixed bandwidth to serve arbitrarily
large number of subscribers
 Users within a cell communicate with the
cell BS
 As users move between cells, calls go
through “hand-off” when switching to new
cell BS

13. Cellular Concept…
• Large-radius cells for large coverage area
with small number of users
• Evolve into small-radius cells when number
of users increase using cell-splitting
• Main ideas of cellular systems
– Small coverage areas (cells)
– Frequency reuse
– Handoff
– Cell splitting to increase capacity

14. Why are Cells Required?
• Original mobile voice networks used
transmitter with large power to cover
very large area
• Capacity severely limited by available
bandwidth
• Spectrum limited, so could not increase
capacity by adding new channels
• Cellular concept was born
– Ushered in modern communication systems

15. Frequency Reuse
• Design cells to be non-overlapping and cover
entire region
• Cells depicted as hexagons
– Conceptual design allowing easy analysis of system
– Close to circular shape achieved by omnidirectional
antennas
• “Footprint”: actual radio coverage of a cell
– Determined from field measurements or propagation
prediction models
– Amorphous in nature
– Use hexagon to approximate shape

16. Frequency Reuse…
• Due to Co-channel Interference (CCI),
cannot use same frequency in adjacent cell
• Cells that use same frequencies must be
separated by distances large enough to keep
interference levels low
• Frequencies assigned to different cells using
frequency reuse plan
• Adjacent cells assigned different
frequencies to avoid interference or
crosstalk

17. Frequency Reuse…
• Objective is to reuse frequency in nearby
cells
• 10 to 50 frequencies assigned to each cell
• Transmission power controlled to limit power at
that frequency escaping to adjacent cells
• The issue is to determine how many cells must
intervene between two cells using the same
frequency

18. Frequency Reuse…
• Cells with same letter use the same set of
frequency channels
• Using hexagonal cells, BS located at center of
cell
• MS at edge of cell receives weak signal from
BS, i.e., low Carrier to Interference ratio (C/I)
A
F
E
D
B
G C
A
F
E
D
B
G C
A
F
E
D
B
G C

19. Frequency Reuse…
• Suppose system has S total channels & k
channels per cell (k < S)
• Channels divided among N cells into disjoint
groups, S = kN, N cells which use all S channels
called “cluster” (N = cluster size, typically 4, 7, 12)
• Clusters replicated in system
• Typically cluster size N = i2
+ ij + j2
– N=7  i=2, j=1
– N=3  i=1, j=1
Move i cells in any direction
Turn 60o
CCW
Move j cells in this direction

20. Frequency Reuse…
A
F
E
D
B
G C
A
F
E
D
B
G CN = 7
i=2, j=1
A
B
C
B
B
C C
C
B
B
A
C
A
AA
N = 3
i=1, j=1

21. Frequency Reuse Pattern
2
3
1
N=3
1
2
4
3
4
1
3
6
5
27
N=4
N=7
1
1
1
1
N=1

22. Cell Geometry
D
R
R
R
Nq
R
D
3==

23. Co-Channel Interference
• Cells using the same frequency cause
interference to each other
• Called co-channel interference (CCI)
• CCI increases as the cluster size N
decreases
• Important factor for signal quality is the
Carrier to Interference Ratio C/I
• Most interference comes from the first tier
of co-channel cells

24. Co-Channel Interference…
1
1
1
1
1
1
1
11
1
1 1
1
Interfering Cell
First tier
Second tier
D
R

25. ∑=
= IK
k
kI
C
I
C
1
C/I is calculated as:
The maximum number of K in the first tier is 6 and knowing that
γγ
α −−
=∝ RRC
∑=
−
−
= IK
k
kD
R
I
C
1
γ
γ
γγ
α −−
=∝ DDI
Wanted signal
Interfering signal
The above equation becomes:
Co-Channel Interference…
KI = # of interfering cells

26. ( )
11
1 1
II
KK
k
k
kk
C
I D q
R
γ
γ
−
−
==
= =
 
 ÷
 
∑∑
Rearranging:
and
R
D
q k
k =
The qk is the co-channel interference reduction factor with kth
co-channel interfering cell.

27. Co-Channel Interference…
• As N decreases the number of frequency
channels per cell increases but C/I
decreases
• C/I is improved by different methods
– Sectored antennas: reduces KI
– Beam tilting: Reduces power to co-channel
cells
– Channel assignment: minimizes activation of
co-channel frequencies, which reduces KI

28. CCI Reduction: Cell Sectoring
• Shown 120 sectored
antennas
• Channel per cell are
divided among 3 sectors
• CCI decreased. Sector 0
gets interference from
sectors 4, 5 and 6 only
• 60 degrees sectored also
possible

29. CCI Reduction: Beam Tilting
By tilting down the antenna beam, the power
outside the cell, causing CCI reduces

30. Channel Assignment
• Fixed Channel Assignment
– Cell allocated predetermined set of channels
– Any call within the cell must use one of the
unused channels assigned to cell
– If all channels used, call is blocked
• Channel Borrowing
– If all channels are used in a cell has, it can,
temporarily, borrow from neighboring cells
– MSC supervises borrowing
– Should not cause high CCI to other cell

31. Channel Assignment …
Dynamic Channel Assignment
• Channels not permanently assigned to cells
• BSC requests channel from MSC when call made
• MSC allocates channel to call based on algorithm that takes
into account
– Probability of future blocking within cell
– Frequency of use of candidate channel
– Reuse distance of channel
• MSC assigns channel that will not interfere with existing calls
• Reduces probability of blocking &Increases channel
utilization

32. Cell splitting
• If higher capacity is needed in a spot, we need to go,
locally, to smaller cluster size N
• Each cell can be split into multiple “microcells” with own
BS
• Rescaling system to smaller cell size
• Transmit power of BS reduced to obtain smaller
coverage area than original BS
• Enables more spatial reuse → greater system capacity
• Cell splitting preserves original frequency reuse plan
• Cell splitting causes increased handoff
• Can use “umbrella” cells where fast-moving mobiles
covered by original cell and slower mobiles covered by
microcells

33. Cell Splitting Example
F D
B
G C
F
E
D
G
C
F
E
D
B E
B
G
C
D
E
F
G
B
C
A
F D
B
G C
F
E
D
G
C
F
E
D
B E
B
G
C
A

34. Cell Splitting Example...

35. Handoff
• Mobiles may move out of coverage area of a cell and into
coverage area of a different cell during a call
• MSC must identify new BS to handle call
– MSC must seamlessly transfer control of call to new BS
– MSC must assign call new forward and reverse channels within
the channels of new BS
• Some important performance metrics in handoff:
– Seamless – user should not know handoff occurring
– Minimum unnecessary Handoff due to short time fading
– Low probability of blocking new calls in the new cell
– Handoff to a good SNR channel so that an admitted call is not
dropped

36. Handoff ...
• Handoff Main Steps
1. Initiation: either mobile or network identifies need for handoff and
begins the process
2. Resource reservation: required resources necessary to support
handoff are allocated
3. Execution: actual handoff takes place and mobile uses new
resources
4. Completion: unneeded resources are freed
• Important handoff parameter:
– SNRold to initiate handoff based on minimum acceptable quality
– SNRnew of the target channel (SNRnew > SNRold )
– D = SNRnew - SNRold dB
1. If D too small, unnecessary handoffs occur
2. If D too large, may be insufficient time to complete handoff before SNRold
becomes too weak and signal is lost

37. Handover decision
receive level
BTSold
receive level
BTSnew
MS MS
HO_MARGIN
BTSold BTSnew

38. Handoff ...
• Intersystem handoff
– Handoff may be to a cell in a different system
– Requires compatibility of different MSCs
– Roaming requirements important
• Some systems prioritize handoff over new calls
– Dropped calls more annoying than blocked calls
– Guard channel
 Some voice channels reserved for handoff
 Reduces total carried traffic
 Can use dynamic channel assignment to increase efficiency
– Queuing handoff requests
 Effectiveness depends on the time interval between when handoff
initiated and when the call will be dropped due to low signal strength

39. Handoff Strategies
• BS typically averages signal strength over moving window of
time to remove rapid fluctuations due to multipath fading
• Handoff will occur using different metrics
– Relative SNR strength
 When SNR at new BS higher than SNR at current BS
– Relative SNR strength with threshold
 When SNR at current BS is below a threshold and SNR at new BS is
higher than at current BS
– Relative SNR strength with hysteresis
 When SNR at new BS is stronger than at current BS by a threshold
– Relative SNR strength with hysteresis and threshold
 SNR at current BS below a threshold and at new BS stronger than at
current BS by a threshold

40. Mobile Assisted Handoff (MAHO)
• Mobile stations measure received SNR
from surrounding BSs
– Inform current BS of measurements
– Handoff initiated when SNR from other BS
exceeds SNR from current BS by a certain
level or for a certain period of time
• Handoff much quicker using MAHO

41. Handoff Problems
• High-speed mobiles require frequent handoffs
– Burdens MSC
– Can use “umbrella cells” to minimize handoff
– Pedestrian users covered in small cells
– High-speed users covered in large umbrella cell
– Minimizes handoffs for high-speed users while ensuring capacity for
pedestrian users
• Cell dragging
– If a user has a good LOS path to BS, SNR might be large even when
user has left the cell
– Causes interference and traffic management problems (user in new
cell but managed by old BS)
• Handoff times
– 1st
generation analog systems: 10 s
– 2nd
generation digital systems: 1-2 s (using MAHO)

42. Types of Handoff
• Hard handoff (Break before Make)
– Whenever mobile enters new cell, must be assigned new channel for
communication
– E.g., FDMA, TDMA
• Soft handoff (Make before Break)
– Mobile can use channels from two or BS simultaneously
– Mobile adds new channel from the target BS(s)
– Signal from multiple BSs are combined (Macro diversity)
– Mobile concurrently transmitting to and receiving from multiple BSs
– BS with low SNR is dropped
– Used mainly with CDMA (IS-95, CDMA2000 & WCDMA)

43. Power Control
• If MS is near the BS or in LOS situation, power
to/from the MS can be reduced
– Helps Reduce CCI
– Save battery power
– Alleviate health concerns
• Coarse power control is adequate
– Implemented through Open Loop Power Control
• In CDMA systems all MS use same frequency
• Fine power control is crucial to mitigate the
near-far effect
– Both Open & Closed Loop Power Control

44. Open-Loop Power Control
• MS measures power on forward link
• If power is high, mobile unit reduces its
uplink power and vice versa
• Power measurements averages the
Rayleigh fading
– Depends on distance and shadowing only
• No feedback from BS
• Not as accurate as closed-loop power
control

45. Closed-Loop Power Control
• Performed on top of the Open Loop Power
Control
• BS measures the uplink power from MS
• BS transmits power control commands on
the forward link
• MS steps its power up or down
accordingly
• In IS-95, the power control rate is 800 Hz

46. Cellular System Channels
• Control channels
– Forward and Reverse
– Setting up and maintaining calls
– Exchange commands an/or messages
between MS and BS or MSC (as needed)
• Traffic channels
– Carry voice and/or data traffic

47. Typical call
• Mobile unit initialization
– Scan and select strongest set up control channel
– Automatically selected BS antenna of cell
• Usually but not always nearest (propagation
environment)
– Handshake to identify user and register location
– Scan repeated to allow for movement
• Change of cell
– Mobile unit monitors for pages (see below)
• Mobile originated call
– Check set up channel is free
• Monitor forward channel (from BS) and wait for idle
– Send number on pre-selected channel

48. Typical call…
• Paging
– MSC attempts to connect to mobile unit
– Paging message sent to BSs depending on called
mobile number
– Paging signal transmitted on set up channel
• Call accepted
– Mobile unit recognizes number on set up channel
– Responds to BS which sends response to MSC
– MSC sets up circuit between calling and called
BSs
– MSC selects available traffic channel within cells
and notifies BSs
– BSs notify mobile unit of channel

49. Typical call…
• Ongoing call
– Voice/data exchanged through respective BSs
and MSC
• Handoff
– Mobile unit moves out of range of cell into
range of another cell
– Traffic channel changes to one assigned to
new BS
• Without interruption of service to user

50. Summary
• Cellular Concept
• Frequency Reuse
• Co-Channel Interference
• Channel Assignment and Cell Splitting
• Handoff Issues and Handoff Types
• Power Control
• Typical Call Scenario