Chapter 3 Cellular Concept

Transcription

1 Chapter 3 Cellular Concept Objectives To resolve spectral congestion and user capacity To provide additional radio capacity radio capacity with no additional increase in radio Methods Large Cells small cells, High power low power Handoff and interference go up Frequency Reuse: result in CCI (Cochannel interference) Frequency Planing Selecting and allocating channel groups for all of the cellular base stations (Channel assignment) To reduce CCI,ADJ (Adjacent Interference) Different groups of channels for neighboring base stations Chapter 3 - Cellular Concept Dr. Sheng-Chou Lin Page

2 Cellular Structure Geometric Shapes: Square, triangle, Hexagon without overlap with equal area Hexagon: for a given distance (center to farthest point) the largest area Cell Diagrams o o o o o o o o o Imaginary Ideal Real easy to draw easy to consider free space isotropic antennas The Real World Chapter 3 - Cellular Concept Dr. Sheng-Chou Lin Frequency Reuse Cluster size N S = kn, S: Total channels available for user k: channels per cell, N: N cells a cluster C = MkN = MS = The total number of duplex channels. A cluster is replicated M times N =, 7, typically Capacity and frequency reuse Capacity = Channels per cell = (total channels) / N Capacity goes up as N decreases, but CCI goes up N= N= Chapter 3 - Cellular Concept 3 Dr. Sheng-Chou Lin Page

3 D R f X+60 f = = f7 f3 J= f f f6 Example Sketch demonstrates N=7 D f5 f7 Frequency Reuse D/R determines required minimum N R f I= f R f distance between two co-channel transmitters X coverage radius where a cell is the best server Given our propagation model and desired C/I, we determined D/R in the last slide. Now we can establish a channel assignment pattern using the smallest number of cells which will separate co-channel cells by at least D/R. N cells are required. N is determined from geometry, as shown at left: D N = ( ) 3 R / N = I + I x J + J Move I cells along any chain Turn 60 counterclockwise and move J cells Chapter 3 - Cellular Concept Dr. Sheng-Chou Lin An Example Total BW = 33MHz (RX+TX channels) One channel BW = 5kHz( per simplex channel) = 50k (per duplex channel) Total channels = 33,000/50 = 660 channels M for control channel, 000/50 = 0 control channels, =60 voice channels. For N = 7, case one (3 control channels + 9 voice channels) cells (3 control channels + 90 voice channels) cells ( control channels + 9 voice channels) cells For N = 7, case two: one control channel each cell ( control channels + 9 voice channels) cells 3 ( control channels + 9 voice channels) cells Chapter 3 - Cellular Concept 5 Dr. Sheng-Chou Lin Page 3

4 The Resource: AMPS Spectrum Frequencies and Channel Numbers Paired Bands Frequency, MHz Uplink (Reverse Path) Downlink (Forward Path) 89.5 A A (non-wireline) B (Wireline) A B Channel Numbers An operator authorized frequency block contains 6 channels In a frequency plan, we assign specific channels to specific cells, following a reuse pattern which restarts with each Nth cell Uplink and downlink bands are paired mirror images A channel includes one uplink and one downlink frequency Chapter 3 - Cellular Concept 6 Dr. Sheng-Chou Lin Cellular Band BAND-A Control Ch. BAND-B Control Ch. BAND-A BAND-B Extended-A Extended-B 83 Ch. No Band-A Voice Channels: - 3 = 3 ch = 50 ch = 33 ch. Tot. No. of Voice Ch. = 395 Band-A Control Channels: = Ch Band-B Voice Channels: = 3 ch = 83 ch. Tot. No. of Voice Ch. = 395 Band-B Control Channels: = ch. Chapter 3 - Cellular Concept 7 Dr. Sheng-Chou Lin Page

5 Capacity Improvement How to improve capacity To minimize required C/I Strategy Channel assignment Channel Assignment: minimize interference Fixed: Voice channels are predetermined for each cell Borrowing strategy: All of its channels are already occupied. Dynamic: MSC (Mobile switch allocates a channel to the required following an algorithm: Future blocking, frequency of use of the candidate channel, reuse distribution of the channel, other cost functions. MSC correct real-time data on channel occupancy, traffic distribution, radio signal strength indications (RSSI) Advantages: channel utilization, Blocking, trunking capacity. Disadvantages: Memory storage, Computational load. Chapter 3 - Cellular Concept 8 Dr. Sheng-Chou Lin If N=3, for example: Channels Channel Set,, 7, 0,... Channel Set, 5, 8,,... Channel Set 3 3, 6, 9,,... N= Channel Assignment Freq. 3 In channel assignment, we dole out the channels to the cells, much like a dealer in a card game deals out cards from the deck until every player has a set. A channel set is a collection of channels which could be assigned at one cell Channels in a channel set normally are N channels apart, where N is the reuse factor Channels in a set must meet combiner minimum frequency spacing requirements Notice that Sets and 3 (i.e., and N) are adjacent frequencies Chapter 3 - Cellular Concept 9 Dr. Sheng-Chou Lin Page 5

6 Handoff Cell- t Mobile automatically transfers the call to a new channel of the new channel base station. Handoff threshold is slightly stronger than a particular signal (minimum signal for acceptable voice quality) Call Starts Center (MSC) Mobile Switching Cell- Call Ends t 3 t Hand-off Required at Boundary Crossings Cell-3 Local Telephone Exchange Chapter 3 - Cellular Concept 0 Dr. Sheng-Chou Lin Handoff Factors Not due to momentary fading unnecessary handoffs are avoided monitor the signal level for a certain period of time average measurement propagation fading effect the length of average time depends on vehicle speed short term fading Dwell time: a call may be maintained within a cell without handoff Factors: propagation, interference, distance between BS and MS, time varying effects Statistics: speed, type of radio coverage (Highway, Microcell ). Signal strength measurement (RSSI): made by BS MSC reverse link strength first generation analog cellular locator receiver: monitor the signal strength of users in neighboring cells BS measurement load MAHO (mobile assisted handoff) Chapter 3 - Cellular Concept Dr. Sheng-Chou Lin Page 6

7 Handoff considerations MAHO (Mobile Assisted Handoff) v.s. RSSI Measurement are made by MS to measure the received power from surrounding BS to report result to the serving BS Ahandoff is initiated power (neighboring cell) > power (current BS) Second generation (Digital TDMA) Much faster since BS load be suited for microcellular, where handoffs are more frequent Practical handoff considerations Mobile velocities for high speed vehicles, a handoff may be never needed during a call, particularly in microcell. Another feature: other than signal strength, CCI and ADJ may be measurement C/I handoff Chapter 3 - Cellular Concept Dr. Sheng-Chou Lin Hand-off Mechanism Adjacent Cell Adjacent Cell Adjacent Cell Adjacent Cell Adjacent Cell Base Station RF Antenna Adjacent Cell. Base Station continuously measure RSSI [C/I]. Based on this measurements decide the Handoff request. 3. Once Handoff request is identified, asks adjacent cells to measure the RSSI on that mobile and send the measurements.. Identifies the candidate cell for Handoff 5. Starts Handoff Chapter 3 - Cellular Concept 3 Dr. Sheng-Chou Lin Page 7

8 For Call Continuation Basic Cellular Call Processing: Why Handoff? to avoid dropping call as mobile leaves coverage range of the serving cell RSSI Drop To Avoid Interference maintain desired C/I ratio avoid giving, receiving interference in other cells Sites A A B B C D C D -50 For Operational Reasons For Load Balancing, Maintenance on VCH, etc. RSSI, dbm -0 C/I Distance, km Chapter 3 - Cellular Concept Dr. Sheng-Chou Lin Basic Cellular Call Processing: Mechanics of the Handoff Process Trigger Screen Select Handoff! Conditions Trigger system to attempt handoff signal strength too low interference or bit error rate too high Measure, Screen alternatives RSSI choose surrounding cells to monitor mobile strength, report results if TDMA, use MAHO - the mobile can measure & report Analyze Measurements, Select Target Cell Is a better choice available? Is changing worthwhile? Implement the Handoff MTX sets up new voice path trunking handoff order sent via blank-and-burst A -95 B -75 C -00 mobile acknowledges and jumps to new voice channel Conversation continues Trigger Time Handoff Voice Channels B C B C A DMS-MTX A DMS-MTX Chapter 3 - Cellular Concept 5 Dr. Sheng-Chou Lin Page 8

9 CDMA vs. AMPS/TDMA Handoffs Soft handoff: unique handoff capability provided by CDMA, since spread spectrum shares the same channel in every cell. Its ability to select between the instantaneous received signals from a variety of BS Soft handoff can only be used between CDMA channels having identical frequency assignments. Soft handoff provides diversity of Forward and Reverse Traffic Channel paths on the boundaries between base stations. MAKE Cell Site B AMPS TDMA B R E A K H A N D O F F Cell Site A AMPS/TDMA Handoffs Break-before-make AMPS takes approximately 00 ms TDMA takes between ms Can diminish call quality Increased chance of dropped calls CDMA Handoffs Make-before-break Directed by the mobile not the base station Undetectable by user Improves call quality Cell Site B CDMA Cell Cell Site Site B A Cell Site A Chapter 3 - Cellular Concept 6 Dr. Sheng-Chou Lin Soft Handoff Considerations Chapter 3 - Cellular Concept 7 Dr. Sheng-Chou Lin Page 9

10 Soft handoff Cell Site A MTX PSTN Cell Site B B S C Soft Handoff : the mobile station starts communications with a target base station without interrupting communications with the current serving base station. Can involve up to three cells simultaneously and use all signals Mobile station compares frames from each cell, and uses the best one Eliminates Ping-Pong effect and chances of dropped calls Chapter 3 - Cellular Concept 8 Dr. Sheng-Chou Lin Softer Handoff alpha gamma beta Handoff is between sectors of the same cell Communications are maintained across both sectors until the mobile station transition has completed May happen frequently MTX is aware but does not participate All activities are managed by the cell site Signals received at both sectors can be combined for improved quality Chapter 3 - Cellular Concept 9 Dr. Sheng-Chou Lin Page 0

11 Interference The major limiting factor in the performance of cellular radio Frequency Plan Restrictions Effects A major bottleneck in increasing capacity Major types and sources Adjacent interference (ADJ) mobile in the same cell mobile in a neighboring cell Cochannel Interference (CCI) other BS operating in the same frequency (co-channel cells) Multiple access interference (MAI) Intermodulation Product (IM) Due to out-of-band users In band interference Noncellular syetem: leaks energy into band. Effects become severe due to near-far effect. Cross talk on voice miss and block calls on control channel be responsible for dropped calls difficult to control in practice due to random propagation effects (Fading phenomenon) Chapter 3 - Cellular Concept 0 Dr. Sheng-Chou Lin Cochannel interference and System capacity C/I (Carrier-to-Interference Ratio) C/I is kept the same even C increases CCI can not combated by simply increasing the carrier, unlike S/N be independent of TX power A function of radius of cell (R) and the distance to the center of nearest cochannel cell (D) (D/R) Cochannel reuse ratio = Q= D/R=SQRT(3N), N: Clutter size Q, N,C/I, Quality, Capacity Q, N,C/I, Quality, Capacity C/I caluclation(optimistic result) Propagation loss D/R number of cochannel cells Chapter 3 - Cellular Concept Dr. Sheng-Chou Lin D R f X+60 f Example Sketch demonstrates N=7 = = f7 f3 J= f f f6 D f5 f7 R f I= f R f distance between two co-channel transmitters X coverage radius where a cell is the best server Page

12 Frequency Reuse Implications of N N is the number of cells in the frequency reuse pattern. N is a very important factor, since it determines: Capacity of A Cell Channels per cell = (total channels) / N As N goes up, capacity becomes progressively worse Interference As N goes up, interference becomes progressively better N Channels per Cell* D/R *Assuming use of 395 voice channels including expanded spectrum Chapter 3 - Cellular Concept Dr. Sheng-Chou Lin SIR v.s.system capacity Calculation () Average received power at a distance d from TX P r = P o ( d / d o ) -n P r P o P Sites n ~ - in urban cellular P r (dbm) = P o (dbm) - 0 n log( d / d o ) d d o Signal - to - Interference S I = = i 0 I = S I i R -n i 0 -n D i I = S : desired signal power dbm = 0log (P/mW) dbw = 0log (P/W) I i : nth cochannel interference Mobile is on the border Chapter 3 - Cellular Concept 3 Dr. Sheng-Chou Lin Page

13 SIR v.s.system capacity Calculation () optimistic result S I = i 0 I = = ( 3N ) n R -n D i -n i 0 = For n = and N = 7 (D/R) n i 0 = 8.7 db = 8 db ( n = and N =6.9) For AMP cellular (S/I) min = 8dB for sufficient voice quality All interferong signals are equal. TX power of each BS is equal n is the same Chapter 3 - Cellular Concept Dr. Sheng-Chou Lin SIR v.s.system capacity Calculation (3) Worse case: mobile unit is at the cell boundary (rarely occurs) D-R from the two nearest CCI cells (D+R/, D. D-R/, D+R) from other interfering cells S = R - I (D-R) - + (D-R/) -+ + (D+R/) - + (D+R) - + (D) - = R - (D-R) - + (D+R) - + (D) - = 9.56 = 7dB for N = 7 For n = (precise) (Approx.) Slightly less than 8dB, N 9, 7/9 capacity reduction can not be tolerable Chapter 3 - Cellular Concept 5 Dr. Sheng-Chou Lin Page 3

14 Outage Probability Outage Probability: P(SIR > SIR o ): SIR o Threshold (N=7, 0o, l = 8dB and r=) Chapter 3 - Cellular Concept 6 Dr. Sheng-Chou Lin Outage Probability Outage Probability: P(SIR > SIR o ): SIR o Threshold (N=7, 0o, l = 8dB and r=) Chapter 3 - Cellular Concept 7 Dr. Sheng-Chou Lin Page

15 Percentage of area Chapter 3 - Cellular Concept 8 Dr. Sheng-Chou Lin Adjacent-Channel Interference (ADJ) Interference will not allow first adjacent channels to be used at same cell site typical receiver IF bandwidth too broad worst case is at cell site: if adjacent channel is much stronger than desired signal (Near-Far effect) adjacent user 0 KHz. signaling tone troublesome false terminations, etc. st. Adjacent channels OK for use in adjacent cells effective handoffs required, never allow adjacent signal to be stronger than desired nd., 3rd., etc. adjacent channels OK in same cell First-Adjacent Channels Not Feasible in Same Cell Frequency First-Adjacent Channels Feasible in Adjoining Cells if handoffs effective Second-Adjacent Channels Feasible in Same Cell Chapter 3 - Cellular Concept 9 Dr. Sheng-Chou Lin Page 5

16 CDMA Multiple access interference (MAI) How can CDMA work on a negative signal-to-noise ratio (i.e., noise higher than signal)? Processing Gain interference signal Signal & interference are both narrow band After spreading, signal & interference both become wide band signal interference After de-spreading, signal become narrow band but interference remain wide band After passing a narrow band filter most of the wide band interference energy get filtered out Chapter 3 - Cellular Concept 30 Dr. Sheng-Chou Lin Near-Far Effect P r Interference becomes sever due to near-far effect Adjacent interference (ADJ) Multiuser access interference (MAI): CDMA Intermodulation Product (IM) P s d far P i d near ADJ becomes severe Eb MAI becomes severe Nt Interference Thermal Noise Chapter 3 - Cellular Concept 3 Dr. Sheng-Chou Lin Page 6

17 An Example of ADJ Why adjacent channel interference Imperfect receiver filter Can be particular serious for Near-Far effect N, ADJ, C/I Solutions to minimize ADJ Careful filtering Channel assignment EX: d (far) = 0 d (near), n =, SIR (near-far) = (P f / P n )= (0) -n - 5dB If filter of RX = 0 db / octave If (SIR) min = 0dB, total S/ADJ = 0+5dB A separation of 6 channels is required 5/ If (SIR) min = 8dB, total S/ADJ = 8+5dB A separation of channels is required 70/0.3 P f P P n d near d far 0dB B/ B B B Chapter 3 - Cellular Concept 3 Dr. Sheng-Chou Lin A Tour of Reuse Factor N N=: Lethal awful C/I: every neighbor is cochannel every neighbor cell is adjacent channel too! center /3 of each cell OK, rest is lost in horrible interference N=: Better, but still lethal Each cell still has cochannel neighbors Each cell has adjacent channel neighbors Chapter 3 - Cellular Concept 33 Dr. Sheng-Chou Lin Page 7

18 A Tour of Reuse Factor N N = 3 : Better, but still lethal Cochannel neighbors are now spaced at D/R of better, but not 8 db... Each cell has 6 adjacent channel neighbors - all the neighbors are adjacent!! N = : Better, but still lethal Cochannel neighbors are now spaced at D/R of 3.6 Each cell has adjacent channel neighbors Chapter 3 - Cellular Concept 3 Dr. Sheng-Chou Lin A Tour of Reuse Factor N 8 3 N = 7 : The first arrangement that works in most propagation environments, giving 8+ db C/I Cochannel neighbors farther away Six at D/R of.58 Each cell has adjacent channel neighbors N = 8 : Better, but not worthwhile Cochannel neighbors farther away Four at D/R of.58 Two at D/R of 6.0 Two at D/R of 6.93 Each cell has adjacent channel neighbors Chapter 3 - Cellular Concept 35 Dr. Sheng-Chou Lin Page 8

19 N = 5 : Better, but not good enough A Tour of Reuse Factor N Cochannel neighbors farther away Two at D/R of 3.0 Four at D/R of.58 Each cell has adjacent channel neighbors N = 6 : Better, but not by much Cochannel neighbors farther away Two at D/R of 3.6 Two at D/R of.58 Two at D/R of 6.0 Each cell has adjacent channel neighbors Chapter 3 - Cellular Concept 36 Dr. Sheng-Chou Lin Intermodulation Distortion Input Non-Linear Device Output f f f f 3f -f f f 3f -f f -f f -f Imagine a non-linear device which is being fed two signals as input Input: frequency and frequency. Because the device is non-linear, its output includes the two input frequencies and additional signals due to intermodulation distortion frequencies of the intermod distortion products are of the form nf + mf and nf -mf where n,m=,,3,... The sum n + m is called the the order of the intermod products Example: 3rd Order Components: f - f, f - f, f + f, f + f 5th Order Components: 3f - f, 3f - f, 3f + f, 3f + f Chapter 3 - Cellular Concept 37 Dr. Sheng-Chou Lin Page 9

20 Intermodulation Distortion Transmitters Receivers C o m b i n e r s D u p l e x e r Amplitude IM under control Frequency Intermodulation distortion can turn a good-sounding cellular system into a sea of phantom interferers, dropped calls, and intermittent crosstalk. IM products everywhere! Frequency Chapter 3 - Cellular Concept 38 Dr. Sheng-Chou Lin Intermodulation Distortion Output Power (dbm) Power Transfer Characteristics of a typical amplifier or other device 3dB Compression point Noise Floor Input Power (dbm) Predicted Power db Compression point Every device (amplifier, etc.) has a relationship between input and output Normally, the output is a linear replica of the input, except when the input is so weak it is lost below the noise floor when the expected output is stronger than the capabilities of the amplifier, and compression occurs Even seemingly passive devices (cables, connectors, antennas) have noise floors and compression points Chapter 3 - Cellular Concept 39 Dr. Sheng-Chou Lin Page 0

21 Concept: Third-Order IM Intercept Point Output Power (dbm) Power Transfer Characteristics of typical amplifier or other device Third order intercept point Third Order intermodulation products Noise Floor Input Power (dbm) Predicted power Strength of 3rd-Order Intermod Products: P 3 = P + P - P 3i (dbm) (for f +/- f ) P 3 = P + P - P 3i (dbm) (for f +/- f ) Where: P = Output f Output port P = Output f P 3i = 3rd order intercept point The third-order IM intercept is a signal level defining the interference-free dynamic range of an amplifier or device defined as the intersection point of: the slope of normal signals and the slope of third-order IM products notice the power of 3rd order intermod products increases 3 times faster than the original signal (its slope is 3 times steeper) INPUT Chapter 3 - Cellular Concept 0 Dr. Sheng-Chou Lin Intermodulation Problem Examples Case : User Receiver Fundamental Overload Imagine a cellular handheld is being used on system B at a signal level of -85 dbm. Next door, there is a cell site of system B with just two channels up, delivering -0 and -5 dbm, respectively. What are the 3rd-order IM levels generated at the front end of the handheld? From calculations at right, the 3rdorder IM products are at -35 and -0 dbm, respectively. If one of these IM products happens to fall on the system B channel, it will be 5 or 50 db stronger than System B!! C/I (system B) = - 85-(- 35) = - 50dB CDMA (system B) f -85 dbm -0 dbm -5 dbm Power control can not work due to interference from different system GIVEN: Input power at f = - 0 dbm Input power at f = - 5 dbm db. -35 dbm 3rd order intercept pt = -5 dbm SOLUTION: P = - 0 dbm, P = -5 dbm P 3 (f ) = (-0) + (-5) - (-5) = - 35 dbm P 3 (f ) = (-5) + (-0) - (-5) = - 0 dbm Chapter 3 - Cellular Concept Dr. Sheng-Chou Lin AMP Page

22 Intermodulation due to Near-Far Effect (Reverse link) Intermodulation Interference (IM) becomes severe Dynamic Power control can be applied at the mobile transmitter to reduce IM f -85 dbm f i = f -35 or -0 dbm -0 dbm -5 dbm f f f i f 3 Ps P i Power control d near d far Chapter 3 - Cellular Concept Dr. Sheng-Chou Lin Intermodulation Problem Examples Case : Mixing in Corroded Receiving Antenna Suppose Antenna B at a certain cell site has corrosion in one element or in a connector on the coax jumper. The corrosion acts as a non-linear conductor with an equivalent 3rd order intercept as shown. Only 0 feet away ( MHz.) is Antenna A, transmitting frequencies f and f each at +0 dbm input. Isolation from Antenna A to Antenna B is approx. -50 db. IM products of -80 dbm are generated at the antenna and fed into the cell site receivers! Solutions Increase isolation between antennas Add a filter at receiver B Cell Site Transmitters Cell Site Receivers +0 dbm Isolation 50 db corrosion Antenna B GIVEN: Input power at f = -0 dbm Input power at f = -0 dbm Equiv. 3rd order intercept pt= +5 dbm SOLUTION: P = - 0 dbm, P = -0 dbm P 3 (f ) = (-0) + (-0) - (+5) = - 80 dbm P 3 (f ) = (-0) + (-0) - (+5) = - 80 dbm Chapter 3 - Cellular Concept 3 Dr. Sheng-Chou Lin f 3 Antenna A filter f f Page

23 Intermodulation Problem Examples Case 3: Mixing in Corroded Transmit Antenna Suppose Antenna A at a certain cell site has corrosion in one element or in a connector on the coax jumper. The corrosion acts as a non-linear conductor with an equivalent intercept as shown IM products of +0 dbm are generated and radiated Only 0 feet away ( MHz.) is Antenna B, with spacing isolation of 50 db IM products arrive at Antenna B with a level of --30 dbm and are fed into the cell receivers Solutions Increase isolation between antennas Add a filter at receiver B Reduce TX power Cell Site Transmitters Cell Site Receivers +0 dbm Isolation 50 db corrosion Antenna A Antenna B GIVEN: Input power at f = +0 dbm Input power at f = +0 dbm Equiv. 3rd order intercept pt= +50 dbm SOLUTION: P = +0 dbm, P = +0 dbm P 3 (f ) = (+0) + (0) - (+50) = +0 dbm P 3 (f ) = (+0) + (0) - (+50) = +0 dbm Chapter 3 - Cellular Concept Dr. Sheng-Chou Lin f 3 TX Filter f f An measurement of Intermodulation Mini-circuit mixer 55 XP- P = P P 3, then input intercept point Input IM 3 = P + P/ P 3 = P + P - P 3i Chapter 3 - Cellular Concept 5 Dr. Sheng-Chou Lin Page 3

24 DPCTH DPCTL RSSI Dynamic Power Control- (DPC) Interference Reduction DPC disable DPC enable Distance from Cell Smallest power to maintain a good quality on the reverse link Prolong battery life Reduce reverse channel interference Especially important for CDMA Dynamic Power Control reduces transmitter power when the path is short: Prevents IM due to receiver overload by strong signals on short paths Reduces interference (CCI+ADJ), since on average, the interfering transmitters are likely to be powered down Power control can be applied at the mobile transmitter, at the cell site transmitter, or at both There are 7 power steps of db each in dynamic power control, 0(max) to 7 (min) TDMA system Chapter 3 - Cellular Concept 6 Dr. Sheng-Chou Lin CDMA Power Control (Up & down link) Chapter 3 - Cellular Concept 7 Dr. Sheng-Chou Lin Page

25 CDMA Power Control Chapter 3 - Cellular Concept 8 Dr. Sheng-Chou Lin Cochannel Interference Locations Uplink/Reverse Path Cochannel interference can occur on either uplink, downlink, or both On uplink, interference occurs at the cell site receiver, from mobiles in surrounding cochannel cells Dynamic Power Control of mobile can give C/I a helpful boost since mobiles in adjacent cells statistically will average lower, while desired user still is powered adequately Chapter 3 - Cellular Concept 9 Dr. Sheng-Chou Lin Page 5

26 Cochannel Interference Locations Downlink/Forward Path On the downlink, interference occurs at the mobile user s receiver due to signals from BS in surrounding cochannel cells Dynamic Power Control of cell voice channels can give C/I a very beneficial extra boost since statistically, the interferer is likely to be power down, while desired user still is powered adequately Chapter 3 - Cellular Concept 50 Dr. Sheng-Chou Lin Adjacent-Channel Interference Locations Uplink/Reverse Path Desired mobile Cell B Cell B Interference occurs at the cell site receiver interference user location is the preliminary variable in determining severity of interference Other important factors Dynamic power control for mobiles can reduce likelihood that interferer will be stronger than desired user Statistically reduces contribution of interfering mobiles except in worst-case near the common edge of the two cells Handoffs: Keep them tight!! don t let interfering mobile drag into Cell A! Will be at full power and closer than desired user, who may be dragging into cell B! Uplink adjacent channel interference cases are less frequent than on downlink, but when they occur, they can be more severe. Chapter 3 - Cellular Concept 5 Dr. Sheng-Chou Lin Page 6

27 Adjacent-Channel Interference Locations Downlink/Forward Path Desired mobile Cell B Cell B Interference occurs at the mobile receiver Handoff is the primary factor in controlling downlink adjacent-channel interference If there is a cell that is do much stronger than the serving cell, why isn t IT the serving cell? Cases of downlink adjacent-channel interference are more frequent but less sever than uplink cases Since both cells transmit continuously; they naturally will be equal in strength at the boundary If sever interference occurs, it s a sign of call dragging: handoffs are too loose. Chapter 3 - Cellular Concept 5 Dr. Sheng-Chou Lin Frequency Plan Constraints What are the ground rules? Ideally, we would like to use every cellular channel in every cell. Why can we? Practical Cell Hardware Restrictions combining multiple transmitters into an antenna - some inconveniences Interference Restrictions Adjacent-channel Interference Can t use adjacent channels in the same cell Co-channel Interference Can t use same channel for multiple conversations in same cell must provide some physical spacing between cells using same channel Cell 395 voice channels What s wrong with this beautiful picture? Chapter 3 - Cellular Concept 53 Dr. Sheng-Chou Lin Page 7

28 Frequency Plan Constraints Transmitter Combining Restrictions RF Block Diagram of a Cell Site Transmitters Receivers Antennas C o m b i n e r s D up l e x e r The number of antennas at a cell site is limited cost and space considerations zoning, aesthetic considerations It is desirable to combine as many transmitters as possible into one antenna. Restrictions: power-handling capability of the antenna (typically 500 watts) 5-50 channels typical limit intermod considerations: transmitter isolation is required Solution: various types of combiners input port for each transmitter low attenuation thru to antenna port high attenuation back into other transmitter ports Chapter 3 - Cellular Concept 5 Dr. Sheng-Chou Lin Tuned Cavity - manual minimum frequency separation ch. required for 7 db isolation requires manual tuning; timeconsuming process insertion loss.5 db for up to 6 inputs Tuned Cavity - Auto-Tune minimum frequency separation ch. required for 7 db isolation fast automatic tuning insertion loss.5 db for up to 6 inputs Hybrid any frequency separation OK Cost of freedom: insertion loss 3.5 db for inputs 7 db for inputs 0.5 db for 8 inputs db for 6 inputs Frequency Plan Constraints Types of Transmitter Combiners INPUT INPUT OUTPUT -3db -3db Chapter 3 - Cellular Concept 55 Dr. Sheng-Chou Lin IN OUT IN OUT n n 0 db Tuned Cavity Frequency vs. Attenuation f c- f c f c+ frequency Page 8

29 C/I is Carrier-to-Interference Ratio AMPS modulation characteristics require 8 db co-channel C/I single interferer, for good quality over multiple interferers ) Between a pair of sites using same three C/I regions exist: Site A C/I better than 8 db neither site gives usable C/I Site B C/I better than 8 db Other sites are needed to serve the region where neither A nor B has good C/I Frequency Plan Restrictions Co-channel Interference over ( 7db channel, C/I = 8 db Rate of signal decay determines how close the next co-channel site can be, and how many additional sites on other channels are needed between By careful inspection of this scenario, it is possible to determine the required separation between co-channel sites to avoid interference RSSI, dbm Site A 0 Frequency Reuse Scenario Good Service C/I = 8 db Other sites Interference Good Service Site B Distance, km Chapter 3 - Cellular Concept 56 Dr. Sheng-Chou Lin Frequency Reuse Determining Required D/R Ratio Setting up a co-channel cell as close as possible without interference When laying out a new system or a coverage expansion, propagation prediction and/or measurement data are used to develop a model for the coverage of an average cell (Okumura, etc.) At some distance R from the cell A, the signal drops to the minimum acceptable level for coverage. R = coverage radius By the distance d INTERF, the signal has dropped an additional number of db equal to the required C/I (8 db) If a new cell B on the same channel is distant from Cell A by the amount R + d INTERF, the desired C/I will exist for Cell A all the way out to the distance R. Distance D = R + d INTERF is the smallest usable separation for co-channel sites in this propagation environment RSSI, dbm R = Radius of Serving Cell D = smallest usable distance to co-channel Cell Frequency Reuse Scenario 0 Site A C/I R Distance d INTERF Site B D = R + d INTERF d INTERF Chapter 3 - Cellular Concept 57 Dr. Sheng-Chou Lin Page 9

30 ACI db Frequency Planning Implications for TDMA Digital System Acceptable Performance under Combined ACI and C/I Area of Acceptable Performance C/I db Frequency plans which work well for analog systems generally will provide good performance on TDMA systems. However, TDMA and digital systems in general have definite bit error rate thresholds which must not be exceeded. The figure at right shows the relationship of adjacent-channel interference (ACI) and co-channel interference (C/I) which should be observed for TDMA systems. Note that negative ACI indicates the adjacent channel interferer is stronger than the desired signal Chapter 3 - Cellular Concept 58 Dr. Sheng-Chou Lin Summary of Frequency Planning Rules Transmitters D Antennas Receivers C o m b i n e r s D u p l e x e r 0 f c- R D/R F f c f c+ frequency Can not use adjacent channels at the same cell Adjacent channels OK in adjacent cells so long as prompt handoff available Two cells using the same channel (cochannel cells) must be separated geographically to preserve at least 8 db C/I at their service boundaries determine required geographic separation D/R from propagation analysis Channels used by transmitters feeding the same antenna must be separated in frequency sufficiently to allow combiners to provide isolation or, hybrid combiners or other non-frequencycritical technique must be used Chapter 3 - Cellular Concept 59 Dr. Sheng-Chou Lin Page 30

31 Capacity Improvement Cell splitting: increases the number of base stations Sectoring: relies on BS antenna placement to reduce CCI Zone microcell: relies on BS antenna placement to reduce CCI Cell splitting Sectoring Zone microcell Chapter 3 - Cellular Concept 60 Dr. Sheng-Chou Lin Sectorization Advantages Cell radius unchanged CCI D/R, cluster size N, frequency reuse, capacity Omni-directional antenna Directional antenna Three 0 o sectors, six 60 o sectors For N=7 and 0 o sectors, number of CCI decreases from 6 to. C/I =.db. C/I =8 db < (sectoring C/I =.db, N=7). = N (omni worst case), Increase in capacity /7 Downtilting the sector antenna to reduce CCI Disadvantages Number of antenna Trunking efficiency, channels 3groups Handoffs, not a major concerns since it occurs within the same cell without intervention from MSC Chapter 3 - Cellular Concept 6 Dr. Sheng-Chou Lin Page 3

32 Radiation Patterns Key Features and Terminology Radiation patterns of antennas are usually plotted in polar form The Horizontal Plane Pattern shows the radiation as a function of azimuth (i.e.,direction N-E-S-W) The Vertical Plane Pattern shows the radiation as a function of elevation (i.e., up, down, horizontal) Antennas are often compared by noting specific features on their patterns: -3 db ( HPBW ), -6 db, -0 db points front-to-back ratio angles of nulls, minor lobes, etc. 70 (W) Typical Example Horizontal Plane Pattern Notice -3 db points 0 (N) db nulls or a Minor minima Lobe Front-to-back Ratio 80 (S) 0 db points Main Lobe 90 (E) Chapter 3 - Cellular Concept 6 Dr. Sheng-Chou Lin Antenna Downtilt What s the goal? Cell A Scenario Scenario Cell B Downtilt is commonly used for two reasons:. Reduce Interference reduce radiation toward a distant co-channel cell concentrate radiation within the serving cell. Prevent overshoot Improve coverage of nearby targets far below the antenna otherwise within null of antenna pattern Are these good strategies? How is downtilt applied? Chapter 3 - Cellular Concept 63 Dr. Sheng-Chou Lin Page 3

33 Sector Antenna Dallas, Texas, USA Chapter 3 - Cellular Concept 6 Dr. Sheng-Chou Lin Sector Antenna FJU, Taiwan Lin-Ko, Taiwan Chapter 3 - Cellular Concept 65 Dr. Sheng-Chou Lin Page 33

34 Rationale for Sectorization Sectorization is a tool for more tightly controlling frequency utilization in a cellular system We saw that if the number of channels remains constant, sectorization actually reduces the capacity of a cell So, why would anyone want to sectorize? In hope of being able to reduce N To substantially improve C/I, even if N is not changed To gain flexibility to control traffic distribution and reduce interference at troublesome boundaries where large cells and small cells meet N=7 Omni N= Sector? 35.6 erlangs 7.03 erlangs Chapter 3 - Cellular Concept 66 Dr. Sheng-Chou Lin Comparison of Typical Coverage Using Omni and Sector Antennas Coverage Comparison Using Sector and Omni Antennas -3 dbm Miles ERP = 00w Ant. Ht. 50 ft. DB-833 vs Omni Whip -95 dbm The figure shows computed coverage in miles for an omnidirectional collinear vertical antenna and a panel antenna typically used for sector applications Computation used Okumura-Hata formula from Lesson 3-95 dbm is typical design limit for edge of a cell -3 dbm is interfering contour which would deliver 8 db C/I at a distant cell edge (-95 dbm) Notice how substantially both coverage and interference are suppressed off the back of the sector antenna Chapter 3 - Cellular Concept 67 Dr. Sheng-Chou Lin Page 3

35 N=7 Omni Plan Sectorization Improves Reuse Density In a system where N=7 omni works well, N= 0 sector may be feasible possible 55% increase in capacity Problems and additional considerations: increased system complexity handoffs & handovers very critical to achieving acceptable performance possibility of specific local propagation conditions unsuitable for sectorization cost of sectorization N= 0º Sector Plan -3 dbm -95 dbm C/I = 8 db N=7/N= Comparison Voice Channels N=7 Omni 5/cell Capacity, Erlangs 35.6 N= 0º Sector 6/sector 78/cell 8./sector 55./cell Total voice channels 3 ( ignoring expanded spectrum ) Chapter 3 - Cellular Concept 68 Dr. Sheng-Chou Lin N = 7, 0 DEG. Sectorized Cell Plan a CHARACTERISTICS: A CLUSTER OF 7 CELLS 3 SECTORS / CELL TOTAL NUMBER OF SECTORS = 7 x 3 = REQUIRES FREQUENCY GROUPS USES DIRECTIONAL ANTENNAS 7c 6c 7a 7b 6a 6b c c 5c b a b 5a 3c c 3a 3b a b 5b Chapter 3 - Cellular Concept 69 Dr. Sheng-Chou Lin Page 35

36 A-BAND N=7 CHANNEL SETS Channel Set Designations A B C D E F G A B C D E F G A3 B3 C3 D3 E3 F3 G3 Control Ch Voice Channels Expanded Spectrum A' Expanded Spectrum A" Set Channel Count Summary 6 Control Normal A A" A' Total Voice Chapter 3 - Cellular Concept 70 Dr. Sheng-Chou Lin Comparison Summary of Popular Frequency Plans Frequency Plan Voice Channels Per Cell Capacity Erlangs per Cell N=7 Omni 3.7 Adjacent Channels? Yes. Good Handoffs!! Complexity Simple N=9 Omni clean Simple N=7 0 o Sector 3.8 clean Moderate N=9 0 o Sector clean Moderate N= 0 o Sector clean Moderate N= 60 o Sector 78. clean Very Difficult N=3 60 o Sector clean Very Difficult Chapter 3 - Cellular Concept 7 Dr. Sheng-Chou Lin Page 36

37 A comparison of Regular antenna and smart antennas Chapter 3 - Cellular Concept 7 Dr. Sheng-Chou Lin Several kinds of intelligence in smart antennas Chapter 3 - Cellular Concept 73 Dr. Sheng-Chou Lin Page 37

38 Gain in Smart Antenna SIR is boost in general ( Experiment has show 0dB increase ) give possibility for reduce frequency reuse distanc Chapter 3 - Cellular Concept 7 Dr. Sheng-Chou Lin Smart Antenna Block Chapter 3 - Cellular Concept 75 Dr. Sheng-Chou Lin Page 38

39 Multi-beams antennas Chapter 3 - Cellular Concept 76 Dr. Sheng-Chou Lin Smart Antenna Demo Chapter 3 - Cellular Concept 77 Dr. Sheng-Chou Lin Page 39

40 Smart Antenna Components Multi-Line Phase Shifter Combiner/Divider Chapter 3 - Cellular Concept 78 Dr. Sheng-Chou Lin Structures Three zone sites A single BS Share the same equipment A mobile is shared by the zone with the strongest signal Any channel may be assigned to any zone MS from one zone to another retains the same channel Useful along high way or urban traffic corridors Advantages Handoff is not required BS is localized similar to sectoring CCI, TX power, Capacity Capacity without reducing trunking efficiency Microcell zone Chapter 3 - Cellular Concept 79 Dr. Sheng-Chou Lin Page 0

41 A Microcell Zone Example Each group of three hexagons represents a cell Omni-cell: N=7, D/R=.6, S/I = 8dB For a constant value of SIR = 8dB. Microcell zone Dz/Rz =.6 S/I 0dB in worst case Omni-cell D/R = 3 N=3 Capacity increase 7/3=.33 No loss in trunking efficiency Are adopted in many cellular and PCS systems One cell One zone Chapter 3 - Cellular Concept 80 Dr. Sheng-Chou Lin Trunking and GOS Trunking concepts Allow a large number of users to share the relatively small number of channels Statistical behavior of users Determine the number of telephone circuits in designing cellular radio Handle a specific capacity at a specific grade of service based on Trunking Theory Queuing Theory GOS (Grade of Service) A measure of the ability to access a trucked system during the busiest hour Specified as the blocking probability. Ex: % Objectives: wireless designer s job is to estimate maximum required capacity and allocate the proper number of channels Chapter 3 - Cellular Concept 8 Dr. Sheng-Chou Lin Page

42 Traffic Engineering Objectives $ 0 Traffic engineering is the intelligent art of keeping both system customers and accountants happy. Traffic engineering finds answers to questions at every stage in the development of a cellular system In Initial Design: How many cells are needed? What size switching resources? How many T-s, how much microwave? Ongoing during Operation: How many radios for each cell or sector? When are new cells needed for capacity? 6 Chapter 3 - Cellular Concept 8 Dr. Sheng-Chou Lin Walking a Fine Line The traffic engineer must walk a fine line between two problems: Overdimensioning too much cost insufficient resources to construct traffic revenue is too low to support costs system operator may fail or get new traffic engineer Underdimensioning blocking poor technical performance (interference) capacity for billable revenue is low revenue is low due to poor quality users unhappy, cancel service system operator may fail or get new traffic engineer Chapter 3 - Cellular Concept 83 Dr. Sheng-Chou Lin Page

43 Basics of Traffic Engineering Terminology & Concept of a Trunk Traffic engineering in telephony is focused on the voice paths which users occupy. They are called by many different names: trunks circuits radios (remember in TDMA, a radio may carry up to 3 Circuits? Some other common terms are: trunk group a trunk group is several trunks going to the same destination, combined and addressed in switch translations as a unit, for traffic routing purposes member one of the trunks in a trunk group Chapter 3 - Cellular Concept 8 Dr. Sheng-Chou Lin Traffic Engineering Erlang: Amount of traffic intensity carried by a channel that compleely occupied Example: 0.5 Erlang of traffic Aradio channel is occupied for 30 minutes during an hour Traffic intensity (each user) = call request rate call holding time i.e. Au = H (Erlangs) Total traffic intensity (U users) = A = u Au Traffic intensity per channel = Ac = u Au / C (C channels) Maximum possible carried traffic is the number of channels C in Erlang AMP cellular: GOS = % blocking = out of 00 calls will be blocked during the busiest hour Trunking Efficiency: Number of users which can be offered a particular GOS with a particular configuration of fixed channels. Chapter 3 - Cellular Concept 85 Dr. Sheng-Chou Lin Page 3

44 Basics of Traffic Engineering Offered Traffic and Call Duration N Trunks A = C x T A = Offered Traffic (Erlangs) C = Average number of calls per unit of time T = Average call duration Offered traffic is the amount of traffic users attempt to transmit through the system. Offered Traffic, Erlangs hour Example: C = 000 call attempts in the busy hour T = 50 seconds average call duration What s the offered traffic? Solution: A = C x T = 000 x ( 50 / 3600 ) =.667 Erlangs Chapter 3 - Cellular Concept 86 Dr. Sheng-Chou Lin Basic Trucked systems Erlang B Formula (Table., Fig.6): No queuing No setup (allocate a channel) time Immediate Acess to a channel if one is available The call is blocked if no channels are available try again later Is called blocked calls cleared Based on M/M/m queue formula using Poisson and others Erlang C Formula(Fig..7): A queue is provided to hold blocked calls Call request may be delayed until a channel becomes available Is called blocked call delayed GOS = Pr(delay > t)=pr (delay>0) Pr(delay>t delay>0) =Pr(delay>0 )exp(-c(c-a)t/h) Average delay D = Pr(delay >0) H/ (C-A) GOS = Pr (delay>0) with Erlang C table Chapter 3 - Cellular Concept 87 Dr. Sheng-Chou Lin Page

45 Equation behind the Erlang-B Table The Erlang-B formula is fairly simple to implement on handheld programmable calculators, in spreadsheets, or popular programming languages. P n (A) = A n n! + A ! A n n! P n (A) = Blocking Rate (%) with n trunks as function of traffic A A = Traffic (Erlangs) n = Number of Trunks Number of Trunks max # of trunks Offered Traffic, A Offered Traffic lost due to blocking average # of busy channels time Chapter 3 - Cellular Concept 88 Dr. Sheng-Chou Lin Erlang-B EX: Channels are grouped GOS = channels A =.6 x 5 channels A= x.36 =.7 60% Loss Total users Chapter 3 - Cellular Concept 89 Dr. Sheng-Chou Lin Page 5

46 Erlang-C Chapter 3 - Cellular Concept 90 Dr. Sheng-Chou Lin Erlang-B Traffic Tables Abbreviated - For P.0 Grade of Service Only #Trunks Erlangs #Trunks Erlangs #Trunks Erlangs #Trunks Erlangs #Trunks Erlangs #Trunks Erlangs #Trunks Erlangs #TrunksErlangs Chapter 3 - Cellular Concept 9 Dr. Sheng-Chou Lin Page 6

47 Examples: Ex. Erlang B: GOS = 0.5% boloking, Number of trunked channels = 0 Each user generates 0. Erlangs of traffic Au = 0. Erlangs A= 3.96 from Erlang-B, Total number of users = A/Au=3.96/0. 39 users Ex Erlang C: A hexagonal cell within a -cell system, radius =.387 km. Total number of channels = 60. The load per user = 0.09 Erlangs, call rate = call/hour, GOS= 5% R=.387 km Area = 5km N=, channels per cell = 60/=5 channels GOS = 5% probability of delay with C= 5, A= 8.8 Erlangs Number of users = 8.8/0.09 = 303 users = 30/ 5km = 60/ km =, call holding time = H = 0.09/ hour=0. seconds Pr(delay>0 delay>0)=exp(-(c-a)t/h)=exp(-(5-8.8)0/0.)=5.% GOS = 0.05 = Pr(delay>0) Pr(delay>0) = =.76% Chapter 3 - Cellular Concept 9 Dr. Sheng-Chou Lin Trunking Efficiency An Important Cellular Application 5 channels 5 ch. 5 ch. 5 ch. blocking probability: % Busy cellular systems often use sectorized cells A cell coverage area is divided into several sectors using directional antennas 3-sector (0-degrees) 6-sector (60-degrees) radio channels assigned per sector Capacity of a sectorized cell is less than capacity of an omni cell with same total number of channels 5 channels: 35.6 Erlangs 3 x 5 channels: 3 x 9.0 erl. = 7.03 Erlangs Why would anyone sectorize? Sectorization eases frequency reuse more than it hurts capacity Chapter 3 - Cellular Concept 93 Dr. Sheng-Chou Lin Page 7

48 Lesson 3 Complete Chapter 3 - Cellular Concept 9 Dr. Sheng-Chou Lin Page 8