Wireless Communications Principles and Practice 2nd Edition T.S. Rappaport Chapter 3: The Cellular Concept – System Design Fundamentals
The Cellular Concept S=KN Where s=duplex channel, K=each cell alloted cell channel(K
19-cell reuse example (N=19)
Frequency reuse factor=1/N N=i^2+ij+j^2 where i,j non negative integers. In fig.3.2 i=3 and j=2(N=19) fig.3.1 i=2 and j=1(N=07)
Figure 3.2 Method of locating co-channel cells in a cellular system. In this example, N = 19 (i.e., I = 3, j = 2). (Adapted from [Oet83] © IEEE.)
Example 2.1 from Rappaport
Channel Assignment Strategies • Fixed Channel Assignment Strategies – Fixed Voice Channel – Call Block – Borrowing Strategies
• Dynamic Channel Assignment Strategies
– Not Fixed Voice Channel – Algorithm to assign Channel – Overhead of Channel occupancy, Traffic distribution,RSSI (Radio Signal Strength Indication) on MSC – Decreased Probability of Block call But increases system load.
Handoffs – the basics •Min.usable signal(-90dbm to-100dbm), •delta for MSC assign late chan. Or delta too small •Depend vehicular speed •Dwell time(call on) factor-interf.,propa.,dist •Dwell time for const speed user(mean) •1G RSSI value to Rx.(locator Rx.to MSC) •2GMAHO mobile measure(suit microcell) •Intersystem handoff(1 to 2 cell. system) Priortize handoff One method- guard channel concept Second method-queuing Delay time & size of queue determine But not zero forced termination
Umbrella Cells
•Diff. antenna size & diff. power level make umbrella and microcell •The speed of each user estimated by short term avg signal strength and assign algorithm to provide umbrella cell or pedestrian handoff. •Cell Dragging- from pedestrian-Solve threshold & radio coverage •1G -to make handoff below threshold take 10sec. •Require delta 6 to 12 db •2G to make handoff require 1 to 2 sec. delta 0 to6 db •Hard handoff-assign diff radio channel •Soft handoff-ability to select between instantaneous Rx signal from variety of base station
Interference and system capacity • Source -another mobile in samecell,call progress in neighbor cell, other base station operate in same frequency • It lead to blocked calls and missed calls due to error in digital signaling. • Two Major –Co channel interference And adjacent channel interference
Smaller N is greater capacity
Q=D/R=Root of 3N Q large----N small S/I =S/(sum of interf.) S/I=(D/R)^n / (io) io is co channel interfering cell and n is path exponent
• Let i0 be the number of co-channel interfering cells. The signalto-interference ratio (SIR) for a mobile receiver can be expressed as S S i0 I Ii
i 1
S: the desired signal power I i : interference power caused by the ith interfering co-channel cell base station • The average received power at a distance d from the transmitting antenna is approximated by close-in reference point n d Pr P0 d0 d 0 or d P0 :measued power TX Pr (dBm) P0 (dBm) 10n log d 0 n is the path loss exponent which ranges between 2 and 4.
• When the transmission power of each base station is equal, SIR for a mobile can be approximated as S I
Rn i0
n D i i 1
• Consider only the first layer of interfering cells S ( D / R)n I i0
n
3N i0
i0 6
• Example: AMPS requires that SIR be greater than 18dB – N should be at least 6.49 for n=4. – Minimum cluster size is 7
• For hexagonal geometry with 7-cell cluster, with the mobile unit being at the cell boundary, the signal-to-interference ratio for the worst case can be approximated as S R 4 I 2( D R) 4 ( D R / 2)4 ( D R / 2) 4 ( D R)4 D 4
Example For given path loss exponent (a)n=4(b)n=3,find the frequency reuse factor and the cluster size that should be used for max. capacity. The S/I ratio of 15db required for satisfactory forward channel performance of cellular system. There are six co channel cells in first tier, and all of them are at the same distance from mobile. Use suitable approximation. -(a)seven cell reuse pattern io=6 Q=D/R=root of 3xN then S/Icheck >15db =18.66db (b)S/I=12.05db <15db Now take N=12(i=J=2),Q=6,S/I=15.56db>15db
Channel Planning for Wireless Systems • Judicious assignment of the appropriate radio channels a more difficult problem in practice. Theoretical analysis provides a guideline. • In general 5% of the available spectrum is reserved for control channels • Frequency reuse of control channels more conservative. • In CDMA N=1, however, practical difficulties forces some kind of frequency planning.
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Adjacent Channel Interference • Interference from channels that are adjacent in frequency, • The primary reason for that is Imperfect Receive Filters which cause the adjacent channel energy to leak into your spectrum. • Problem is severer if the user of adjacent channel is in close proximity. Near-Far Effect • Near-Far Effect: The other transmitter(who may or may not be of the same type) captures the receiver of the subscriber. • Also, when a Mobile Station close to the Base Station transmits on a channel close to the one being used by a weaker mobile: The BS faces diffi ulty i dis ri i ati g the desired o ile user fro the leed over of the adjacent channel mobile.
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Near-Far Effect: Case 1
• The Mobile receiver is captured by the unintended, unknown transmitter, instead of the desired base station
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Near-Far Effect: Case 2
• The Base Station faces difficulty in recognizing the actual mobile user, when the adjacent channel bleed over is too high.
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Minimization of ACI (1) Careful Filtering ---- min. leakage or sharp transition (2) Better Channel Assignment Strategy • Channels in a cell need not be adjacent: For channels within a cell, Keep frequency separation as large as possible. • Sequentially assigning cells the successive frequency channels. • Also, secondary level of interference can be reduced by not assigning adjacent channels to neighboring cells. • For tolerable ACI, we either need to increase the frequency separation or reduce the passband BW.
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2.5.3 Power Control for Reducing Interference • Ensure each mobile transmits the smallest power necessary to maintain a good quality link on the reverse channel – long battery life – increase SIR – solve the near-far problem
Power Control for Reducing Interference • Power levels transmitted by every subscriber unit are under constant control by the Base Station. • Mobile Station should transmit minimum power to maintain quality link on reverse channel. • This has benefits of longer Battery Life at the Mobile Station Reduced reverse SIR. • In CDMA systems, it is extremely important to control the power, as the neighboring cells are using the same channel.
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2.6 Trunking and Grade of Service • Erlangs: One Erlangs represents the amount of traffic density carried by a channel that is completely occupied. – Ex: A radio channel that is occupied for 30 minutes during an hour carries 0.5 Erlangs of traffic.
• Grade of Service (GOS): The likelihood that a call is blocked. • Each user generates a traffic intensity of Au Erlangs given by Au H H: average duration of a call. : average number of call requests per unit time • For a system containing U users and an unspecified number of channels, the total offered traffic intensity A, is given by A UAu • For C channel trunking system, the traffic intensity, Ac is given as A UA / C c
u
Key Definitions for Trunked Radio
2.7 Improving Capacity in Cellular Systems • Methods for improving capacity in cellular systems – Cell Splitting: subdividing a congested cell into smaller cells. – Sectoring: directional antennas to control the interference and frequency reuse. – Coverage zone : Distributing the coverage of a cell and extends the cell boundary to hard-to-reach place.
2.7.1 Cell Splitting • Split congested cell into smaller cells. – Preserve frequency reuse plan. – Reduce transmission power. Reduce R to R/2
microcell
Illustration of cell splitting within a 3 km by 3 km square
• Transmission power reduction from Pt1 to Pt 2 • Examining the receiving power at the new and old cell boundary Pr [at old cell boundary ] Pt1R n Pr [at new cell boundary] Pt 2 ( R / 2) n
• If we take n = 4 and set the received power equal to each other Pt1 16 The transmit power must be reduced by 12 dB in order to fill in the original coverage area. Problem: if only part of the cells are splited Pt 2
• •
– Different cell sizes will exist simultaneously
• Handoff issues - high speed and low speed traffic can be simultaneously accommodated
2.7.2 Sectoring • Decrease the co-channel interference and keep the cell radius R unchanged – Replacing single omni-directional antenna by several directional antennas – Radiating within a specified sector
• Interference Reduction
position of the mobile
interference cells
2.7.3 Microcell Zone Concept • Antennas are placed at the outer edges of the cell • Any channel may be assigned to any zone by the base station • Mobile is served by the zone with the strongest signal. • Handoff within a cell – No channel reassignment – Switch the channel to a different zone site
• Reduce interference – Low power transmitters are employed
In-building deployment is the next great growth phase
Zone Cell Concept
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