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 conceptExample: daily conversation by lowering voice, a room could accommodate more simultaneous conversations

Frequency ReuseIf the S channels are divided among N cells and each cell has k channels, the total number of available radio channels,

S=kNIf 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/NThe 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

Channel Assignment StrategiesFixed: Each cell is allocated a predetermined set of

voice channels Channel borrowing schemes supervised by

MSCDynamic: 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

Handoff

Handover (European usage)Definition: a process of transfer one base station or a channel to anotherNecessary: 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

HandoffDecide when to hand off: too early may lead to too many handoffs, too late lead to call droppingMeasurement of received signals: may use the different between the received signal power and the minimum required signal powerHandoff area: the boundary area between cells, where handoff may be necessaryMoving speed: useful in handoff decisionCell residence time (dwell time): the time spent by a mobile in a cell, useful for GOS design

Handoff StrategiesMobile-initiated handoff strategies Mobile makes a handoff decision based on its power

measurementNetwork-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 MSCCombined handoff schemes Mobile assist handoff Inter-system handoff

Call Admission Control (CAC) and Handoff Prioritization

Decide whether a new call is accepted and how handoff calls handledGuard 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 thresholdQueueing priority schemes Queueing handoff requests while blocking new

callsObjective: minimizes the call dropping while keeps the call blocking under control

Handoff StrategiesHandoff types Hard handoff: a call served by one BS at any time Soft handoff; can be simultaneously served by multiple

BSsHierarchical 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

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

System Capacity Cellular system is interference-limited: increasing one’s transmitting power may increase interference to othersSystem capacity: maximum total number of customers can be supported in the whole systemLimitation on system capacity: Interference Minimum SIR: a minimum required SIR

for reasonable voice conversation

System Capacity Simplified analysisAssume the same cell size, same transmitting power from each BSR: cell radius, D: frequency reuse distance (the distance between the centers of co-channel cells)Co-channel reuse ratio,

System Capacity Let i0 denote the number of co-channel interference cell

System Capacity

Small value of Q provides larger capacityLarger value of Q improves transmission quality (less interference)

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 severeIn 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

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 channelsIn 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 BSWill use the statistical behavior of mobile usersQueueing may be used for requests

Grade of Service (GoS) A quality of service (QoS) parameter: for voice calls onlyTrunking 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 timeTrunking 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

Grade of Service (GoS)

Blocking probability (blocked-call-clear policy)

Grade of Service (GoS)

GoS with queueing policy

Capacity Improvements

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

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 microcellsPCS 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

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 2SIR can be increased significantlyDisadvantage: Handoff rate increase: sector to sector The number of antenna increases Trunking efficiency decreases

Adaptive sectoring is possible: adapt to change of traffic