ECE 476/ECE 501C/CS 513 - Wireless Communication Systems Winter 2004 Lecture 3: Cellular Fundamentals Chapter 3 - The Cellular Concept - System Design Fundamentals I. Introduction Goals of a Cellular System High capacity Large coverage area Efficient use of limited spectrum Large coverage area - Bell system in New York City had early mobile radio Single Tx, high power, and tall tower Low cost Large coverage area - Bell system in New York City had 12 simultaneous channels for 1000 square miles Small # users Poor spectrum utilization What are possible ways we could increase the number of channels available in a cellular system? Lecture 3, Page 1 of 20
Cellular concept Frequency reuse pattern Figure 3.1, page 59 Cells labeled with the same letter use the same group of channels. Cell Cluster: group of N cells using complete set of available channels Many base stations, lower power, and shorter towers Small coverage areas called cells Each cell allocated a % of the total number of available channels Nearby (adjacent) cells assigned different channel groups to prevent interference between neighboring base stations and mobile users Same frequency channels may be reused by cells a reasonable distance away reused many times as long as interference between same channel (co-channel) cells is < acceptable level As frequency reuse # possible simultaneous users # subscribers but system cost (more towers) To increase number of users without increasing radio frequency allocation, reduce cell sizes (more base stations) # possible simultaneous users Lecture 3, Page 2 of 20
The cellular concept allows all mobiles to be manufactured to use the same set of freqencies *** A fixed # of channels serves a large # of users by reusing channels in a coverage area *** II. Frequency Reuse/Planning Design process of selecting & allocating channel groups of cellular base stations Two competing/conflicting objectives: 1) maximize frequency reuse in specified area 2) minimize interference between cells Cells base station antennas designed to cover specific cell area hexagonal cell shape assumed for planning simple model for easy analysis circles leave gaps actual cell footprint is amorphous (no specific shape) - where Tx successfully serves mobile unit base station location cell center omni-directional antenna (360 coverage) - not necessarily in the exact center (can be up to R/4 from the ideal location) cell corners sectored or directional antennas on 3 corners with 120 coverage. - very common III. System Capacity S : total # of duplex channels available for use in a given area; determined by: amount of allocated spectrum channel BW modulation format and/or standard specs. (e.g. AMPS) k : number of channels for each cell (k < S) N : cluster size # of cells forming cluster S = k N M : # of times a cluster is replicated over a geographic coverage area Lecture 3, Page 3 of 20
System Capacity = Total # Duplex Channels = C C = M S = M k N (assuming exactly MN cells will cover the area) If cluster size (N) is reduced and the geographic area for each cell is kept constant: The geographic area covered by each cluster is smaller, so M must to cover the entire coverage area (more clusters needed). S remains constant. So C. The smallest possible value of N is desirable to maximize system capacity. Cluster size N determines: distance between co-channel cells (D) level of co-channel interference A mobile or base station can only tolerate so much interference from other cells using the same frequency and maintain sufficient quality. large N large D low interference but small M and low C! Tradeoff in quality and cluster size. The larger the capacity for a given geographic area, the poorer the quality. Frequency reuse factor = 1 / N each frequency is reused every N cells each cell assigned k S / N N cells/cluster connect without gaps specific values are required for hexagonal geometry N = i 2 + i j + j 2 where i, j 1 Typical N values 3, 4, 7, 12; (i, j) = (1,1), (2,0), (2,1), (2,2) Lecture 3, Page 4 of 20
Move i cells along any chain of hexagons, then turn 60 degrees and move j cells. Show the cluster arrangement for a cluster size of 4. Example: Given a Frequency Division Duplex cell system using two 25 khz channels for frequency division duplex operation. Assume 75 voice channels are used for each cell in a cluster size of N = 7 with one control channel per cell. Assume omni-directional antennas (1 antenna/cell). What amount of spectrum is needed for such a system? Lecture 3, Page 5 of 20
If a cellular provider wished to have the possibility of 100,000 simultaneous calls in progress over its coverage area of 1000 km 2, what cell size would need to be used in the system design? IV. Channel Assignment Strategies Goal is to minimize interference & maximize use of capacity lower interference allows smaller N to be used greater frequency reuse larger C Two main strategies: Fixed or Dynamic Fixed each cell allocated a pre-determined set of voice channels calls within cell only served by unused cell channels all channels used blocked call no service several variations MSC allows cell to borrow a VC (that is to say, a FVC/RVC pair) from an adjacent cell donor cell must have an available VC to give Lecture 3, Page 6 of 20
Dynamic channels NOT allocated permanently call request goes to serving base station goes to MSC MSC allocates channel on the fly allocation strategy considers: likelihood of future call blocking in the cell reuse distance (interference potential with other cells that are using the same frequency) channel frequency All frequencies in a market are available to be used Advantage: reduces call blocking (that is to say, it increases the trunking capacity), and increases voice quality Disadvantage: increases storage & computational load @ MSC requires real-time data from entire network related to: channel occupancy traffic distribution radio signal strength indications (RSSI's) from all channels V. Handoff Strategies Handoff: when a mobile unit moves from one cell to another while a call is in progress, the MSC must transfer (handoff) the call to a new channel belonging to a new base station new voice and control channel frequencies very important task often given higher priority than new call It is worse to drop an in-progress call than to deny a new one Minimum useable signal level lowest acceptable voice quality call is dropped if below this level specified by system designers typical values 90 to 100 dbm Lecture 3, Page 7 of 20
Quick review: Decibels S = Signal power in Watts Power of a signal in decibels (dbw) is P signal = 10 log 10 (S) Remember db is used for ratios (like S/N) dbw is used for Watts dbm = db for power in milliwatts = 10 log 10 (S x 10 3 ) dbm = 10 log 10 (S) + 10 log 10 (10 3 ) = dbw + 30-90 dbm = 10 log 10 (S x 10 3 ) 10-9 = S x 10 3 S = 10-12 Watts = 10-9 milliwatts -90 dbm = -120 dbw Signal-to-noise ratio: N = Noise power in Watts S/N = 10 log 10 (S/N) db (unitless raio) choose a (handoff threshold) > (minimum useable signal level) so there is time to switch channels before level becomes too low as mobile moves away from base station and toward another base station Lecture 3, Page 8 of 20
Fig. 3.3, pg. 63 Handoff Margin = P handoff threshold P minimum usable signal db carefully selected too large unnecessary handoff MSC loaded down too small not enough time to transfer call dropped! A dropped handoff can be caused by two factors not enough time to perform handoff delay by MSC in assigning handoff high traffic conditions and high computational load on MSC can cause excessive delay by the MSC no channels available in new cell Lecture 3, Page 9 of 20
Handoff Decision signal level decreases due to signal fading don t handoff mobile moving away from base station handoff must monitor received signal strength over a period of time moving average time allowed to complete handoff depends on mobile speed large negative received signal strength (RSS) slope high speed quick handoff statistics of the fading signal are important to making appropriate handoff decisions Chapters 4 and 5 1 st Generation Cellular (Analog FM AMPS) Received signal strength (RSS) of RVC measured at base station & monitored by MSC A spare Rx in base station (locator Rx) monitors RSS of RVC's in neighboring cells Tells Mobile Switching Center about these mobiles and their channels Locator Rx can see if signal to this base station is significantly better than to the host base station MSC monitors RSS from all base stations & decides on handoff 2 nd Generation Cellular w/ digital TDMA (GSM, IS 136) Mobile Assisted HandOffs (MAHO) important advancement The measures the RSS of the FCC s from adjacent base stations & reports back to serving base station if Rx power from new base station > Rx power from serving (current) base station by pre-determined margin for a long enough time period handoff initiated by MSC MSC no longer monitors RSS of all channels reduces computational load considerably enables much more rapid and efficient handoffs imperceptible to user Lecture 3, Page 10 of 20
A mobile may move into a different system controlled by a different MSC Called an intersystem handoff What issues would be involved here? Prioritizing Handoffs Issue: Perceived Grade of Service (GOS) service quality as viewed by users quality in terms of dropped or blocked calls (not voice quality) assign higher priority to handoff vs. new call request a dropped call is more aggravating than an occasional blocked call Guard Channels % of total available cell channels exclusively set aside for handoff requests makes fewer channels available for new call requests a good strategy is dynamic channel allocation (not fixed) adjust number of guard channels as needed by demand so channels are not wasted in cells with low traffic Queuing Handoff Requests use time delay between handoff threshold and minimum useable signal level to place a blocked handoff request in queue a handoff request can "keep trying" during that time period, instead of having a single block/no block decision prioritize requests (based on mobile speed) and handoff as needed calls will still be dropped if time period expires Lecture 3, Page 11 of 20
VI. Practical Handoff Considerations Problems occur because of a large range of mobile velocities pedestrian vs. vehicle user Small cell sizes and/or micro-cells larger # handoffs MSC load is heavy when high speed users are passed between very small cells Umbrella Cells Fig. 3.4, pg. 67 use different antenna heights and Tx power levels to provide large and small cell coverage multiple antennas & Tx can be co-located at single location if necessary (saves on obtaining new tower licenses) large cell high speed traffic fewer handoffs small cell low speed traffic example areas: interstate highway passing thru urban center, office park, or nearby shopping mall Cell Dragging low speed user w/ line of sight to base station (very strong signal) strong signal changing slowly user moves into the area of an adjacent cell without handoff Lecture 3, Page 12 of 20
causes interference with adjacent cells and other cells Remember: handoffs help all users, not just the one which is handed off. If this mobile is closer to a reused channel interference for the other user using the same frequency So this mobile needs to hand off anyway, so other users benefit because that mobile stays far away from them. Typical handoff parameters Analog cellular (1 st generation) threshold margin 6 to 12 db total time to complete handoff 8 to 10 sec Digital cellular (2 nd generation) total time to complete handoff 1 to 2 sec lower necessary threshold margin 0 to 6 db enabled by mobile assisted handoff benefits of small handoff time - greater flexibility in handling high/low speed users - queuing handoffs & prioritizing - more time to rescue calls needing urgent handoff - fewer dropped calls GOS increased can make decisions based on a wide range of metrics other than signal strength - such as also measure interference levels - can have a multidimensional algorithm for making decisions Soft vs. Hard Handoffs Hard handoff: different radio channels assigned when moving from cell to cell all analog (AMPS) & digital TDMA systems (IS-136, GSM, etc.) Many spread spectrum users share the same frequency in every cell CDMA IS 95 Since a mobile uses the same frequency in every cell, it can also be assigned the same code for multiple cells when it is near the boundary of multiple cells. The MSC simultaneously monitors reverse link signal at several base stations Lecture 3, Page 13 of 20
VII. Co-Channel Interference MSC dynamically decides which signal is best and then listens to that one - Soft Handoff - passes data from that base station on to the PSTN This choice of best signal can keep changing. Rx does nothing for handoffs except just transmit, MSC does all the work Advantage unique to CDMA systems - As long as there are enough codes available. Interference is the limiting factor in performance of all cellular radio systems What are the sources of interference for a mobile receiver? Interference is in both voice channels control channels Two major types of system-generated interference: 1) Co-Channel Interference (CCI) 2) Adjacent Channel Interference (ACI) Lecture 3, Page 14 of 20
First we look at CCI Frequency Reuse Many cells in a given coverage area use the same set of channel frequencies to increase system capacity (C) Co-channel cells cells that share the same set of frequencies VC & CC traffic in co-channel cells is an interfering source to mobiles in cells Possible Solutions? A) Increase base station Tx power to improve radio signal reception? this will also increase interference from co-channel cells by the same amount no net improvement B) Separate co-channel cells by some minimum distance to provide sufficient isolation from propagation of radio signals? if all cell sizes, transmit powers, and coverage patterns same co-channel interference is of Tx power co-channel interference depends on: R : cell radius D : distance to base station of nearest co-channel cell if D / R then spatial separation relative to cell coverage area improved isolation from co-channel RF energy Q = D / R : co-channel reuse ratio hexagonal cells Q = D / R = Fundamental tradeoff in cellular system design: small Q small cluster size more frequency reuse larger system capacity great But also: small Q small cell separation increased co-channel interference (CCI) reduced voice quality not so great Lecture 3, Page 15 of 20 3 N
Tradeoff: Capacity vs. Voice Quality Table 3.1, pg. 69 Signal to Interference ratio S / I, Eq. (3.5) : S I = i o S i= 1 I i where S : desired signal power I i : interference power from i th co-channel cell i o : # of co-channel interfering cells Approximation with some assumptions Eq. (3.8) : S I n R = i o n ( Di ) i= 1 where D i : distance from i th interferer to mobile Rx power @ mobile (D i ) n n : path loss exponent - free space or line of sight (LOS) (no obstruction) n = 2 - urban cellular n = 2 to 4, signal decays faster with distance away from the base station Lecture 3, Page 16 of 20
- having the same n throughout the coverage area means radio propagation properties are roughly the same everywhere - if base stations have equal Tx power and n is the same throughout coverage area (not always true) then the above equation can be used. Now if we consider only the first layer (or tier) of co-channel cells assume only these provide significant interference And assume interfering base stations are equidistant from the desired base station (all at distance D) then Eq. (3.9) : S I = i o n n ( D ) n n Q ( 3N ) ( 3N ) R = R = = = n ( D) i i i i 0 0 0 0 n 2 What determines acceptable S / I? voice quality subjective testing AMPS S / I 18 db (assumes path loss exponent n = 4) Review: What is this S/I as a ratio (not in db)? Review: What is 10dB + 3dB + 5dB? Lecture 3, Page 17 of 20
Solving (3.9) for N Most reasonable assumption is i o : # of co-channel interfering cells = 6 2 n (( S I ) i ) ( 63.1 6) 2 n N = 0 = = 6.49 7 3 3 N = 7 (very common choice for AMPS) Many assumptions involved in (3.9) : same Tx power hexagonal geometry n same throughout area D i D (all interfering cells are equidistant from the base station receiver) optimistic result in many cases propagation tools are used to calculate S / I when assumptions aren t valid S / I is usually the when a mobile is at the cell edge - low signal power from its own base station & high interference power from other cells - more accurate approximations are necessary in those cases (like eq. (3.10) in the text) Lecture 3, Page 18 of 20
- Fig. 3.5, pg. 71 N =7 and S / I 17 db Eq. (3.5), (3.8), and (3.9) are (S / I) for forward link only, i.e. the cochannel base Tx interfering with desired base station transmission to mobile unit so this considers interference @ the mobile unit What about reverse link co-channel interference? less important because signals from mobile antennas (near the ground) don t propagate as well as those from tall base station antennas obstructions near ground level significantly attenuate mobile energy in direction of base station Rx Lecture 3, Page 19 of 20
also weaker because mobile Tx power is variable base stations regulate transmit power of mobiles to be no larger than necessary Next Lecture: Finish Chapter 3 - Adjacent channel interference, trunking efficiency, sectoring and cell splitting. Lecture 3, Page 20 of 20