Trunking & Grade of Service Trunked radio system  radio system where a large # of users share a limited pool of channels Channel allocated on demand & returned to channel pool upon call termination Exploit statistical (random) behavior of users so that fixed # of channels can accommodate large # of users Trunking theory used by telephone companies to allocate limited # of voice circuits for large # of telephone lines Efficient use of equipment resources  $$ savings Disadvantage is that some probability exists that mobile user will be denied access to a channel Blocked call : access denied  “blocked call cleared” Delayed call : access delayed & call put into holding queue for specified amount of time (5-15 seconds) ECE 4730: Lecture #6

Trunking & Grade of Service Grade of Service (GOS) Measure of user access to a trunked radio system during busiest hour of the week Specified as probability (Pr) that call is blocked or delayed Busiest hour  typically 4-6 pm on Thu or Fri (cellular) Erlang (erl) : unitless measure of traffic intensity Example: 0.5 erl = 1 channel occupied 30 minutes during 1 hour Table 3.3, pg. 76  Trunking theory definitions ECE 4730: Lecture #6

Trunking & Grade of Service ECE 4730: Lecture #6

Trunking & Grade of Service Traffic Intensity (A) Also called “Offered” Traffic Intensity Offered?  not necessarily carried by system (blocked/delayed) Each user Au = l H (erl) System with U users  A = U Au = U l H (erl) Intensity/channel  AC = U Au / C (erl/channel) Maximum carried traffic load = total # available channels = C (erl) ECE 4730: Lecture #6

Trunking & Grade of Service Erlang B formula Blocked Call Cleared (denied)  BCC GOS = Pr [blocked call] = A = total offered traffic C = # channels in trunking pool (e.g. a cell or sector) Program in calculator or use computer to calculate ECE 4730: Lecture #6

Trunking & Grade of Service Erlang C formulas Blocked Call Delayed  BCD  put into holding queue Probability that call is delayed greater than t seconds Probability that call is initially denied access and put into holding queue Program in calculator or use computer to calculate ECE 4730: Lecture #6

Trunking & Grade of Service Graphical form of Erlang Curves Fig. 3.6 & 3.7 - pgs. 81, 82 Family of curves Pr vs. A for many values of C Provide approximate solutions Errors due to “eyeball” interpolation Numerical formulas provide exact values ECE 4730: Lecture #6

Number of Trunked Channels ( C ) Erlang B Graph Number of Trunked Channels ( C ) ECE 4730: Lecture #6

Trunking & Grade of Service Example: How many users can be supported in a cell containing 50 channels for a 5% GOS (BCC) if the average user calls twice/hr with an average call duration of 5 minutes? Fig. 3.6  For GOS = 5% and C = 50 then A = 45 erl Twice/hr  l = 2 Call duration  H = 5 minutes Au = l H = 2 (5 min / 60 min) = 1/6 erl (unitless!) A = U Au  U = A / Au = 45 / (1/6) = 270 users See additional examples 3.4-3.7 in book ECE 4730: Lecture #6

Trunking Efficiency Trunking Efficiency  measure of # users supported by specific configuration of fixed channels Table 3.4, pg. 79  Lets assume 1% GOS 1 group of 20 channels  12 erl or (12/20) · 100 = 60% efficiency 2 groups of 10 channels  2 · 4.46 = 8.92 erl or (8.92/20) · 100 = 44.6% efficiency ***Larger channel group supports [(12/8.92)  1] · 100 = 35% more traffic!!*** Allocation of channel groups can substantially change # users supported by trunked system As trunking pool size  then trunking efficiency  ECE 4730: Lecture #6

Trunking Efficiency ECE 4730: Lecture #6

Improving Cellular System Capacity Cell Sectoring Method that  capacity by reducing CCI Replace omni-directional antennas at base station with several directional antennas 3 sectors  3 @ 120° antennas 6 sectors  6 @ 60° antennas Cell channels broken down into sectored groups CCI reduced b/c only some of neighboring co-channel cells radiate energy in direction of main cell Figs. 3.10 & 3.11, pgs. 90 & 91 Cell splitting (discussed next) keeps D / R unchanged (same CCI) but increases frequency reuse/area ECE 4730: Lecture #6

Cell Sectoring ECE 4730: Lecture #6 3 sectors  3 @ 120° antennas

Cell Sectoring N = 7 cell cluster 6 CCI cells in first tier io = 2 interfering cells ECE 4730: Lecture #6

Cell Sectoring ECE 4730: Lecture #6 How is capacity increased? By reducing CCI the cell system designer can choose smaller cluster size (N ) Smaller N  greater frequency reuse  larger system capacity Much less costly than cell splitting Only requires more antennas @ base station vs. multiple new base stations for cell splitting Primary disadvantage is available channels in a cell subdivided into sectored groups Trunked channel pool   trunking efficiency  *** Overall system capacity increased at the expense of reducing capacity of individual cells (w/sectors) *** Why? Cluster size N is reduced  frequency reuse increases!! ECE 4730: Lecture #6

Cell Sectoring Other Advantages : ECE 4730: Lecture #6 More antenna gain  sector antenna focuses signal energy Forward/reverse link budgets improved More Tx power delivered to coverage area Better building penetration Flexibility in controlling CCI and dropped calls from poor handoff execution Downtilt antennas in certain sectors to reduce CCI in specific cells Uptilt antennas in certain sectors to increase coverage at cell boundary to improve chance of successful handoff ECE 4730: Lecture #6

Cell Sectoring Other Disadvantages : ECE 4730: Lecture #6 Must design network coverage with sectoring decided in advance Can’t effectively (easily) use sectoring to increase capacity after setting cluster size N Can’t be used to gradually expand capacity as traffic  like cell splitting ECE 4730: Lecture #6

Improving Cellular System Capacity Cell Splitting Subdivide congested cell into several smaller cells Must decrease antenna height & Tx power so smaller coverage results and CCI level is held constant Each smaller cell keeps  same # of channels as the larger cell!! Capacity  b/c channel re-use  per unit area Smaller cells  “micro-cells” Fig. 3.8, pg. 87 ECE 4730: Lecture #6

Cell Splitting Base stations placed at cell corner for illustration purposes ECE 4730: Lecture #6

Other Graphical Examples Cell Splitting Other Graphical Examples ECE 4730: Lecture #6

Cell Splitting Advantages : Disadvantages : ECE 4730: Lecture #6 Only needed for cells that reach max. capacity  not all cells Implement when Pr [blocked call] > acceptable GOS System capacity can gradually expand as demand  Disadvantages : # handoffs/unit area  Umbrella cell for high velocity traffic may be needed **More base stations  $$ for real estate, towers, etc.** ECE 4730: Lecture #6