The Cellular Concept: System Design Fundamentals
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Transcript of The Cellular Concept: System Design Fundamentals
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