1 cellular wireless networks
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PrinciplesOperationCapacity
Interference1G to 4G systems
Cellular Wireless Networks
Maria Leonora GuicoTcom 126 2nd Sem Lecture 1
Acknowledgements to:1.Dr. Lawrie Brown (UNSW@ADFA) for the Lecture slides on “Data and Computer Communications”, 8/e, by William Stallings, Chapter 14 “Cellular Wireless Networks”.2.Lecture Slides on Chapter 4,” Communication Networks “ by Leon Garcia and Widjaja
Cellular Wireless Networks key technology for mobiles, wireless nets etc developed to increase mobile phone capacity based on multiple low power transmitters area divided into cells
in a tiling pattern to provide full coverage each with own antenna each with own range of frequencies served by base station adjacent cells use different frequencies to avoid crosstalk
Cellular Geometries
Cell Site vs Cell•The cell site is a location or a point, the cell is a wide geographical area. • Most cells have been split into sectors or individual areas to make them more efficient. • Antennas transmit inward to each cell and cover a portion or a sector of each cell, not the whole thing. Antennas from other cell sites cover the other portions. •The cell site equipment provides each sector with its own set of channels. •In this example, the cell site transmits and receives on three different sets of channels, one for each part or sector of the three cells it covers.
Cell site vs Cell (2)•Most people see the cell as the blue hexagon, being defined by the tower in the center, with the antennae pointing in the directions indicated by the arrows.• In reality, the cell is the red hexagon, with the towers at the corners.• The confusion comes from not realizing that a cell is a geographic area, not a point. We use the terms 'cell' (the coverage area) and 'cell site' (the base station location) interchangeably, but they are not the same thing."
Basic Concepts
Frequency Reuse A region is partitioned into cells Each cell is covered by base station Power transmission levels controlled to minimize inter-cell interference Spectrum can be reused in other cells
Handoff Procedures to ensure continuity of call as user moves from cell to another Involves setting up call in new cell and tearing down old one
Frequency Reuse must manage reuse of frequencies power of base transceiver controlled
allow communications within cell on given frequency limit escaping power to adjacent cells allow re-use of frequencies in nearby cells typically 10 – 50 frequencies per cell example for Advanced Mobile Phone Service (AMPS)
N cells all using same number of frequencies K total number of frequencies used in systems each cell has K/N frequencies K=395, N=7 giving 57 frequencies per cell on average
6
1
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5
4
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7
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Frequency Reuse Adjacent cells may not use
same band of frequencies Frequency Reuse Pattern
specifies how frequencies are reused
Figure shows 7-cell reuse: frequencies divided into 7 groups & reused as shown
Also 4-cell & 12-cell reuse possible
Note: CDMA allows adjacent cells to use same frequencies
FrequencyReusePatterns
No. of subscribers who can use the same set of frequencies (channels) in non-adjacent cells at the same time in a small area (city) is dependent on the total number of cells in the area
Number of simultaneous users is called the frequency reuse factor (FRF) FRF = N C
where: FRF = frequency reuse factor (unitless) N = total number of full-duplex channels in an area
C = total number of full-duplex channels in a cell
Frequency Reuse Factor
Frequency ReuseIn characterizing frequency reuse, the following parameters are commonly used:
D = minimum distance between centers of cells that use the same band of frequencies (called cochannels)R = radius of a celld = distance between centers of adjacent cells (d = √3R)N = number of cells in repetitious pattern (reuse factor), each cell in pattern uses a unique band of frequencies.
With a hexagonal cell pattern, the following values of N possible N = I2 + J2 + (I x J), I, J = 0, 1, 2, 3, …
Possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, etc
Reuse Distance
D N (reuse pattern or cluster size)
3.46R 4
4.6R 7
6R 12
7.55R 19
Reuse Distance (2)
Cluster size and First Tier Geometry of a hexagon is such that the number of cells
per cluster can only have values that satisfy the equationN = i2 +ij + j2
where N = number of cells per cluster
i and j = nonnegative integer values
Process of finding tier with nearest co-channel cells (first tier) is:
1. Move i cells through the center of successive cells.2. Turn 60o in a counterclockwise direction.3. Move j cells forward through the center of successive cells.
First Tier Co-channel Cells(Sample figure for 19-cell cluster)
Increasing Capacity (1) Add new channels
not all channels used to start with Frequency borrowing
taken from adjacent cells by congested cells or assign frequencies dynamically
Cell splitting non-uniform topography and traffic distribution use smaller cells in high use areas
Increasing Capacity (2) cell sectoring
cell divided into wedge shaped sectors (3–6 per cell) each with own channel set directional antennas
microcells move antennas from tops of hills and large buildings to tops of
small buildings and sides of large buildings use reduced power to cover a much smaller area good for city streets, roads, inside large buildings
Cell Splitting
Cell Sectoring In basic form, antennas are omnidirectional Replacing a single omni-directional antenna
at base station with several directional antennas, each radiating within a specified sector.
Cell Sectoring (2) Achieves capacity improvement by essentially rescaling the
system. By using sectorized antennas (120 degrees, 60 degrees) co-
channel interference is reduce. Less co-channel interference, number of cells in a cluster can be
reduced which leads to more channels per cell. Larger frequency reuse factor, larger capacity
Micro Cell Zone Concept Large control base station is replaced by
several lower powered transmitters on the edge of the cell.
The mobile retains the same channel and the base station simply switches the channel to a different zone site and the mobile moves from zone to zone.
Since a given channel is active only in a particular zone in which mobile is traveling, base station radiation is localized and interference is reduced.
Typical Parameters of Traditional Cells
Macrocell Microcell
Cell Radius 1 to 20 km 0.1 to 1 km
Transmission Power 1 to 10 W 0.1 to 1 W
Average Delay Spread 0.1 to 10 uS 10 to 100 ns
Maximum Bit Rate 0.3 Mbps 1 Mbps
Cell Splitting vs Cell Sectoring- Using Cell-Splitting and Sectoring to Improve Capacity/Coverage of Cellular Systems
- Cell splitting is achieved by installing smaller cells (microcells) in saturated macrocellular regions. More channels become available to users that appear in the saturated region (based on demand and economic consideration)
As a service area becomes full of users, this approach
is used to split a single area into smaller ones.
In this way, urban centers can be split into as many
areas as necessary to provide acceptable/sound
service levels, while larger, less expensive cells can be used to cover remote rural
regions
Handoff/Handover- Process of transferring a call from one base
station to another when a user's radio signal becomes weaker at the first and stronger at the second base station.
- "Weaker" and "stronger" is quantified by a signal threshold level, which is above the minimum signal level for acceptable voice communications.
- Selecting this threshold level is critical to:1)ensure unnecessary handoffs do not occur;
and2)call dropping does not occur
Handoff (2)
• Handoffs require a change in frequency for the mobile phone; must be seamless to subscriber
• Blocking occurs when there are no usable channels available in the target cell to switch on
Handoff (3)Stages of handoff:1. Initiation – either mobile unit or the network determines
need for handoff and initiates necessary network procedures
2. Resource reservation – reserve voice and control channels needed to support handoff
3. Execution – actual transfer of control from one base station to another base station
4. Completion – unnecessary network resources are relinquished and made available to other mobile units
Kinds of handoff:• Hard handoff: break-before-make process• Soft handoff: no perceivable interruption of service (200
ms). Mobile unit establishes contact with new base station before giving up current radio channel
Handoff Threshold Handoff threshold: typically, -90~-100 dBm (1~10uW)
Roaming MS moves to a new cell
The new cell is served by a new MSC i.e. handoff that results in MSC change
Mobile unit moves from one cell to another – possibly from one company’s service area into another company’s service area (requiring roaming agreements)
Consider a system with a fixed number of full-duplex channels available in a given area, with each service area divided into clusters and allocated a group of channels, divided among N cells (cells have same no. of channels but do not cover same size area)
Total number of cellular channels available in a cluster is: F = GNwhere: F = no. of full-duplex channels in a cluster
G = number of channels in a cell N = number of cells in a cluster
When a cluster is duplicated m times in a given service area, the total no. of full-duplex channels is: C = mFwhere: C = total channel capacity in a given area m = number of clusters in a given area
Channel Capacity
CapacityTotal number of users, n
where:
W = total bandwidth
N = frequency reuse factor
B = channel bandwidth
m = number of cells required to cover an area
( )BNWmn /=
Two major kinds: Co-channel interferenceAdjacent-channel interference
Interference in Cellular Systems
Co-channel Interference
• Interference between two cells using the same set of frequencies
• Certain minimum distance must separate co-channels to reduce co-channel interference
Co-channel Interference
• Cell radius is proportional to transmit power
• More radio channels can be added to a system by either: 1. decreasing the transmit power per cell
2. making cells smaller
3. filling vacated coverage areas with new cells
Co-channel Interference• Where all cells are approximately the same
size, co-channel interference is dependent on radius ( R ) of the cells and the distance to the center of the nearest co-channel cell (D).
• Increasing the D/R ratio (co-channel reuse ratio, Q) increases the spatial separation between co-channel cells relative to the coverage distance, hence reducing co-channel interference
Co-channel Interference• For hexagonal geometry, Q = D/R
where Q = co-channel reuse ratio (unitless) D = distance to center of the nearest co-channel
cell (kilometers) R = cell radius (kilometers)
• Smaller Q value, larger channel capacity (smaller cluster size)
• Large Q value improves co-channel interference and overall transmission quality
• In actual cellular systems, trade-off is made between the two conflicting objectives
Adjacent Channel Interference• Occurs when transmissions from adjacent
channels (channels next to one another in the frequency domain) interfere with each other
• Results from imperfect filters in receivers• Most prevalent when adjacent channel is
transmitting very close to a mobile unit’s receiver at the same time the mobile unit is trying to receive transmissions from the base station on an adjacent frequency (near-far effect)
• Minimized by using precise filtering and careful channel assignments (maintain reasonable frequency separation between channels)
• If reuse factor is small, separation between adjacent channels may not be sufficient
Adjacent Channel Interference
Sample Problems
Capacity Computations 1. A total of 33 MHz are allocated to a system which uses 2x25 kHz for
full duplex (i.e., each channel is 50 kHz). What is the number of channels for the system?
Since 4 and 7 are popular number of cells per cluster/system. What is the number of channels per cell?
a. For reuse N = 4:
b. For reuse N = 7:
2. Now assume 1 MHz of the 33 MHz is allocated to control channels. Each channel is still 50 kHz.
What is the total number of voice (traffic) channels (excluding control
traffic)?
Again, we take N=4 and N=7. What is the number of channels per cell: For N = 4?
For N = 7?
A total of 33 MHz are allocated to a system which uses 2x25 kHz for full duplex (i.e., each channel is 50 kHz).
Number of channels per system
Number of channels per cell:
a. For reuse N = 4:
For reuse N = 7:
33 0002 25
660, kHzkHz
channels×
=
6604
165= channels / cell
6607
95= channels / cell
Now assume 1 MHz of the 33 MHz is allocated to control channels. Each control channel is still 50 kHz
Total number of voice (traffic) channels is now
For N = 4 => 640/4 = 160 voice channels + control channels.
For N = 7 => 640/7 = 92 channels + control.
32 0002 25
640, kHzkHz
channels×
=
Increased Capacity with Cell splitting
1. Determine the channel capacity for a cellular telephone area comprised of 7 macrocells with 10 channels per cell.
2. What is the channel capacity if each macrocell is split into four minicells?
3. What is the channel capacity if each minicell is further split into four microcells?
Increased Capacity with Cell splitting1. Determine the channel capacity for a cellular telephone area
comprised of 7 macrocells with 10 channels per cell.
2. What is the channel capacity if each macrocell is split into four minicells?
3. What is the channel capacity if each minicell is further split into four microcells?
10 channels X 7 cells = 70 channels/area cell area
10 channels X 28 cells = 280 channels/area cell area
10 channels X 112 cells = 1120 channels/area cell area
Frequency Reuse Example Assume a system of 32 cells with a cell radius of 1.6 km, a total frequency bandwidth that supports 336 traffic channels, and a reuse factor of N = 7.
If there are 32 total cells, determine the following:1.What geographic area is covered?2.How many channels are there per cell?3.What is the total number of concurrent calls that can be handled?Repeat for a cell radius of 0.8 km and 128 cells.
Frequency Reuse Example
Generations of Cellular Networks
1G 2G 3G 4G2.5G
Analog Digital
Circuit-switching Packet-switching
1G Systems Goal: To develop a working system that could provide basic
voice service Time frame: 1970-1990 Technology: FDMA/FDD Example Systems:
Advanced Mobile Phone System (AMPS-USA) Total Access Communication System (TACS-UK) Nordic Mobile Telephone (NMT-Europe)
Incompatible analog systems
First Generation Analog original cellular telephone networks analog traffic channels early 1980s in North America Advanced Mobile Phone Service (AMPS) also common in South America, Australia, and
China replaced by later generation systems
2G Systems
Goal: Digital voice service with improved quality and also provide better data services
Time Frame: 1990- 2000 Technology: TDMA/TDD, CDMA Example Systems:Global System for Mobile (GSM-Europe) IS-136(TDMA) IS-95 (CDMA)
Second Generation CDMA provide higher quality signals, higher data rates,
support digital services, with overall greater capacity key differences includedigital traffic channelsencryptionerror detection and correctionchannel access time division multiple access (TDMA)code division multiple access (CDMA)
Code Division Multiple Access (CDMA)
have a number of 2nd gen systems for example IS-95 using CDMA
each cell allocated frequency bandwidth is split in twohalf for reverse, half for forwarduses direct-sequence spread spectrum (DSSS)
Code Division Multiple Access (CDMA) Advantages
frequency diversitynoise bursts & fading have less effect
multipath resistancechipping codes have low cross & auto correlation
privacy inherent in use of spread-spectrum
graceful degradationmore users means more noise leads to slow signal degradation until unacceptable
Code Division Multiple Access (CDMA) Disadvantages self-jammingsome cross correlation between users if not
perfectly synchronized near-far problemsignals closer to receiver are received with less
attenuation than signals farther awayTransmissions from remote units more difficult to
recover
IS-95 second generation CDMA scheme primarily deployed in North America transmission structures different on forward and
reverse links
2.5G Systems
Goal: To provide better data rates and wider range of data services and also act as a transition to 3G
Time frame: 2000-2002 Systems: IS-95BHigh Speed Circuit Switched Data (HSCSD)General Packet Radio Service (GPRS)Enhanced Data rates for GSM Evolution (EDGE)
3G Systems Goal: High speed wireless data access and unified
universal standard Time frame: 2002- Two competing standards
One based on GSM, IS-136 and PDC known as 3GPP Other based on IS-95 named 3GPP2
Completely move from circuit switching to packet switching
Enhanced data rates of 2-20Mbps
Third Generation Systems high-speed wireless communications to support multimedia, data, and
video in addition to voice 3G capabilities:
voice quality comparable to PSTN 144 kbps available to users over large areas 384 kbps available to pedestrians over small areas support for 2.048 Mbps for office use symmetrical and asymmetrical data rates packet-switched and circuit-switched services adaptive interface to Internet more efficient use of available spectrum support for variety of mobile equipment allow introduction of new services and technologies
4G Systems
Future systems Goal: High mobility, High data rate, IP based networkHybrid network that can interoperate with other
networks
Driving Forces trend toward universal personal telecommunications universal communications access – capability of using
one’s terminal in a wide variety of environments to connect to info serices
GSM cellular telephony with subscriber identity module, is step towards goals
personal communications services (PCSs) and personal communication networks (PCNs) also form objectives for third-generation wireless
technology is digital using time division multiple access or code-division multiple access for spectrum efficiency and high capacity
PCS handsets low power, small and light
CDMA Design Considerations – Bandwidth and Chip Rate
dominant technology for 3G systems is CDMA 3 CDMA schemes, share some design issues
bandwidth (limit channel to 5 MHz) 5 MHz reasonable upper limit on what can be allocated
for 3G 5 MHz is enough for data rates of 144 and 384 kHz
chip rate given bandwidth, chip rate depends on desired data rate,
need for error control, and bandwidth limitations chip rate of 3 Mbps or more reasonable
CDMA Design Considerations – Multirate
provision of multiple fixed-data-rate channels to user different data rates provided on different logical channels logical channel traffic can be switched independently through
wireless fixed networks to different destinations flexibly support multiple simultaneous applications efficiently use available capacity by only providing the capacity
required for each service use TDMA within single CDMA channel or use multiple CDMA codes
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