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Wireless Communications A little history and evolution of mobile radio Marconi - 1897, invented wireless concept 1960’s &1970’s – Bell laboratories developed the cellular concept

1970’s -- development of highly reliable, miniature solid state radio frequency hardware

Wireless communication era was born Cellular phone users

1984 - 25000 1994 - 16 million 1997 - 50 million 2000 - number of wireless users = number of wired users

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Examples of mobile radio systems

Used in everyday life - •Garage door openers • Remote controllers for home entertainment • Cordless Telephones • Hand held Walkie talkies • pagers / Beepers • Cellular Telephones Mobile – Describes a radio terminal attached to a high speed mobile platform. e.g A Cellular phone in a fast moving vehicle Portable – Describes a radio terminal that can be hand-held and used by someone at walking speed. e.g cordless telephone Subscriber – Mobile or Portable user Base stations – link mobiles through a backbone network

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Types of Mobile Radio Transmission System Simplex – Communication is possible only in one direction, e.g. Paging Systems Half Duplex – Two way communication, but use the same radio channel for both transmission and reception. User can only transmit or receive information Full Duplex – Simultaneous two way radio transmission and reception between subscriber and base station. Two simultaneous but separate channels (FDD) or Adjacent timeslots on a single radio channel (TDD)

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Cordless Telephone Systems

• Full duplex communication systems

• Few hundred meters

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Paging Systems

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Paging systems are communication systems that send brief messages to a subscriber -

•Numeric messages • Alpha- Numeric Message • Voice Message • News Headlines • Stock Quotes • Faxes

Coverage Area -

• 2 to 5 km • within individual buildings • world wide coverage

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Cellular Telephone Systems

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Base station -mobile network

FVC – Forward Voice Channel RVC – Reverse Voice Channel FCC – Forward Control Channel RCC – Reverse Control Channel

FVCRVC

RCC

FCC

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Functions of cellular system

• Provides wireless connection to the PSTN for any user location within the radio range of the system

• High capacity is achieved by limiting the coverage of each base

station transmitter to a small geographical area called a Cell and by the same radio channels being reused by another base station located some distance array – Frequency reuse

• Switching system called handoff enables call to forced un-

interrupted when the user uses one cell to another

• Typical MSC handles 100,000 cellular users and 5000 simultaneous conversion at a time

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Schematic of telephone call made to mobile user

MSC

Mobile Switching

Center

PSTN

Step 2 Step 1

Incoming Telephone Call to Mobile X

Base Stations

Step 6

Step 5

Step 4

Mobile X

Step 3

Step 7

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Brief outline of cellular process Telephone call placed to a mobile user Step 1 – the incoming telephone call to Mobile X is received at the MSC Step 2 – the MSC dispatches the request to all base stations in the cellular system Step 3 – the base stations broadcast the Mobile Identification Number (MIN) – telephone number of Mobile X, as a paging message over the FCC throughout the cellular system Step 4 – the mobile receives the paging message sent by the base station it monitors and responds by Identifying itself over the reverse control channel

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Step 5 – the base station relays the acknowledgement sent by the mobile and informs the MSC of the handshake Step 6 – the MSC instructs the base station to move the cell to AM issued voice channel within in the cell Step 7 – the base station signals the mobile to change frequencies to an un-used forward and reverse voice channel pair. At the point another data message (alert) is transmitted over the forward voice channel to instruct the mobile to ring Now the call is in progress. The MSC adjusts the transmitted power of the mobile and changes the channel of the mobile end and base stations in order to maintain call quality. This is called handoff.

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Telephone call placed by mobile

MSC

Mobile Switching

Center

PSTN

Step 3

Step 2

Step 1

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Step 1 – when a mobile originates a call, it sends the base station its telephone number (MIN), electric serial number (ESN) and telephone number of called party. It also transmits a station class mark (SCM) which indicates what the maximum power level is for the particular user. Step 2 – the cell base station receives the data and sends it to the MSC Step 3 – the MSC validates the request, makes connection to the called party through the PSTN and validates the base station and mobile user to move to an un-used forward and reverse channel pair to allow the conversation to begin.

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Roaming

• All cellular systems provide a service called roaming. This allows subscribes to operate in service areas other than the one from which service is subscribed.

• When a mobile enters a city or geographic area that is different from

its home service area, it is registered as a roamer in the new service area.

• Periodically, the MSC issues a global command over each FCC in

the system, asking for all mobiles which are previously un-registered to report their MIN and ESN over the RCC for billing purposes.

• If a particular manner has roaming authorization for billing purposes,

MSC requests the subscriber as a valid roamer.

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Frequency spectrum allocation for US cellular radio service Channel number Center Frequency (MHZ) 1 ≤ N ≤ 799 0.03 N + 825.0 990 ≤ N ≤ 1023 0.03 (N – 1023) + 825 1 ≤ N ≤ 799 0.03 N + 870.0 990 ≤ N ≤ 1023 0.03 (N – 1023) +870.0 channels 800 – 989 are un-used

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Trends in Cellular Radio Personal Communications

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The Cellular Concept – System Design Fundamentals

• The cellular concept was a major breakthrough in solving the problem of spectral congestion and user capacity

• Replaces single high power transmitter (Large cell) with many low

power transmitters (Small cells), each providing coverage to only a small portion of the service area.

Frequency Reuse Each cellular base station is allocated a group of radio channels. Base stations in adjacent cells are assigned channel groups which contain completely different channels than neighboring cells.

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Choice of Hexagonal cell

Choices Factors

• Equal Area • No overlap between Cells

S S S

A1 A2 A3

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For a given S A3 > A1 A3 > A2 Here A3 provides maximum coverage area for a given value of S Actual cellular footprint is determined by the contour of a given transmitting antenna By using hexagon geometry, fewest number of cells cover a given geographic region.

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Channel capacity Let a cellular system have total of S duplex channels for use. If S channels are divided into N cells (in a cluster) into unique and disjoint channel groups which each has the same number of channels, total number of available radio channels is: S = KN Where K is the number of channels / cell If a cluster is replicated M times within the system, the total number of duplex channels, C, or the capacity is C = MKN = MS N – cluster size = 4,7 or 12

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Design of cluster size N In order to connect without gaps between adjacent cells (to Tessellate) N = i2 + ij + j2 Where i and j are non- negative integers Example i = 2, j = 2 N = 22 +2(1)+12 = 4+2+1 = 7

To find the nearest co- channel neighbor of particular cell (1) move i cells along any chain of hexagons and then (2) turn 60 degrees counterclockwise and move j cells

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Example If a particular FDD cellular telephone system has a total bandwidth of 33 mhz, if the phone system uses two 25 KHZ simplex channels to provide full duplex voice and control channels, compute the number of channels per cell if N = 4, 7, 12. Solution Total bandwidth = 33 MHZ Channel bandwidth = 25 KHZ x 2 = 50 KHZ Total available channels = 33 MHZ / 50 KHZ = 660 N = 4 channel per cell = 660 / 4 = 165 channels N = 7 channel per cell = 660 / 7 = 95 channels N = 4 channel per cell = 660 / 12 = 55 channels

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Channel assignment strategies Fixed channel assignments

• Each cell is allocated a predetermined set of voice channels

• If all the channels in that cell are occupied, the cell is blocked and the subscriber does not receive service

• Variation includes borrowing strategy, a cell is allowed to borrow

channels from a neighboring cell if all its own channels are already occupied. This borrowing is supervised by the MSC

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Dynamic channel assignments

• Voice channels are not allocated to different cells permanently

• Each time a cell request is made, the serving base station requests a channel from the MSC

• The switch then allocate a channel to the requested cell based on a

decision algorithm taking into account different factors – frequency re-use of candidate channel, cost factors

Dynamic channel assignment is more complex (real time) but reduces likelihood of blocking

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Handoff strategies Handoff – when a mobile moves into different cell while a conversation is in progress, the MSC automatically transfers the cell to a new channel belonging to the new base station

• Important task in any cellular radio system

• Handoffs must be performed successfully, as infrequently as possible and not visible to users

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Pn – Pm = ∆ - should not be too large ∆ too large – too many handoffs or too small ∆ too small – channel of cell being lost

Pn Pm

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Dwell time

• Time over which a cell may be maintained within a cell, without hand off

• Each base station constantly monitors the signal strength of all its

reverse voice channels to determine the relative location of each mobile user with respect to the base station tower

• Mobile assisted hand-off (MAHO) – every mobile station measures

the received power from surrounding base stations and continuously reports the results of these measurements to the serving base station – Faster hand-off rate

• Inter-system handoff – one cellular system to different cellular

system

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Interference and system capacity Major limiting factor in performance of cellular radio systems – two main types Co-channel interference and Adjacent channel interference Co-channel interference Cells that use the same set of frequencies are called co-channel cells. Interference between the cells are called co-channel interference Signal to interference ratio (SIR) or S / I for a mobile receiver is given by S / I = SIR = S/ ∑

=

S

i 1

Ii

S = Derived signal power from designated base station Ii = Interference power caused by the ith interfacing co-channel cell

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Assumptions For any given antenna (base station) the power at a distance d is given by

Pr = Po (d / do ) – n n is path loss exponent

PrPod

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Hence, S / I = R – n / ∑=

io

i

Di1

)(- n

io = total number of first layer interfacing cells If the mobile is at the center of the cell Di = D S / I = R – n / (D)- n 1

1∑

=

io

i

= (R / D)-n / io

For a hexagonal geometry D / R = N3 = Q - co-channel reuse ratio S / I = ( N3 ) n / io

Maximum co-channel interface – when mobile is at cell boundry. N = 7 S / I ≈ R-4 / [ 2(D-R)-4 +2(D+R)+4+2D-4]

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Adjacent Channel Interference Interference resulting from signals which are adjacent in frequency to the desired signal Due to imperfect receiver filters which allow nearby frequencies to leak into pass band Can be minimized by careful filtering and channel assignments by keeping the frequency separation between each channel in a green cell as large as possible, the adjacent channel interference may be reduced considerably

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Trunking and grade of service

• Cellular radio system rely on trunking to accommodate a large number of users in a limited radio spectrum – How a large population can be accommodated by a limited number of services

• Trunking – each user is allocated a channel on a per cell basis and

upon termination of the cell, the previously occupied channel is immediately returned to the pool of available channels

• First initiated by Danish mathematician called Erlang

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Grade of Service (GOS) Measure of ability of the user to access a trunked system during the busiest hour during a week, month or a year 4 to 6 pm on Thursday or Friday evening Traffic intensity (Aµ Erlang) of each user is: Aµ = λ H λ - Average number of call requests per unit time H – Average duration of a call

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Total traffic intensity For a system entering U users, the total offered traffic intensity A is given as A = U Aµ Erlangs If there are C channels in the system, average traffic intensity per channel is A c = U A µ / C

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Blocked calls cleared system No queuing for call requests If no channels are available, the requesting user is blocked without access and is free to try again later. Assuming a finite number of available channels C, and using queuing theory, we obtain GOS = Probability (call is blocked) = [Ac / C! ]/ [ A

c

k∑

=0

k / k!]

AMPS cellular system is designed for GOS = 0.02 This is called the Erlang B formula (Appendix A)-Table 2.6 of book.

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Blocked calls delayed system Queue is provided to hold calls which are blocked. If a channel is not available immediately, the call request may be delayed until a channel becomes available Pr [Delay > 0 ] = Ac / [ A c + C! ( 1 – A / C )] [∑

−

=

1

0

c

k

Ak / k ! ]

Pr [Delay > ts ] = Pr [Delay > 0 ] e – (C-A) t / H Average Delay D for all calls in a queued system is given by: D = Pr [Delay > 0 ] H / C-A This is called Erlang C formula -Table 2.7 of book

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Example A hexagonal cell with a 4-cell system has a radius of 1.387 km. A total of 60 channels are used within the entire system. If the load / user is 0.029 Erlangs, λ = 1 call per hr., compute the following for an Erlang C system that has a 5% probability of a delayed cell.

a. How many users per square will the system support? b.What is the Pr [Delay > 10s ]

Solution: Cell radius = R = 1.387 km Area covered per cell = 2.598 (1.387)2 = 5 sq km Number calls per cluster = 4 Total number of channels per cell = 60 / 4 = 15 channels

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a. From Erlang C chart , GOS = 0.05, C = 15, Traffic intensity A = 9.0 E Number of users = Total traffic intensity / Traffic per user = 9.0 / 0.029 = 310 users Number of user per system = 310 / 5 = 62 users per system b.Pr [Delay > 10] = Pr [Delay > 0 ] e –(C-A)t / H = 0.05 x e-(15 – 9 ) 10 / H H = A µ / λ = 0.029 hr = 0.029 x 60 x 60 seconds = 104.4 seconds Pr [Delay > 10] = 0.05 e – (15 – 9) 10 / 104.4 = 0.0281 = 2.81%

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Improving capacity in cellular systems As demand for wireless services increases, number of channels assigned to a cell is not enough to support the required number of users Solution is to increase channels per unit coverage area

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Cell splitting

• Subdivides a congested cell into smaller cells, each with its own base station

• Increases the capacity of a cellular system Sectoring

• Achieves capacity improvement by essentially re-scaling the system • Cell radius R is unchanged but the co-channel ratio D / R is

decreased • Capacity improvement is achieved by reducing the number of cells in

a cluster and this increases frequency re-use • Replacing a single omni-directional antenna at base station with

several directional antennas, each radiating within a specified sector

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Micro Cell Zone Concept

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• 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

• The channels are distributed in time and space by all three

zones are re-used in co-channel calls is normal fashion • Advantage is that while the call maintains a particular coverage

radius, co-channel interference is reduced due to zone transmitters on edge of the call.

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Practice Problems [2.19] The US AMPS system is allocated 50 MHZ of spectrum in the 800 MHZ range and provides 832 channels. 42 of those channels are control channels. The forward channel frequency is exactly 45 MHZ greater than the reverse channel frequency. (a) Is the AMPS system simplex, half-duplex or duplex? What is the bandwidth for each channel and how is it distributed between the base station and the subscriber? (b)Assume a base station transmits control information on channel 352 operating at 880.56 MHZ. What is the transmission frequency of a subscriber unit transmitting on channel 352? (c)The A side and B side cellular carries evenly split the AMPS channels. Find the number of voice channels and number of control channels for each carrier? (d)For an ideal hexagonal cellular layout which has identical cell sites, what is the distance between the enters of the two nearest co-channel cells for 7 cell re-use? For 4 cell re-use?

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Solution a. AMPS system is duplex Total bandwidth = 500 MHZ Total number of channels = 832 Bandwidth for each channel = 50 MHZ / 832 = 60 KHZ 60 KHZ is split into two 30 KHZ channels (forward and reverse channel). The forward channel is 45 MHZ > reverse channel. b. for Ffw = 880.560 MHZ Frev = Ffw – 45 = 835.560 MHZ

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c. Total number of channels = 832 = N Total number of control channels Ncon = 42 Total number of voice channels Nuo = 832 – 42 = 790 Number of voice channels for each carrier = 790 / 2 = 395 channels Number of control channels for each carrier = 42 / 2 = 21 channels d. N = 7 Q = D / R = N3 = 21= 4.58

ð D = 4.58 R N = 4 Q = 12 = 3.46

ð D = 3.46 R

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