The Cellular Concept: System Design Fundamentals

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=i2 + ij + j2 - 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 i0 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