EEE 264-2:Capacity Increase Techniques and Calculations

Increasing capacity of cellular systems Capacity (or number of users) of a cellular system can be enlarged using frequency reuse Capacity can also be improved using cellular layout and antenna design techniques such as: Cell splitting Antenna sectoring

Cell splitting design Cell splitting subdivides a congested cell into smaller cells, each with its own base station. Original large cell with radius R is split into medium cells with radius R/2. Medium cell is further split into small cells with radius R/4.

Cell splitting from radius R to R/2 and R/4 Large cells R/4 Medium cells Small cells

System capacity increase using cell splitting-Example Consider a R - R/2 cell splitting system with R = 1 km. Suppose each base station is allocated 60 channels regardless of cell size. Find the number of channels contained in a 3 x 3 km2 area around (small ) cell “A” for the following cases: Without cell splitting With cell splitting

Cell splitting example with R = 1 km

Solution To cover a 3 x 3 km2 area centered around cell A, we need to cover 1.5 km to right, left, top and bottom Number of large cells in this 3 x 3 km2 area ~ 4 Number of small cells in this 3 x 3 km2 area = x number of large cells = 4 x 4 = 16 small cells With large cells, the number of channels = 4 x 60 = 240 With 16 small cells, the number of channels = 60 x 16 = 960 ( R R/2 2 )

System capacity increase using directional antennas (sectoring) In basic form, cellular antennas are omnidirectional Directional antennas can increase the system capacity relative to that of omnidirectional antennas Sectorization can be done in multiples of 60˚

Antenna sectorization 1 2 1 3 2 1 3 2 6 4 1 3 5 2 3 6 4 5 a. 3 sectors of 120˚ each b. 6 sectors of 60˚ each

Sectoring calculations Signal to interference ratio (S/I) ( S I ) omni = 1 6 qk q 3N , k = path loss exponent 120˚ 2 = 3 For 120 degrees sectoring , N = Number of cells/cluster For 60 degrees sectoring ( S I ) 60˚ = 1 qk = 6 omni †

Example For N = 7, base stations using omnidirectional antennas can just satisfy the 18 db requirement. Determine if the use of 120˚ sectoring and N = 7 would better satisfy the 18 db requirement for path loss exponent k = 4. ( S I )

( ) ( ) Radio Propagation Models Solution q = 3 N = 3 x 7 = 4.6 = qk = 1 2 1 2 = qk = = 220.5  23.4 db 4.6 4 120˚ Since this is greater than 18 db, it will work.

Handoff management MSC is an appropriate device to oversee the handoff operation Transfer of mobile control from current base station BS to new target BS Initiation phase Employs a decision making strategy based on measured received signal.

Handoff management Execution phase Involves the allocation of new channels to mobile, and exchange of control messages. MSC obtains status information for all base stations periodically Intraswitch handoff — between cells controlled by same MSC Interswitch handoff — between cells controlled by different MSCs.

Handoff Strategies -MCHO Mobile controlled handoff (MCHO) Desirable since it does not burden the network. However, it increases complexity of mobile terminal.

Handoff Strategies -NCHO Network controlled handoff (NCHO) BSs or APs (Access points) monitor signal quality from mobile. MSC then chooses the candidate BS or AP and initiates handoff. Mobile plays passive role in process.

Handoff Strategies - MAHO Mobile assisted handoff (MAHO) Employed by GSM system Mobile records signal levels from various BSs using a periodic beacon generated by BSs Mobile relays power levels from different BSs to MSC via current BS MSC makes handoff decision.

Types of handoff - Hard Hard handoff - (break before make) Mobile has radio link with only one BS at anytime Old BS connection is terminated before new BS connection is made.

Types of handoff - Soft Soft handoff (make before break) Mobile has simultaneous radio link with more than one BS at any time New BS connection is made before old BS connection is broken Used by CDMA systems

Types of handoff - Backward/Forward Backward handoff Handoff is predicted and initiated via the existing BS link Loss of power in existing BS link is a problem Forward handoff Handoff is begun via the new BS radio link Delay is a problem

Intraswitch handoff process MSC Fixed Terminal AP_0 AP_1 Link before handoff Link after handoff MS

Signaling sequence for intraswitch handoff MSC (3) SEQ_PKT (8) UP_READY NEW_AP_READY (4) HO_MUST (2) LAST_PKT (7) NO_MORE (6) LAST_UP AP_0 (5) UP_NO_MORE AP_1 FBK NEW_AP_READY (4) HO_MUST (5) READY MS

Example handoff process Mobile is currently located in cell served by AP_0 and is moving toward the cell being served by AP_1. When mobile reaches the cell boundary of AP_0, the MSC initiates and executes handoff algorithm. MSC knows that AP_1 has a channel available to accept the handoff.

Handoff process: Step 1 Send the message MSC directs the mobile to handoff to AP_1 Send the message (1) NEW_AP_READY To the mobile via AP_0, with identity of candidate AP_1 included.

Handoff process: Step 2 AP_0 responds with message (2) LAST_PKT to the MSC Message contains the sequence number of packet sent to the mobile MSC send the sequence number of the following downlink packet to AP_1 using the message (3) SEQ_PKT

Handoff process: Step 3 The message (4) HO_MUST indicates the last downlink packet from MSC to AP_0 When AP_0 receives this message, it flags the termination of connection by sending the message (5) VP_NO_MORE to MSC

Handoff process: Step 4 Mobile switches its operating frequency and sends the message (6) READY to AP_1 This message contains the sequence number of the last packet correctly received by mobile

Handoff process: Step 5 AP_1 starts downlink transmission and buffers all uplink packets from mobile It also sends the message (7) LAST_UP to MSC requesting approval of uplink transmission

Handoff process: Step 6 MSC waits for message (8) NO_MORE from AP_0 Than MSC switches uplink connection from AP_0 to AP_1 and sends the message (9) UP_READY to AP_1