1. The Cellular Concept: System Design Fundamentals What if there is no power degradation for a transmitted signal? Transmission range is limited: the possibility of cellular concept Example: daily conversation by lowering voice, a room could accommodate more simultaneous conversations

2. Frequency Reuse If the S channels are divided among N cells and each cell has k channels, the total number of available radio channels, S=kN If a cluster is replicated M times, the total number of duplex channels, C=MkN=MS, where the factor N is called the cluster size The frequency reuse factor is given by 1/N The geometry of hexagon of the nearest co-channel neighbors, N=i 2 + ij + j 2 - move I cells along any chain of hexagon - turn 60 degrees counter-clockwise and move j cells

3. Channel Assignment Strategies Fixed:  Each cell is allocated a predetermined set of voice channels  Channel borrowing schemes supervised by MSC Dynamic:  Channels are not allocated to different cells permanently, each call requests channels from MSC  Require the MSC to collect real-time data on  Channel occupancy  Traffic distribution  RSSI: Radio Signal Strength Indications of all channels  Mobile speed and direction

4. Handoff Handover (European usage) Definition: a process of transfer one base station or a channel to another Necessary: when a mobile moves from a cell to another, power from serving base station in the old cell may become weak, the base station in the new cell has stronger power in serving the call  Identifying the new serving base station  Voice and control signaling at the new BS

5. Handoff Decide when to hand off: too early may lead to too many handoffs, too late lead to call dropping Measurement of received signals: may use the different between the received signal power and the minimum required signal power Handoff area: the boundary area between cells, where handoff may be necessary Moving speed: useful in handoff decision Cell residence time (dwell time): the time spent by a mobile in a cell, useful for GOS design

6. Handoff Strategies Mobile-initiated handoff strategies  Mobile makes a handoff decision based on its power measurement Network-initiated  BS monitors the signal power on the reverse voice channels  Locator receiver is controlled by MSC and monitors the signal strength of mobiles in neighboring cells  Handoff decision is made by MSC Combined handoff schemes  Mobile assist handoff  Inter-system handoff

7. Call Admission Control (CAC) and Handoff Prioritization Decide whether a new call is accepted and how handoff calls handled Guard channel scheme: a number of channels set aside for handoff calls  New calls are accepted only when the number of busy channels is less than a threshold Queueing priority schemes  Queueing handoff requests while blocking new calls Objective: minimizes the call dropping while keeps the call blocking under control

8. Handoff Strategies Handoff types  Hard handoff: a call served by one BS at any time  Soft handoff; can be simultaneously served by multiple BSs Hierarchical handoff strategy  Microcell and macrocell concept: PCS cell as microcell while AMPS (high tower BS) as macrocell, satellite as macrocell etc.  Slower mobile is served by microcell while fast mobile is served by macrocell-reduce handoff rate  Handoff prediction may be useful

9. Interference Adjacent channel interference  Out-of band user interference  Receiver imperfection Co-channel interference  Frequency reuse leads to co-channel interference  CDMA uses the same frequency band  Downlink interference is more serious problem  Major bottleneck in increasing system capacity  May lead to dropped calls  Co-channel cells: the ones using the same channel

10. System Capacity Cellular system is interference-limited: increasing one ’ s transmitting power may increase interference to others System capacity: maximum total number of customers can be supported in the whole system Limitation on system capacity:  Interference  Minimum SIR: a minimum required SIR for reasonable voice conversation

11. System Capacity Simplified analysis Assume the same cell size, same transmitting power from each BS R: cell radius, D: frequency reuse distance (the distance between the centers of co- channel cells) Co-channel reuse ratio,

12. System Capacity Let i 0 denote the number of co-channel interference cell

13. System Capacity Small value of Q provides larger capacity Larger value of Q improves transmission quality (less interference)

14. Power Control In TDMA, co-channel interference is controlled by power control  MS power is also controlled for ongoing calls  BS interference seems to be more severe In CDMA, MS transmission powers are controlled: all mobile use the same channel  Need to control the interference  Need to control the near-far effect: nobody should be power-dominating

15. Trunking Trunking: a concept from POTS, a kind of multiplexing or resource sharing, a method allowing a large number of users to share a relatively small number of channels In cellular systems, channels at BS are shared by any user in the cell on a per call basis: a user is granted a channel (if available) upon request, after the call termination, the channel will be returned to the channel pool at BS Will use the statistical behavior of mobile users Queueing may be used for requests

16. Grade of Service (GoS) A quality of service (QoS) parameter: for voice calls only Trunking theory (and Queueing theory) GoS: a measure of the ability of user to access a trunked system during the busiest hour. It is typically given as the blocking probability or the probability of a call experiencing a delay greater than a certain tolerable queueing time Trunking efficiency: a measure of number of users which can be offered a particular GoS with a particular configuration of fixed channels, which can be found by the ratio of total traffic supported with the GoS to total number of channels

17. Grade of Service (GoS) Blocking probability (blocked-call-clear policy)

18. Grade of Service (GoS) GoS with queueing policy

19. Capacity Improvements Cell-splitting: raising your voice does not help, but lowering your voice does Sectoring: directing your voice also helps (use your hand when you talk to your neighbors)

20. Cell Splitting Cell splitting: a process of subdividing a congested area into smaller cells, each with its own BS of lower antenna and lower transmitting power  microcells PCS cells can be regarded as the consequence of AMPS cell splitting  MS shrinks in size  Less interference  Lower power consumption Rescaling the system: decreasing R and keeping the Q unchanged

21. Sectoring Keeping cell radius R unchanged and decreasing frequency reuse factor Q=D/R or reducing the number of interfering cells (co-channel cells) Using directional antenna! The number of interfering BS will decrease: 120 degree sectoring reduces from 6 to 2 SIR can be increased significantly Disadvantage:  Handoff rate increase: sector to sector  The number of antenna increases  Trunking efficiency decreases Adaptive sectoring is possible: adapt to change of traffic