Wireless Communications

Transcription

1 Wireess Communications Ceuar Concept Hamid Bahrami Reference: Rappaport Chap3 Eectrica & Computer Engineering

2 Statements of Probems Soving the probem of Spectra congestion System Capacity A system-eve design considers using ces Low power transmitter Sma service areas More base stations Reduced interference between base stations

3 Design Difference

4 Frequency Reuse Introduction Frequency reuse: a technique reying on aocation and reuse of channes throughout a coverage region Increase the channe capacity Minimize the interference Low power transmission Effective coverage area Approach Ce: a sma geographic area, in which each ceuar base station is aocated with a set of radio channes Compete different channes among neighboring ces Limiting the coverage area within a ce

5 Frequency Reuse Concept D= distance between ces B B G C G C A A F D F E D E Custer Ces abeed with the same etter use the same group of channes. The hexagona ce shape is conceptua and universay adopted. Base station transmitters coud be in the center of the ce or on ce vertices.

6 Rea-Life Coverage

7 Frequency Reuse Custer Size A arger custer size causes the ratio between the ce radius and the distance between co-channe ces to decrease, eading to weaker co-channe interference A sma custer size indicates that co-channe ces are ocated much coser together

8 Frequency Reuse Definitions Capacity, frequency reuse factor, custer S: the number of dupex channes avaiabe for use per custer in a ceuar system k: the number of channes in each ce k<s N: the number of ces per custer 1/N: frequency reuse factor C: the tota number of dupex channes (capacity) in a ceuar system M: the number of custers S = kn C = MkN = MS

9 Exampe: Rappaport page 97, probem 3.4 If 20 MHz of tota spectrum is aocated for a dupex wireess ceuar system and each simpex channe has 25 khz RF bandwidth. Find The number of dupex channes The tota number of channes per ce size if N=4 reuse is used. 20M = 400 channes 25K 2 400/4 = 100 channes per ce

10 Exampe: Garg, page 130 We consider a ceuar system in which tota avaiabe voice channes to hande the traffic are 960. The area of each ce is 6km 2 and the tota coverage is 2000km 2 - The system capacity if the custer size is N=4. - The system capacity if the custer size is N=7. Area of a custer # of custers # of channes /ce System capacity N=4 4*6=24 km / /4=240 84*960=80640 N=7 7*6=42 km / /7=137 48*960=46080 Comments: N from 7 to 4, the capacity [from to 80640] No free unch what woud be the cost?

11 Frequency Reuse Locating Co-channe Ces N = 3: i = 1, j = 1 1) Move i ces aong any chain 2) Turn 60 degrees counter-cock wise and move j ces 2 N = i + ij + j 2

12 Co-channe Interference Interference between signas from co-channes Increasing the signa-to-noise ratio (SNR) to overcome therma noise does not work Physicay separate co-channe ces Co-channe reuse ratio D Q = = 3N R Condition: same ce size, same transmitter power R: radius of the ce D: distance between centers of the nearest co-channe ces D D-R D+R D-R D+R D

13 Co-channe Interference Custer size N Co-channe reuse ratio Q i=1, j=1 3 3 i=1, j= i=0, j= i=2, j= D Q = = 3N R Tradeoff!!! Large vaue of Q à smaer eve of interference à improve the transmitter quaity Sma vaue of Q à smaer N à improve the capacity

14 Co-channe Interference SIR Signa-to-interference ratio (SIR) for a forward channe S I = i o S i= 1 S: the desired signa power from the desire BS I i : the interference power cause by the i th interfering co-channe ce BS I i P r = P 0 d d o n d P = r ( dbm) P0 ( dbm) 10n og do The average receiver power at a distance d with the path oss exponent n

15 Co-channe Interference SIR When the transmit power of each base station is equa The path oss exponent is the same throughout the coverage ( ) = = = = o i o i n i n i i i D R I S I S 1 1 Consider the first ayer of interfering ces A the interference BSs are equidistant from the desire BS The distance is equa to D ) / ( i N i R D I S n n = =

16 Exampe: find the custer size N Exampe: assuming that 6 ces are cose enough to create significant interference and they are approximatey equidistant from the desired BS. For the US AMPS system that uses FM and 30 khz channes, it indicates that sufficient voice quaity is provided when S/I is greater than or equa to 18 db. What woud be the custer size? (n=4) S I = 3N i 0 n 10og 10 S I =18dB S I = N = = 6.49 n 3N = i 0

17 Exampe: Worst Case S I = i o S i= 1 I i D Q = = 3N R S I S I = = 2( D 2( Q R) 1) 4 4 R 1 + 2( Q 4 + 2( D + R) + 1) D + 2Q For N=7, Q=4.6, The worst case S/I for n=4 is about 17 db (53.37) 4 4 D D-R D+R D-R D+R D

18 Exampe: If a S/I ratio is 15 db is required for satisfactory forward channe performance of a ceuar system, what is the frequency reuse factor and custer size that shoud be used for maximum capacity if the path oss n=3? Assume that there are six co-channe ces in the first tier, and a of them are at the same distance from the mobie. D Q = = 3N R S I = 3N i 0 n

19 Adjacent Channe Interference Interference resuting from signas which are adjacent in frequency to the desired signa Imperfect receiver fiters Near-far effect When a mobie user cose to a BS transmits on a channe cose to one being used by a weak mobie Minimized by carefu fitering; proper channe assignment; and power contro

20 Power Contro The power eves transmitted by every subscriber unit are under constant contro by the serving BS Smaest power to maintain a good quaity ink on the reverse channe Advantage Battery ife Reduce the reverse channe S/I

21 Channe Assignment and Panning Air Interface Standard Assigning the radio channes to each BS Contro channes 5% + voice channes 95% The avaiabe mobie radio spectrum is divided into channes Assigning contro channes is done more conservativey (smaer frequency reuse) Chaenge in practice Appropriate frequency reuse ratio Radio propagation Imperfect coverage

22 Channe Assignment Pan Keeping the frequency separation between channes in a given ce as arge as possibe Sequentiay assigning successive channes in the frequency band to different ces Objectives Increasing capacity Minimizing adjacent channe interference

23 Channe Assignment Strategies Fixed channe assignment strategy Each ce is aocated a predetermined set of voice channes Ony the unused channes in the ce coud be served If a channes are occupied, the ce is bocked (no service) Borrowing strategy: a ce is aowed to borrow channes from a neighboring ce, controed by the Mobie Switching Center (MSC)

24 Channe Assignment Strategies Dynamic channe assignment strategy The serving base station requests a channe from the MSC when a ca is required The MSC aocates a channe by taking into account The ikeihood of future bocking within the ce [to increase the trunking capacity] The frequency of use of the candidate channe The reuse distance of the channe [to avoid the co-channe interference] Other cost Requirement: rea-time management

25 Handoff Strategies Handoff: when a mobie moves into a different ce whie a conversation is in process, the MSC automaticay transfers the ca to a new channe beonging to the new base station Requirements Successfuy, Infrequenty and Imperceptibe to the users Specify an optimum signa eve at which to initiate a handoff - Received Signa Strength (RSS) Δ = P r handoff P r min usabe

26 Handoff Strategies Ensure The drop in the measured signa eve is not due to momentary fading The mobie is moving away from the serving BS Monitor: the signa eve for a certain period of time Dwe time: the time over which a ca may be maintained within a ce without handoff Each wireess technoogy uses its own methods to impement the handoffs

27 Handoff Strategies 1G handoff strategy Signa strength measurements are made by the BS and supervised by the MSC Each BS monitors the signa strengths of a of its voice channes The ocator receiver in each BS scans and determines signa strengths of mobies users in neighboring ces The MSC decides if a handoff is necessary 2G Mobie Assisted Handoff (MAHO) Every mobie station measures the received power and reports to the serving BS Handoff when the power received form the BS of a neighboring ce begins to exceed the power received from the current BS by the certain eve or for a certain time

28 Handoff Strategies Prioritizing handoffs Guard channe concept A fraction of the tota avaiabe channes in a ce is reserved excusivey for handoff Reducing the tota carried traffic, but offering efficient spectrum utiization Queuing of handoff Decreasing the probabiity of forced termination of a ca The deay time and size of the queue is determined from the traffic pattern of the particuar service area

29 Handoff Strategies Practica Issue 1: high speed vehices pass through the coverage region of a ce within a matter of seconds Soution: Umbrea ce Issue 2 (ce dragging): pedestrian users might provide a very strong signa to the BS. Even though the user has traveed we beyond the designed range of the ce, the received signa at the BS may be above the handoff threshod. Thus, no handoff occurs. Soution: more inteigent handoff strategies to decide on handoff threshods and radio coverage parameters (e.g. dwe time), Abiity to make handoff decision based on wide range of metrics other than signa strength Issue 3: hard handoff vs. soft handoff

30 Improving Coverage and Capacity Statement of the probem The need of increasing number of users The number of channes assigned is insufficient Techniques Frequency reuse Ce spitting Sectoring Coverage zone

31 Ce Spitting The process of subdividing a congested ce into smaer ces, when the traffic oad carried by the origina ce exceeds its capacity Each smaer ce has its own BS Reduction in antenna height Reduction in transmitter power

32 Ce Spitting Prove that spitting ces with radius R/2 coud make the smaer transmitted power possibe.

33 Ce Spitting To increase the capacity [ogic?] Increase the number of times that channes are reused Increase the number of channes per unit area [smaer radius ces/microces] More ces impies more ce boundaries wi be crossed more often, hence increasing trunking and handoff It is not necessary to spit a the existing ces at same time. Ony those ces which have traffic overoads are candidates for spitting

34 Sectoring A process of repacing a singe omni-directiona antenna (idea) with directiona antennas (practica) The tota channes per sector = tota channes/ce the number of sectors/ce SIR coud be increased, How about capacity? 120 o sector o sector

35 Sectoring Sectoring reduces CELL capacity because the channe resource is distributed more thiny among sectors However, ess co-channe interference à it is possibe to reduce the custer size à potentiay increase the system capacity More ces partitions impies more ce boundaries wi be crossed more often, hence increasing trunking and handoff

36 A Microce Zone Concept Statement of the probem The increased number of handoffs when sectoring is empoyed Concept For seven ce reuse Each of the three zone sites are connected to a singe BS and share the same radio equipment No handoff necessary inside three zones Improve the capacity by decreasing interference

37 Repeaters Range extension Cover the service gap Hard-to reach Within buidings, in vaeys, in tunnes Radio retransmissions Bidirectiona in nature Simutaneousy send signas to and receive signas from a serving BS

38 Mobie Foward Channe (Downink) Reverse Channe (Upink) Radio tower Ce-Site MTSO to PSTN Power On/Log-On Handset reads a certain information from its memory This info is usuay programmed by the service provider to give the user a unique mobie assignment number (MAN) Handset scans the Forward Contro Channes for the strongest signas It monitors the detected contro channes for the eve of signa

39 Scan Heo, this is me! Who is me? Ce Site & MTSO It tunes itsef to a suitabe channe and goes into ide state In ide state, it constanty monitors the data/signa strength If the signa is beow a threshod, the unit goes into scanning mode unti it finds another channe Searching.. A check is sent across the contro channes to verify the area code or the ID number (registration) The MTSO continuay monitors the mobie unit

40 Incoming Cas System finds the mobie unit through a paging channe A page is sent by the MTSO to a base stations The Mobie Identification Number (MIN) or MAN is broadcast to the paging message\mobie handset receives the message and identifies itsef Each MU uses microprocessor to receive incoming cas and to shift to appropriate frequency As a ca comes in, the BS controers assigns a channe and then directs a frequency synthesizer in the handset to shift to the appropriate frequency "Ring" "Ring" Car 55 Car 55, where are you? Here I am I've got a ca for you, use channe 16 OK Page car 55 Here it comes Radio tower Radio tower Page car 55 Page car 55 Page car 55 Radio tower Page car 55 Ce Site & MTSO

41 Outgoing Cas Ca initiation request is made by the Mobie Unit MTSO vaidates request and makes connection. Vaidation comes back in the form of instruction to use a specific channe I want to make a ca! Use channe #20 OK Go ahead Radio tower Radio tower Radio tower Radio tower Ce Site & MTSO

42 Traffic Management Statement of Probems Dia-Tone deay A arge number of users compete for a sma number of servers An assumption that the user wi wait unti a server is avaiabe Service denia A arge number of users compete for a sma number of servers An assumption that no deay wi be encountered. The user is either give access to trunk or is advised by a busy signa or a recording that none are avaiabe The user may frequenty reinstate the ca attempt Def: to determine the usage of a transmission route

43 Traffic Usage Caing Rate or Ca Intensity: the number of times a route or traffic path is used per unit time (cas per hour) per traffic path during the busy hour Ca hoding time: the average duration of occupancy of a traffic path by a ca

44 Traffic Intensity Def: the average number of cas simutaneousy in progress during a particuar period of time Unit: Erangs 1 Erang = one ca in process during an hour Exampe: a radio channe that is occupied for 30 minutes during an hour carries 0.5 Erangs of traffic Traffice intensity = the sum of theduration of circuit hoding time monitoring period

45 Exampe: Check your understanding about the definition In a switching office, an equipment component has an average hoding time of 5 seconds; about 450 attempts to use this equipment for a one-hour period. Assuming there is no overfow the system can hande a cas. How much usage in Erangs has accumuated on this piece of the equipment? Answer: The duration of monitoring period is one hour The sum of hoding time: 450 * (5/3600) Traffic intensity: 450*5/3600/1=0.625 Erangs

46 Exampe: Check your understanding about the definition In a wireess network, each subscriber generates two cas per hour on the average ca hoding time of 120 seconds. What is the traffic intensity? Answer: The duration of monitoring period is one hour The sum of hoding time: 2 * (120/3600) Traffic intensity: 2*120/3600/1= Erangs

47 Exampe: working ike a traffic manager In order to determine voice traffic on a ine, we coected the foowing data during a period of 90 minutes. Cacuate the traffic density. Ca no Duration of cas (seconds) Answer: The duration of monitoring period: 1.5 hours The sum of hoding time: =800 seconds =0.22 hour Traffic intensity 0.22/1.5=0.148 Erangs

48 Exampe: working ike an engineer, not a mathematician The average mobie user has 500 minutes of use per month; 90% of traffic occurs during work days. There are 20 week days per month. Assuming that in a given day, 10% of traffic occurs during the busy hour (BH), determine the traffic per subscriber per BH. Answer: The tota hours during work days: (500/60)*0.9=7.5 hours / The average hours per work day: 7.5/20=0.375 hours The average hours during the BH per day: 0.375*0.1= hours Traffic per subscriber per BH is about Erangs

49 Trunking and Grade of Service Trunking: Accommodate a arge number of users in a imited radio spectrum A arge number of users to share the reativey sma number of channes in a ce Providing access to each user from a poo of avaiabe channes Erang: the measure of traffic intensity A radio channe that is occupied for thirty minutes during an hour carries 0.5 Erangs of traffic

50 Trunking and Grade of Service Grade Of Service (GOS): the abiity of a user to access a trunked system during the busiest hour A benchmark to define the desired performance of a particuar trunked system Specifying a desired ikeihood of a user obtaining channe access given a specific number of channes avaiabe in the system

51 Trunking and Grade of Service Set up time: the time required to aocate a trunked radio channe to a requesting user Bocked ca (Lost ca): ca which cannot be competed at time of request due to congestion Hoding time H: average duration of a typica ca Traffic intensity A: measure of channe time utiization, which is the average channe occupancy, in Erangs Load: traffic intensity across the entire trunked radio system Grade of service: a measure of congestion, the probabiity of a ca being bocked (B); the probabiity of a ca being deayed for a certain time duration (C) Request rate λ: the average number of ca requests per unit time

52 Trunking and Grade of Service Two types of trunked systems 1 st type: no queuing for ca requests Pr A [ bocking] = = GOS C k =1 C A / C! 2 nd type: a queue is provided to hod bocked cas k / k! C: the number of trunked channes A: the tota offered traffic Pr [ deay > 0] = A C + C! A C / C! ( 1 A/ C) A k C k 1 k = 0! = GOS