EE552/452 Wireless Communications (and Networks) Spring 2007 PDF

EE 552/452, Spring, 2007 Wireless Communications (and Networks) Zhu Han Department of Electrical and Computer Engineering Class 5 Jan. 30th, 2007  Outline  Trunking and Grade of Service...

EE 552/452, Spring, 2007 Wireless Communications (and Networks) Zhu Han Department of Electrical and Computer Engineering Class 5 Jan. 30th, 2007  Outline  Trunking and Grade of Service  Technique to improve the capacity  Call procedure  Introduction to radio wave propagation EE 552/452 Spring 2007  Trunking  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 EE 552/452 Spring 2007 Terms of Trunking Theory EE 552/452 Spring 2007  Trunking Theory  Each user generates a traffic intensity of Au Erlangs. – Au=H, where H is the average duration of a call and  is the average number of call requests per unit time for each user – A=UAu, where U number and A is the total offered traffic intensity – Ac=UAu/C, where C is the number of channels – Offered traffic >= the traffic carried by the trunked system  First type of trunked system – Blocked calls cleared: no queuing for call requests, no setup time – M/M/m/m: memory-less, Poisson arrival, exponential service, finite channels. AC – Erland B formula Pr(blocking )  C! C k GOS A  k 0 K! EE 552/452 Spring 2007  Erlang B Example EE 552/452 Spring 2007  Erlang B EE 552/452 Spring 2007  Blocked Calls Delayed  Blocking calls are delayed until channels are available, queuing  Erlang C AC Pr( delay  0)  C 1 Ak A  C!(1  ) C A C k 0 k!  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) EE 552/452 Spring 2007  Erlang C EE 552/452 Spring 2007  Examples  Example 3.4  Example 3.5  Example 3.6  Example 3.7 EE 552/452 Spring 2007  Approaches to Increasing Capacity  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 EE 552/452 Spring 2007  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.  Cell Splitting from radius R to R/2  Table 3.4 example EE 552/452 Spring 2007  Cell Splitting Example  Power reduction is 16 times and 12 dB for half cell radius for propagation factor is 4.  antenna downtilting  Umbrella cell  Example 3.8 EE 552/452 Spring 2007  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 EE 552/452 Spring 2007  Sectoring Examples  Only two cochannel cell  S/I improvement 7.2dB  Capacity 12/7  First type handoff  Trunking efficiency low  Urban area not good  Example 3.9 EE 552/452 Spring 2007  Repeater  Extend coverage range  Directional antenna or distributed antenna systems EE 552/452 Spring 2007  Micro Cell Zone  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 EE 552/452 Spring 2007  Micro Cell Zone 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. EE 552/452 Spring 2007  Example  2.33 times capacity gain EE 552/452 Spring 2007  Homework  Some questions are tough  Due 2/8. Do not work until the last minute, you cannot finish. – 3.1 – 3.5 – 3.7 – 3.9 – 3.10 – 3.15 – 3.27 – 3.28 – 3.29 EE 552/452 Spring 2007  Questions? EE 552/452 Spring 2007