Wireless networks - Lecture 14: Fundamentals of cellular networks (Part 4). The main topics covered in this chapter include: trunking and grade of service; improving coverage and capacity; block calls cleared; trunking efficiency; microcell zone concept;...
Wireless Networks Lecture 14 Fundamentals of Cellular Networks (Part IV) Dr Ghalib A Shah Outlines Trunking and Grade of Service ► Measuring Traffic Intensity ► Trunked Systems • Blocked Calls Cleared • Blocked Calls Delayed ► Erlang Charts Improving Coverage and Capacity ► ► ► ► Cell Splitting Sectoring Repeaters for Range Extension Microcell Zone Concept Last lecture review Interference and system capacity ► Co-channel interference and capacity ► Adjacent channel interference and capacity Channel Planning for Wireless System Trunking ► Allows a large number of users to share a small number of channels ► Channel allocated per call basis from a pool of available channels ► Relies on statistical behavior of users so that a fixed number of channels (circuits) may accommodate a large random user community ► Trunking theory is used to determine number of channels for particular area (users) ► Tradeoff between the number of available channels and likelihood of call blocking during peak calling hours Trunking Theory ► Developed by Erlang, Danish Mathematician, how a large population can be accommodated by a limited number of servers, in late 19th century ► Today, used to measure traffic intensity ► Erlang represents the amount of traffic intensity carried by a completely occupied channel • i.e one call-hour per hour or one call-minute per minute • 0.5 Erlang: Radio channel occupied 30 minutes during hour Grade of Service GOS is a benchmark used to define performance of a particular trunked system ► Measure of the ability of a user to access trunked system during the busiest hour • Busy hour is based on the demands in an hour during a week, month or year • Typically occur during rush hours between pm to pm GOS is typically given as likelihood of call blocking or delay experienced greater than certain queue time Traffic intensity Traffic intensity is measured as call request rate multiplied by call holding time User traffic intensity of Au Erlang is (1) Au=λH Where H is average call duration or holding time and λ is average number of call requests For system of U users and unspecified channels, the total offered traffic intensity A is (2) A = UAu In a C channel trunked system, traffic equally distributed, traffic intensity per channel Ac is (3) Ac= UAu/C Note that traffic is not necessarily the carried traffic but offered to the trunked system If offered load increases the system capacity, the carried traffic becomes limited In Erlang, max possible carried traffic is the number of channels C AMPS is designed for a GOS of 2% blocking ► i.e out of 100 calls will be blocked due to channel occupancy There are two types of commonly used trunked systems ► Blocked Calls Cleared ► Blocked Calls Delayed Block Calls Cleared User is given immediate request if a channel is available If no channel available, the requesting user is blocked and free to try later Assume call arrivals as Poisson Distribution the Erlang B formula determines the probability that call is blocked with no queuing, is a measure of GOS for trunked system Erlang B Trunking GOS Capacity of an Erlang B System Number of Channels C Capacity (Erlangs) For GOS = 0.01 = 0.005 = 0.002 = 0.001 0.153 0.105 0.065 0.046 0.869 0.701 0.535 0.439 1.36 1.13 0.900 0.762 10 4.46 3.96 3.43 3.09 20 12.0 11.1 10.1 9.41 24 15.3 14.2 13.0 12.2 40 29.0 27.3 25.7 24.5 70 56.1 53.7 51.0 49.2 100 84.1 80.9 77.4 75.2 10 Erlang C 13 if no channels are available immediately, the call is delayed, probability that call is forced to wait more than t seconds is Average delay D in all calls in queued system is 14 Trunking Efficiency A measure of the number of users which can be offered a particular GOS with particular configuration of channels The way channels are grouped can alter the number of users handled For example, From table ► 10 trunked channels at GOS of 0.01 can support 4.46 Erlang of traffic ► Whereas groups of channels can support 2x1.36=2.72 Erlangs of traffic, 60% lesser 15 Improving Coverage and Capacity As demand increases, number of channels per cell become insufficient Cellular design techniques needed to provide more channels per unit coverage area Various techniques developed to expand the capacity of system ► Cell splitting ► Sectoring ► Micro cell zone concept 16 Cell Splitting Achieve capacity improvement by decreasing R and keeping D/R (cell reuse ratio) unchanged Divide the congested cells into smaller cells ► Smaller cells are called micro cells If radius of cell is cut to half, approximately four cells would be required ► Increased number of cells would increase the number of clusters, which in turn increase the capacity Allows a system to grow by replacing larger cells with smaller cells without upsetting the allocation scheme 17 18 For new cells to be smaller in size, tx power must be reduced By which factor? If n =4 then the received powers equal to each other becomes Power must be reduced by 12 dB in order to maintain S/I requirements 19 Thus low speed and high speed users can simultaneously handled Channels in old cell must be broken down into two groups corresponding to smaller and larger cells At beginning of cell splitting, fewer channels to smaller power groups As demand grows, more channels will be required and thus more micro cells In the end, the whole system will be replaced with micro cells 20 Sectoring Keep cell radius unchanged and decrease D/R Increases SIR so that cluster size may be reduced ► SIR is improved using directional antennas ► Hence increasing frequency reuse without changing transmission power Cell is partitioned into 120o sectors or 60o sectors as shown in Fig 21 22 23 Instead of interference from cells, only sectors interfere thus S/I can be found to be 24.2 dB, where it is 17 dB in worst case presented before This S/I improvements allow designers to decrease cluster size N and hence enhances capacity Drawbacks ► Increased number of handoffs 24 Microcell Zone Concept A cell is divided into zones with a single BS sharing the same radio equipment Zones are connected through coaxial cable, fiber optics or microwave links to the BS Superior to sectoring since antennas are placed at outer edges of the cells and any channel may be assigned to any zone by BS As mobile travels from one zone to other, it retains same channel, BS simply switches the channel to a different zone Co-channel interference is minimized becuase ► Large BS is replaced by several low powered tx ► Improves S/I 25 26 27 ... number of users to share a small number of channels ► Channel allocated per call basis from a pool of available channels ► Relies on statistical behavior of users so that a fixed number of channels... Efficiency A measure of the number of users which can be offered a particular GOS with particular configuration of channels The way channels are grouped can alter the number of users handled ... call-minute per minute • 0.5 Erlang: Radio channel occupied 30 minutes during hour Grade of Service GOS is a benchmark used to define performance of a particular trunked system ► Measure of