Lecture 1
Communications Systems
Mobile Cellular Communications Systems
 Prof. Masoud Alghoniemy
◼ EE building, 4th floor
◼ Office hours: before lecture at the office and by appointment
◼ E-mail: alghoniemy@alexu.edu.eg
 Textbook: wireless Communications: Principles and Practice
 Author: Rappaport T. S.
 Publisher: Pearson Education
 Course tone:
◼ Easy problems
◼ Understand the concepts
2
Course Outline
 Introduction
 The Cellular Concept - system design fundamentals
 Wireless Channels
3
Lecture Outline
 Introduction
 Cellular Telephone Systems
 System Capacity
 Channel Assignment Strategies
 Handoff Strategies
 Co-channel Interference
4
I. Introduction
 Examples of mobile communication systems
◼
◼
◼
◼
◼
◼
Garage door openers
Remote controllers
Cordless phones
Hand-held walkie-talkie
Pagers “beepers”
Cellular phones
 Nowadays: mobile → cellular
 Cost, complexity, performance and type of services are
vastly different
5
Wireless systems definitions
 Base station (BS): a fixed station used for radio communication with
mobile station and located at the center/ or on the edge of a coverage
region.
 Mobile station (MS): mobile user’s unit
 Control channel (CC): radio channel used for transmission of call
setup, request, initiation, and other control purposes.
 Forward channel (FC): radio channel used for transmission from the
BS to the mobile unit. (Downlink/ FL)
 Reverse channel (RC): radio channel used for transmission from the
mobile unit to the BS. (Uplink/ RL)
 Full Duplex systems: allow simultaneous two-way communication
(transmission and reception) (FDD/TDD)
6
Wireless systems definitions
 Half-duplex systems: at any given time, the user can only either
transmit or receive information using the same radio channel.
 Simplex systems: communication is only possible in one direction
(e.g., paging systems without acknowledgment)
 Handoff: the process of transferring a mobile station (unit) from one
base station/ channel to another.
 Base Station Controller (BSC): manages the radio network functions
and interfaces between the MSCs and BSs
 Mobile Switching Center (MSC): switching center which coordinates
the routing of calls in a large service area.
 Transceiver: a device capable of simultaneously transmitting and
receiving radio signals
7
MS, BS, BSC, MSC, and PSTN
Home phone
PSTN
…
MSC
…
BSC
BSC
BSC
…
…
…
BS MS
BS MS
MSC
BS MS
BS MS
BS MS
…
BSC
…
BS MS
BS MS
BS MS
8
II. Cellular Telephone Systems
 Provides a wireless connection to the Public Switched Telephone Network
(PSTN) for any user location within the radio range of the system
 Large geographic coverage area
 Efficient use of limited frequency spectrum
 High user mobility,
sophisticated handoff mechanisms
 High system capacity
Large number of simultaneous users
◼ obtained by limiting coverage of each base station to small area (cell)
◼ frequency spectrum can be reused by other non-adjacent cells in network
9
Cellular Telephone Systems
 Base station (BS)
◼
◼
◼
serves mobile users in each cell
bridge between mobile unit and BSC/MSC
connected to MSC via phone line or Line of Sight (LOS) microwave link
 Mobile Switching Center (MSC)
◼
connects cellular system to Public Switched Telephone Network (PSTN)
◼
controls base stations. “cellular network brain”
 call initiation/setup
 base station handoffs
 controlling power levels in mobile units
 billing information
 roaming user ID and verification
Typically handles 5000 simultaneous calls supporting 100,000 cellular
subscribers (at most 5% of subscribers are assumed to be active at anyone time)
◼
10
Common Air Interface (CAI)
◼ Communication between the BS and the mobiles
◼ Standard mechanism used by all mobiles.
◼ Defines 4 different channels to be used by a mobile unit
 Forward/reverse voice channels - FVC/RVC
◼ Full Duplex communication
 Forward/reverse control channels - FCC/RCC
◼ call initiation & setup
◼ makes up to 5% of total number of available channels
▪
One cell contains 10 to 60 voice channels and only 1 to 3
control channel pairs (F+R)
◼ MSC broadcasts call request from PSTN over all Forward Control
Channels (FCC) of all base stations – to find a mobile user
11
The old days!
 Large coverage area
✓
✓
✓




Single Tx, high power, and tall tower
Low cost
Very good coverage
Small number of users
Poor spectrum utilization
No frequency reuse
Example: Bell mobile system in NY city in 1970

Could only support 12 simultaneous calls over a thousand square miles
 A breakthrough was made by introducing the cellular concept
◼ Offered very high capacity in a limited spectrum allocation
12
Nowadays: The Cellular Concept
 A fixed number of channels is allowed to serve an arbitrarily large
number of subscribers by reusing the channels throughout the region
 Small coverage areas called “cells”
 Cells labeled with the same letter use the same group of channels
(frequencies).
 No two adjacent cells
should use the same frequency
✓ Many base stations,
lower power, and shorter towers
13
Cell Shape
 Hexagonal cell shape is conceptual and is assumed for planning
 permits easy and manageable analysis → circles leave gaps
 Has the largest area for a given distance between the center to
the furthest point
 actual cell “footprint” is amorphous (no specific shape)
◼ Is determined by field measurements where BS
successfully serves mobile units
𝑹
𝑹
𝑹
𝑹
Ideal cell
used cell shape
Not used cell shape
actual cell
14
Base Station Antenna
 Base station antennas designed to cover specific cell area
 cell center → omni-directional antenna (360° coverage)
 not necessarily in the exact center (can be up to R/4 from the
ideal location)
Sectored antenna
Omni antenna
15
Formation of a Cluster
𝑁=3
 Cell Cluster: group of N-cells
using complete set of the available
channels
 𝑵: cluster size
 Cells in a cluster should be assigned
a different set of channels/
frequencies
 Cells in neighboring clusters can
reuse the same set of frequencies in
the same spatial pattern
𝑁=4
16
 Each cell is allocated 𝒌, a percentage (%) of the total number of
available channels, 𝑺. (frequencies)
◼ If the total number of available channels is 𝑆. Then, each cell is
assigned k ≒ S / N channels
 Nearby (adjacent) cells assigned different channel groups
◼ to prevent interference between neighboring base stations and mobile
users
 Same frequency channels may be reused
by cells at a “reasonable” distance away
◼ reused many times as long as
interference between same channel
(co-channel) cells is < acceptable level
𝑁=3
17
Frequency reuse factor
 Frequency reuse factor = 1 / N
◼ each frequency is reused every N cells
Spectrum utilization of 1/𝑁 within each cell
Each cell uses 1/N of the available channels (spectrum)
 N cells/cluster
𝑁=7
18
Formation of a Cluster
 Due to the hexagonal structure of the cells, cluster size 𝑁 is not
arbitrary
 Cluster size 𝑁 can only take values 3, 4, 7, 12, 13, 19, and 27, etc.
according to the formula
𝑁 = 𝒊𝟐 + 𝒊𝒋 + 𝒋𝟐 , where 𝑖, 𝑗 are integers ≥ 0
.
𝒊
𝒋
Cluster size 𝑵
1
2
1
0
3
4
1
2
3
3
2
2
2
3
7
12
19
27
4
3
37
typical values for the cluster size N
Formation of a Cluster
 To find the nearest co-channel neighbors of a particular cell
◼ (1) Move i cells along any chain of hexagons
◼ (2) turn 60 degrees and move left j cells.
20
Frequency reuse
 As frequency reuse↑ → more base stations with less power → number
of possible simultaneous users↑ → number of subscribers ↑→ but
system cost ↑ (more towers)
 The cellular concept allows all mobiles to be manufactured to use the
same set of frequencies
Frequency reuse permits a fixed number of channels to serve
a large number of users by reusing channels in a coverage area
21
III. System Capacity
𝒌
𝟏
2
𝟑
𝑵
𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦
𝑺
 S : total number of duplex channels available for use in a given area
(cluster); determined by:
◼ Total amount of allocated spectrum & channel BW
 k : number of channels allocated to each cell (k < S)
 N : cluster size → number of cells forming a cluster

S=kN
22
System Capacity
 M : number of times a cluster is replicated over a geographic coverage area
equals to the number of frequency reuse
 System Capacity (C) = Total number of Duplex Channels
“ in a specified area even if reused ”
C=MS=MkN
(assuming exactly MN cells will cover the area)
✓
If cluster size (𝑁 ↓) is reduced while cell size 𝐢𝐬 kept constant → frequency reuse
(𝑀 ↑) must increase to cover the whole area → capacity (𝐶 ↑)

As (𝑀 ↑) & (𝑁 ↓) → interference from co-channel cells ↑
The smallest possible value of N is desirable to maximize system capacity.
◼ The actual value of N is determined by how much interference a BS or MU
can tolerate.
23
Example
 FDD cellular system has a BW of 33 MHz. Each user uses two 25 kHz
simplex channels to provide full-duplex channel. Compute the number of
available channels/cell in the following cases:
(a) 𝑵 = 𝟒 cell reuse
(b) 𝑵 = 𝟕 cell reuse
(c) 𝑵 = 𝟏𝟐 cell reuse
Solution:
User’s duplex channel bandwidth = 25 𝑘𝐻𝑧 × 2 = 50 𝑘𝐻𝑧
Total number of available channels = total BW/user BW =
33000
50
= 𝟔𝟔𝟎 channels
660
= 165 channels
4
660
7, the number of available channels per cell, 𝑘 =
= 95 channels
7
660
12, the number of available channels per cell, 𝑘 =
= 55 channels
12
(a) For 𝑁 = 4 , Number of available channels per cell, 𝑘 =
(b) For 𝑁 =
(c) For 𝑁 =
24
 If 1MHz of the allocated spectrum is dedicated to control channels.
Determine an equitable distribution of control and voice channels in each
cell for
(a) 𝑵 = 𝟒 cell reuse
(b) 𝑵 = 𝟕 cell reuse
(c) 𝑵 = 𝟏𝟐 cell reuse
Solution: Total number of control channels =
1𝑀𝐻𝑧
50𝑘𝐻𝑧
=
1000
50
= 𝟐𝟎 channels
total umber of voice channels = 660 − 20 = 𝟔𝟒𝟎 channels
In practice, each cell is allocated one control channel (1 CC)
𝑵
Control channels/ cell
Voice channels/ cell
4
1 CC/ cell (4 out of 20)
4 cells: 640/4=160 VC/ cell
7
1 CC/ cell (7 out of 20)
4 cells: 91 VC/ cell
3 cells: 92 VC/ cell
12
1 CC/ cell (12 out of 20) 8 cells: 53 VC/cell
4 cells: 54 VC/cell
25
IV. Channel Assignment Strategies “frequency planning”
 Design process of selecting & allocating channel groups of cellular base
stations
 Two competing/conflicting objectives:
1.
maximize frequency reuse 𝑴 ↑, 𝑵 ↓ in specified area to maximize
capacity (C)
 Requires more reuse of channel frequencies
2.
minimize interference between cells
 Requires less reuse of channel frequencies 𝑴 ↓
Recall: smaller N → greater frequency reuse → larger capacity (C) → higher
interference
26
IV. Channel Assignment Strategies “frequency planning”
 Two main strategies for channel assignment:
◼ A-Fixed channel assignment
◼ B-Dynamic channel assignment
 Impacts the performance of the system
27
A-Fixed Channel Assignment
𝒌
𝟏
2
𝟑
𝑵
𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦
𝑺
◼ Each cell is permanently allocated a pre-determined set of
voice channels
 calls within a cell are only served by unused cell channels
 all channels used → blocked call → no service
◼ Several variations:
 MSC allows cell to borrow a VC (that is to say, FVC/RVC
pair) from an adjacent cell
 donor cell must have an available VC to donate
28
B-Dynamic Channel Assignment
◼ Channels are NOT allocated permanently
◼ All the available channels in the market are accessible to all
cells
◼ Call request → goes to serving base station → goes to MSC
◼ MSC allocates channels “on the fly”
 allocation strategy considers:
◼ likelihood of future call blocking in the cell
◼ reuse distance (interference potential with other cells that
are using the same frequency)
29
Dynamic Channel Assignment
 Advantage: reduces call blocking (that is to say, it increases the
trunking capacity), and increases voice quality
 Disadvantage: increased storage & computational load @ MSC
more than the fixed channel assignment.
◼ Requires: real-time data from entire network related to:
 channel occupancy
 traffic distribution
 Radio Signal Strength Indications (RSSI's) from all channels
30
V. Handoff Strategies
 Handoff: when a mobile unit moves from one cell to another
while a call is in progress, the MSC must transfer (handoff) the
call to a new channel belonging to a new base station
◼ new voice and control channel frequencies
◼ very important task → often given higher priority than new call
 It is worse to drop an in-progress call than to deny a new one
31
Signal Strength
Signal strength
(in dB)
Cell i
Cell j
-60
-70
-80
-90
-60
-70
-80
-90
-100
-100
Select cell i on left of boundary
Select cell j on right of boundary
Ideal boundary
32
Typical Cell Shape
Signal strength
(in dB)
Cell i
Cell j
-60
-60
-70
-80
-90
-100
-70
-80
-90
-100
Signal strength contours indicating actual cell tiling.
This happens because of terrain, presence of obstacles
and signal attenuation in the atmosphere.
33
Handoff Region
Signal strength
due to BSi
Signal strength
due to BSj
Pj(x)
Pi(x)
E
Pmin
BSi
◼
◼
◼
◼
MS
BSj
X1
X3
X5
Xth
X4
X2
lowest acceptable voice quality → Minimum usable signal level
call is dropped if below this level
specified by system designers
typical values → -90 to -100 dBm (𝒅𝑩𝒎 = 𝒅𝑩 + 𝟑𝟎)
34
 choose a (handoff threshold) > (minimum useable signal level)
◼ So, there is time to switch channels before level becomes too low
as mobile moves away from base station and toward another base station
35
 Handoff Margin △
△ = Phandoff threshold - Pminimum usable signal dB
◼ carefully selected
◼ △ too large → unnecessary handoff → MSC loaded
◼ △ too small → not enough time to transfer → call dropped!
 A dropped handoff can be caused by two factors:
◼ not enough time to perform handoff
 delay by MSC in assigning handoff
 high traffic conditions and high computational load on
MSC can cause excessive delay by the MSC
◼ no channels available in the new cell
36
Handoff Decision
◼ signal level decreases due to:
 signal fading → don’t handoff
✓ mobile moving away from base station → initiate handoff
◼ must monitor received signal strength (RSS) over a period of time
“use moving average filter” to avoid wrong handoff decision.
◼ time allowed to complete handoff depends on mobile speed
 large negative received signal strength (RSS) slope → high speed →
quick handoff
◼ statistics of the fading signal are important in making appropriate
handoff decisions
37
Handoff in 1st Generation Cellular
 1st Generation Cellular (Analog FM → AMPS)
◼ Received signal strength (RSS) of RVC measured at base
station & monitored by MSC
◼ A spare Rx in base station (locator Rx) monitors RSS of
RVC's in neighboring cells
 Tells Mobile Switching Center (MSC) about these mobiles and
their channels
◼ Locator Rx can see if signal to this base station is significantly
better than to the host base station
◼ MSC monitors RSS from all base stations & decides on
handoff
◼ Disadvantage: MSC is loaded
38
Handoff in 2nd Generation Cellular
 2nd Generation Cellular w/ digital TDMA (GSM, IS-136)
◼ Mobile Assisted Handoffs (MAHO)
 important advancement
 The mobile measures the RSS of the FCC’s from adjacent
base stations & reports back to serving base station
 if Rx power from new base station > Rx power from serving
(current) base station by pre-determined margin for a long
enough time period → handoff initiated by MSC
Signal strength
due to BSi
Signal strength
due to BSj
Pj(x)
Pi(x)
E
Pmin
BSi X
1
X3
MS
X5
Xth
X4
X2BSj
39
Advantages of MAHO
◼ MSC no longer monitors RSS of all channels
 reduces computational load considerably
 enables much more rapid and efficient handoffs
 imperceptible to users
40
Typical Handoff Parameters
 Analog cellular (1st generation)
 threshold margin △ ≈ 6 to 12 dB
 total time to complete handoff ≈ 8 to 10 sec
 Digital cellular (2nd generation) (MAHO)
 lower necessary threshold margin △ ≈ 0 to 6 dB
 total time to complete handoff ≈ 1 to 2 sec
41
Intersystem Handoff
 A mobile may move into a different system controlled by a
different MSC
◼ Called an intersystem handoff
◼ May happen when the MSC cannot find another cell with
the same system to transfer the call in-progress.
 What issues would be involved here?
◼ Local call → long distance call (higher cost)
◼ Compatibility issues between the two systems
◼ Handoff request may be blocked by the new system
42
Prioritizing Handoffs ‘reduce drop-offs’
 Issue: Perceived Grade of Service (GOS) – service quality as
viewed by users
 “quality” is measured in terms of dropped or blocked calls (not
voice quality)
 assign higher priority to handoff vs. new call request
 a dropped call (forced termination) is more annoying than an
occasional blocked call
 Solution 1: Guard Channels preserved for handoffs
◼ Percentage (%) of total available cell channels exclusively set
aside for handoff requests
◼ issue: makes fewer channels available for new call requests
◼ a good strategy is dynamic channel allocation (not fixed)
 adjust number of guard channels as needed by demand
 So, channels are not wasted in cells with low traffic
43
Prioritizing Handoffs ‘reduce drop-offs’
 Solution 2: Queuing Handoff Requests
◼ To decrease the probability of forced termination of a call.
◼ Trade-off between decreasing the probability of forced
termination and total carried traffic.
◼ Possible because of time delay between handoff threshold and
minimum usable signal level can be used to place a blocked
handoff request in a queue
◼ a handoff request can "keep trying" during that time period,
instead of having a single block/no block decision
◼ prioritize requests based on mobile speed and traffic pattern
◼ calls will still be dropped if time period expires
44
Practical Handoff Considerations
 Problems occur because of large variations of users' velocities
◼ pedestrian vs. high-speed vehicle user
 High-speed vehicles crosse the cell in a matter of seconds
→ too many handoff requests
 Pedestrians may never need handoffs.
 The introduction of Small and/or micro-cells → larger number of
handoffs
◼ Results in MSC becomes burdened when high speed users are passed
between very small cells
 Another difficulty is the inability of the service provides to obtain new
cell cites → cell is congested → more handoffs
45
Solution: Umbrella Cells
 Use different antenna heights and Tx power levels, at the same
tower, to provide large and small cell coverage
 multiple antennas & Tx can be co-located at single location if
necessary (saves on obtaining new tower licenses)
◼
◼
large area coverage (large cell) → high speed traffic → fewer handoffs
small area coverage (small cell) → low speed traffic
◼ example areas: interstate highway passing thru urban center, office
park, or nearby shopping mall
46
Cell Dragging Problem
 Results from pedestrian users (low speed) with line of sight to base
station (very strong signal) usually in urban areas
 Strong signal changing slowly
 user moves into the area of an adjacent cell without handoff
 causes interference with adjacent cells and other cells
 Remember: handoffs help all users, not just the one which is
handed off.
 If this mobile is closer to a reused channel → interference for
the other user using the same frequency
 So, this mobile needs to hand off anyway, so other users benefit
because that mobile stays far away from them.
 Solution: careful adjustment of handoff thresholds and radio coverage
parameters.
47
Soft vs. Hard Handoffs
◼ Hard handoff: different radio channels are assigned when
moving from cell to cell
 all analog (AMPS) & digital TDMA systems (IS-136, GSM,
etc.)
48
Soft vs. Hard Handoffs
 Soft handoff: SPREAD SPECTRUM users share the same
frequency in every cell. (CDMA systems (IS-95))
◼ Does not mean a physical change in the assigned channels, rather a
different BS handles the radio communication task.
◼ The MSC simultaneously monitors reverse link signal at several base
stations
◼ MSC dynamically decides which signal is best and then listens to that
one
◼ MSC passes data from that base station on to the PSTN
 This choice of best signal can keep changing.
 Mobile user does nothing for handoffs except just transmit, MSC
does all the work
 Advantage unique to CDMA systems
 As long as, there are enough codes available.
49
VI. Interference and System Capacity
 Interference is the limiting factor in performance of all cellular radio
systems
 Interference affects:
◼ voice channels: cross-talk
◼ control channels: missed or blocked calls
 Sources of interference:
◼
◼
◼
◼
Another mobile in the same cell
A call in-progress in a neighboring cell
Other BS operating in the same frequency band
Any non-cellular system leaking energy into the cellular band
 Two major types of system-generated interference:
1) Co-Channel Interference (CCI)
2) Adjacent Channel Interference (ACI)
50
Co-channel Interference (CCI)- Forward Link
 Cause: Frequency Reuse
◼ Many cells in a given coverage area use the same set of channel
frequencies to increase system capacity (C)
◼ Co-channel cells → cells that share the same set of frequencies
◼ Both VC & CC traffic in co-channel cells is an interfering source
to mobiles in several different cells
A
RL: UPLINK
FL: DOWNLINK
A
51
Possible Solutions?

1) Increase base station Tx power: to improve radio signal
reception?
 this will also increase interference from co-channel cells by the
same amount
 if all cell sizes, transmit powers, and coverage patterns are
almost the same → co-channel interference is independent of
Tx power
 no improvement
✓ 2) Physically separate: co-channel cells by some minimum
distance to provide sufficient isolation due to propagation of
radio signals?
✓
but if you increase the distance between co-channel cells (D),
you also need to increase cell radius (R) to maintain the same
frequency reuse pattern
52
Co-channel Reuse Ratio (Q)
 co-channel interference depends on:
◼ R : cell radius
◼ D : distance to base station of nearest co-channel cell
✓ if (D / R) ↑ then spatial separation relative to cell coverage area ↑
◼ improved isolation from co-channel
RF energy and reduces CCI
 Q=D/R:
co-channel reuse ratio for hexagonal geometry
𝑄=
𝐷
𝑅
= 3𝑁
Cluster
R
F7
F6
F2
F3
F1
F1
𝑸
(i,j)
N
(1,1)
3
3
(1,2)
7
4.58
(2,2)
12
6
F5
F4
F7
F6
13
F3
F1
F1
F5
(1,3)
F2
F4
6.24
53
Quality vs. Capacity
 Fundamental tradeoff in cellular system design:
◼ 𝑄 = 𝐷/𝑅 =
3𝑁
✓ small 𝑄 = 3𝑁 → small cluster size (N)→ more frequency reuse
→ larger system capacity (𝐶) → GREAT
✓ large 𝑄 = 𝐷/𝑅 → large cell separation (D/R)→ decreased co-channel
interference (CCI) → improved transmission quality → GREAT
◼ Tradeoff: Capacity vs. Voice Quality
The higher the capacity for a given geographic area, the poorer the
quality and vice-versa. (makes sense!!)
54