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

2

5

4

3

7

2

6

1

3

1

7

2

4 5

4

6

37

5

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