Wireless Communications Principles and Practice - Chapter 3: The Cellular Concept – System Design Fundamentals

Wireless Communications Principles and Practice Chapter 3: The Cellular Concept – System Design Fundamentals The Cellular Concept • The limited capacity of the first mobile radio- telephone services: - The spectrum usednot much sharing - A lot of bandwidth dedicated to a single call. - Coverage not good The Cellular Concept • The limited capacity of the first mobile radio- telephone services: - interference: impossible to reuse the same frequency The Cellular Concept • Introduction: - Developed by Bell Labs 1960’s-70’s - The cellular concept was a major breakthrough in solving the problem of spectral congestion and user capacity. - Offered very high capacity in a limited spectrum allocation without any major technological changes. - Frequency Reuse: a fixed number of channels to serve an arbitrarily large number of subscribers. The Cellular Concept • Cellular Network Architecture The Cellular Concept • Cellular Network Architecture − Areas divided into cells. − A system approach, no major technological changes. − Each served by base station with lower power transmitter. − Each gets portion of total number of channels. − Neighboring cells assigned different groups of channels, Interference minimized. − Hexagon geometry cell shape. Frequency reuse Frequency reuse • Adjacent cells assigned different frequencies to avoid interference or crosstalk • Objective is to reuse frequency in nearby cells – 10 to 50 frequencies assigned to each cell – Transmission power controlled to limit power at that frequency escaping to adjacent cells – The issue is to determine how many cells must intervene between two cells using the same frequency Frequency reuse • Each cell allocated a group k channels • A cluster has N cells with unique and disjoint channel groups, N typically 4, 7, 12 • Total number of duplex channels S = kN • Cluster repeated M times in a system, Total number of channels that can be used (capacity) • C = MkN = MS Frequency reuse • C = MkN = MS • S is a total of duplex channels available for use. • Smaller cells -> higher M -> higher C + Channel reuse higher capacity + Lower power requirements for mobiles – Additional base stations required – More frequent handoffs Effect of cluster size N Channels unique in same cluster, repeated over clusters Keep cell size same – Large N : weaker interference, but lower capacity – Small N: higher capacity, more interference need to maintain certain S/I level frequency reuse factor: 1/N – Each cell within a cluster assigned 1/N of the total available channels Frequency reuse • In order to connect without gaps between adjacent cells • N = i2 + ij + j2 where i and j are non-negative integers • Example N=7, i = 2, j = 1 – N = 22 + 2.1 + 12 = 4 + 2 + 1 = 7 • i = 1, j = 2 (True) – move i cells along any chain or hexagon. – then turn 60 degrees counterclockwise and move j cells. 19-cell reuse example (N=19) 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.) Frequency reuse • Example If a total of 33 MHz of bandwidth is allocated to a particular FDD cellular telephone system which uses two 25 kHz simplex channels to provide full duplex voice and control channels. Compute the number of channels available per cell if a system uses: a. 4 – cell reuse b. 7 - cell reuse c. 12 - cell reuse Frequency reuse Solution Frequency reuse • Example If 1 MHz of the allocated spectrum is dedicated to control channels, determine an equitable distribution of control channels and voice channels in each cell for each of three systems. Frequency reuse • Solution Frequency reuse • Solution 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 call is blocked, and the subscriber does not receive service. – Variation includes a borrowing strategy: a cell is allowed to borrow channels from a neighboring cell if all its own channels are occupied. – This is supervised by the Mobile Switch Center: Connects cells to wide area network; Manages call setup; Handles mobility Channel Assignment Strategies • Dynamic Channel Assignments – Voice channels are not allocated to different cells permanently. – Each time a call request is made, the serving base station requests a channel from the MSC. – The switch then allocates a channel to the requested call based on a decision algorithm taking into account different factors: frequency re-use of candidate channel and cost factors. – Dynamic channel assignment is more complex (real time), but reduces likelihood of blocking Handoffs – the basics • Reasons for handover – Moving out of range: When a mobile moves into a different cell while a comversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. – Load balancing Handoffs – the basics − Important task in any cellular radio system − Must be performed successfully, infrequently, and imperceptible to users. − Identify a new base station − Channel allocation in new base station − High priority than initiation request (block new calls rather than drop existing calls) Handoffs – the basics ∆=handoff threshold - Minimum acceptable signal to maintain the call ∆ too small: – Insufficient time to complete handoff before call is lost. – More call losses ∆ too large: – Too many handoffs – Burden for MSC Styles of Handoff Network Controlled Handoff (NCHO) – In first generation cellular system, each base station constantly monitors signal strength from mobiles in its cell – Based on the measures, MSC decides if handoff necessary – Mobile plays passive role in process – Burden on MSC Styles of Handoff Mobile Assisted Handoff (MAHO) – Present in second generation systems – Mobile measures received power from surrounding base stations and report to serving base station – Handoff initiated when power received from a neighboring cell exceeds current value by a certain level or for a certain period of time – Faster since measurements made by mobiles, MSC don’t need monitor signal strength Mobile Controlled Handoff Styles of Handoff Hard handoff - (break before make) – FDMA, TDMA – Mobile has radio link with only one BS at anytime – Old BS connection is terminated before new BS connection is made. Soft handoff (make before break) – CDMA systems – Mobile has simultaneous radio link with more than one BS at anytime – New BS connection is made before old BS connection is broken – Mobile unit remains in this state until one base station clearly predominates Prioritizing Handoffs Dropping a call is more annoying than line busy Guard channel concept – Reserve some channels for handoffs – Waste of bandwidth – But can be dynamically predicted Queuing of handoff requests – There is a gap between time for handoff and time to drop. – Better tradeoff between dropping call probability and network traffic. Umbrella Cells Interference and System Capacity Major limiting factor in performance of cellular radio systems Sources of interference: – Other mobiles in same cell – A call in progress in a neighboring cell – Other base stations operating in the same frequency band – Non-cellular system leaking energy into the cellular frequency band Interference and System Capacity Effect of interference: – Voice channel: cross talk – Control channel: missed or blocked calls Two main types: – co-channel interference – adjacent channel interference Co-Channel Interference And System Capacity • Frequency reuse implies that in a given coverage area there are several cells that use the same set of frequencies. - Co – channel cells - Co – channel interference Co-Channel Interference And System Capacity Co-Channel Interference And System Capacity Cells that use the same set of frequencies are called co- channel cells. Interference between the cells is called co-channel interference. Co-channel reuse ratio: – R: radius of cell – D: distance between nearest co-channel cells Small Q : small cluster size N -> large capacity Large Q : good transmission quality tradeoff must be made in actual cellular design Smaller N is greater capacity Some important formulas • I0 is be the number of co – channel interfering cells. • S/I (or SIR): The signal-to-interference • P0 : The power received at a close – in reference point • d0 : A small distance from the transmitting antenna. • Pr : The power received at a distance d Some important formulas Example Solution Co-channel cells for 7-cell reuse Worst Case Interference • With N=7, n = 4, S/I >= 18 dB (2.9) • But: S/I : 17 dB (2.11) Adjacent Channel Interference • Interference resulting from signals where are adjacent in frequency to the desired signal. • Due to imperfect receiver filters that allow nearby frequencies to leak into pass band. • Can be minimized by careful filtering and assignments, and by keeping frequency separation between channel in a given cell as large as possible, the adjacent channel interference may be reduced considerably. The United States AMPS System • Initially: 666 duplex channels • Added: 166 , Now: 832 duplex channels • The extended: 667 – 799 and 990-1023 • The channels to two competing operators. • The 416 channels: - 395 : voice channels. - 21: control channels. The United States AMPS System • Block A: - Voice channels : 1 – 312 667-716 and 991 – 1023 (Extended) - Control channels: 313- 333 Block B: - Voice channels: 355 – 666 717 – 799 (Extended) - Control channels: 334 - 354 Trunking and Grade of Service • Trunking: the channel is allocated on demand and recycle after usage, tradeoff between the number of channels and blocking probability • Grade of service: – Likelihood of a call is blocked or the delay greater than a threshold during the busiest time. • Trunking theory – Erlang, a Danish Mathematician studied how a large population could be accommodated by a limited number of servers. – Erlang capacity: the percentage of line/channel occupied over time Key Definitions for Trunked Radio Trunking Theory • Each user generates a traffic intensity of Au Erlangs. λ : The average number of call requests per unit time. H: The average duration of a call. The total offered traffic intensity A U: Number user in System. The trunked system: C channel Ac= UAu/C Erlang B Trunking GOS – Blocked calls cleared: no queuing for call requests, no setup time. Erlang B Trunking GOS Erlang B Blocked Calls Delayed • Blocking calls are delayed until channels are available, queuing • Erlang C • Probability of delay lager than t Exponential service distributions Pr[delay>t] = Pr[delay>0]Pr[delay>t|delay>0] = Pr[delay>0]exp(-(C-A)t/H) • The average delay D D=Pr[delay>0]H/(C-A) Erlang C Example 1 • C=1, GOS = 0.005 -> A= 0.005 • C=5, GOS = 0.005 -> A= 1.13 • C=10, GOS = 0.005 -> A= 3.96 • C=20, GOS = 0.005 -> A= 11.1 • C=100, GOS = 0.005 -> A= 80.9 Solution Solution Example 2 • C= 19, GOS = 0.02 -> A=12 • C= 57, GOS=0.02 -> A= 45 • C= 100, GOS = 0.02 ->A=88 Solution Solution Solution Solution Improving Capacity in cellular Systems Problems • The demand for wireless service increases • The number of channels assigned to a cell becomes insufficient to support the required number of users Improving Capacity in cellular Systems • Frequency borrowing – Frequencies are taken from adjacent cells by congested cells • Cell splitting – Cells in areas of high usage can be split into smaller cells • Cell sectoring – Cells are divided into a number of wedge-shaped sectors, each with their own set of channels • Microcells – Antennas move to buildings, hills, and lamp posts Cell splitting • Subdivide a congested cell into smaller cells • Each with its own base station, reduction in antenna and transmitter power • More cells -> more clusters -> higher capacity • Achieves capacity improvement by essentially rescaling the system. Cells are split to add channels with no new spectrum usage Cell Splitting from radius R to R/2 Cell splitting Cell splitting Cell splitting Cell splitting Example Cell Splitting increases capacity Solution Solution Solution Sectoring • In basic form, antennas are omnidirectional • Replacing a single omni-directional antenna at base station with several directional antennas, each radiating within a specified sector. • Achieves capacity improvement by essentially rescaling the system. • Less co-channel interference, number of cells in a cluster can be reduced • Larger frequency reuse factor, larger capacity Sectoring Sectoring Sectoring improves S/I Sectoring improves S/I •Only two cochannel cell The penalty of sectoring • The number of handoffs increases. (Fortunately, many modern base stations support sectorization and allow mobiles to be handed off from sector to sector within the same cell without intervention from the MSC, so the handoff problem is often not a major concern.) • Decrease in trunking efficiency, The Zone Cell Concept Superior to sectoring, any base station channel may be assigned to any zone by the base station Same channel No handoff Only the active zone Zone Cell Concept 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. Zone Cell Concept