The figure shows an example of incoming call connection setup at the air interface how the various logical channels are used in principle. The MS is called via the PCH and requests a signaling channel on the RACH. It gets SDCCH through an IMMEDIATE ASSIGNMENT message on the AGCH. Then follow authentication, start of ciphering, and start of setup over the SDCCH. An ASSIGNMENT COMMAND message gives the traffic channel to the MS, which acknowledges its receipt on the FACCH of the traffic channel. The FACCH is also used to continue the connection setup.
GSM handoffs Intra-BSS: if old and new BTSs are attached to same base station MSC is not involved Intra-MSC: if old and new BTSs are attached to different base stations but within same MSC Inter-MSC: if MSCs are changed
GSM Intra-MSC handoff1. Mobile station monitors signal quality and determines handoff is required, sends signal measurements to serving BSS2. Serving BSS sends handoff request to MSC with ranked list of qualified target BSSs3. MSC determines that best candidate BSS is under its control4. MSC reserves a trunk to target BSS5. Target BSS selects and reserves radio channels for new connection, sends Ack to MSC6. MSC notifies serving BSS to begin handoff, including new radio channel assignment
GSM Intra-MSC handoff7. Serving BSS forwards new radio channel assignment to mobile station8. Mobile station retunes to new radio channel, notifies target BSS on new channel9. Target BSS notifies MSC that handoff is detected10. Target BSS and mobile station exchange messages to synchronize transmission in proper timeslot11. MSC switches voice connection to target BSS, which responds when handoff is complete12. MSC notifies serving BSS to release old radio traffic channel
GSM Inter-MSC handoff1. MS sends signal measurements to serving BSS2. Serving BSS sends handoff request to MSC3. Serving MSC determines that best candidate BSS is under control of a target MSC and calls target MSC4. Target MSC notifies its VLR to assign a TMSI5. Target VLR returns TMSI6. Target MSC reserves a trunk to target BSS7. Target BSS selects and reserves radio channels for new connection, sends Ack to target MSC8. Target MSC notifies serving MSC that it is ready for handoff
GSM Inter-MSC handoff9. Serving MSC notifies serving BSS to begin handoff, including new radio channel assignment10. Serving BSS forwards new radio channel assignment to mobile station11. Mobile station retunes to new radio channel, notifies target BSS on new channel12. Target BSS notifies target MSC that handoff is detected13. Target BSS and mobile station synchronize timeslot14. Voice connection is switched to target BSS, which responds when handoff is complete15. Target MSC notifies serving MSC16. Old network resources are released
Geometric Representation Cells are commonly represented by hexagons. Why hexagon? How about circle? How about square, or triangle?
Channel Reuse The total number of channels are divided into K groups. K is called reuse factor or cluster size. Each cell is assigned one of the groups. The same group can be reused by two different cells provided that they are sufficiently far apart.
Coordinate System Use (i,j) to denote a particular cell. Example: Cell A is represented by (2,1).
Distance Formula D = 3(i + ij + j ) R 2 2 = 3K R R where D K = i + ij + j 2 2 Reuse factor
Air Interface: MS to BTS Uplink/Downlink of 25MHz 890 -915 MHz for Up link 935 - 960 MHz for Down link Combination of frequency division and time division multiplexing FDMA 124 channels of 200 kHz TDMA Burst Modulation used Gaussian Minimum Shift Keying (GMSK)
Number of channels in GSM Freq. Carrier: 200 kHz TDMA: 8 time slots per freq carrier No. of carriers = 25 MHz / 200 kHz = 125 Max no. of user channels = 125 * 8 = 1000 Considering guard bands = 124 * 8 = 992 channels
Frequency Reusage If a mobile company got the bandwidth of 12MHz with the guardband of 5 KHz and the seperation band of 10 KHz How many channels will be available for communication in the above scenario ? How many channels will be available if we have a cluster of K = 19 in a BSC of 380 cells? How many channels will be available if we reuse frequency at level one?
Outgoing call setup User keys in the number and presses send Mobile transmits request on uplink signaling channel If network can process the call, BS sends a channel allocation message Network proceeds to setup the connection Network activity: MSC determines current location of target mobile using HLR, VLR and by communicating with other MSCs Source MSC initiates a call setup message to MSC covering target area
Incoming call setup Target MSC initiates a paging message BSs forward the paging message on downlink channel in coverage area If mobile is on (monitoring the signaling channel), it responds to BS BS sends a channel allocation message and informs MSC Network activity: Network completes the two halves of the connection
LAI Location Area Identifier of an LA of a PLMN (A Public Land Mobile Network is a generic name for all mobile wireless networks that use land based radio transmitters or base stations.) Based on international ISDN numbering plan Country Code (CC): 2,3+ decimal digits Mobile Network Code (MNC): 2,3 decimal digits Location Area Code (LAC) : maximum 5 decimal digits, or maximum twice 8 bits, coded in hexadecimal Is broadcast regularly by the BTS on broadcast channel
Cell Identifier (CI) Within LA, individual cells are uniquely identified with Cell Identifier (CI). It is maximum 2*8 bits LAI + CI = Global Cell Identity
Cellular Concept Base stations (BS): implement space division multiplex Each BS covers a certain transmission area (cell) Each BS is allocated a portion of the total number of channels available Cluster: group of nearby BSs that together use all available channels Mobile stations communicate only via the base station, using FDMA, TDMA, CDMA…