A.6 Handover decision algorithm in the MSC
3GPP45.008GSM/EDGE Radio subsystem link controlTS
The MSC shall select the cell to which an MS is to be handed over by the following criteria:
‑ Handover for radio criteria shall be handled taking into account the following order of priority:
‑ RXQUAL;
‑ RXLEV;
‑ DISTANCE;
‑ PBGT.
e.g. if there are more handover bids to a cell than there are free traffic channels, then the bids with cause "RXQUAL" shall take highest priority.
‑ In order to avoid overload in the network, for every cell and with reference to each of 16 adjacent cells, it shall be possible to define (by O&M) for each adjacent cell one of at least 8 priority levels. These shall be considered together with the list of candidates and the interference levels in the choice of the new cell. For example, if there are two cells which meet the criteria for handover, then the cell with the highest priority shall be used. This enables umbrella cells, for instance, to be given a lower priority, and only handle calls when no other cell is available.
‑ Channel congestion on the best cell shall cause the choice of the second best cell, if available, and so on. If no cell is found and call queuing is employed in the MSC, then the MSC shall queue the request on the best cell for a period equal to H_INTERVAL (H_INTERVAL < T_Hand_RQD shall be set by O&M). This handover shall have priority over the queue handling new calls.
Table A.1: Parameters and thresholds stored for handover purposes
|
L_RXLEV_UL_P |
RXLEV threshold on the uplink for power increase. Typical range ‑ 103 to ‑ 73 dBm. |
|
U_RXLEV_UL_P |
RXLEV threshold on the uplink for power reduction. |
|
L_RXQUAL_UL_P |
RXQUAL threshold on the uplink for power increase. |
|
U_RXQUAL_UL_P |
RXQUAL threshold on the uplink for power reduction. |
|
L_RXLEV_DL_P |
RXLEV threshold on the downlink for power increase. Typical range ‑ 103 to ‑ 73 dBm. |
|
U_RXLEV_DL_P |
RXLEV threshold on the downlink for power reduction. |
|
L_RXQUAL_DL_P |
RXQUAL threshold on the downlink for power increase. |
|
U_RXQUAL_DL_P |
RXQUAL threshold on the downlink for power reduction. |
|
L_RXLEV_UL_H |
RXLEV threshold on the uplink for handover process to commence. Typical range ‑ 103 to ‑ 73 dBm. |
|
L_RXQUAL_UL_H |
RXQUAL threshold on the uplink for handover process to commence. |
|
L_RXLEV_DL_H |
RXLEV threshold on the downlink for handover process to commence. Typical range ‑ 103 to ‑ 73 dBm. |
|
L_RXQUAL_DL_H |
RXQUAL threshold on the downlink for handover process to commence. |
|
MS_RANGE_MAX |
Threshold for the maximum permitted distance between MS and current BTS. Range (2, 35 Km); step size 1.0 Km. |
|
RXLEV_UL_IH |
RXLEV threshold on uplink for intracell (interference) handover. Typical range ‑ 85 to ‑ 40 dBm. |
|
RXLEV_DL_IH |
RXLEV threshold on downlink for intracell (interference) handover; typical range ‑ 85 to ‑ 40 dBm. |
|
RXLEV_MIN(n) |
Minimum RXLEV required for an MS to be allowed to handover to cell "n". |
|
RXLEV_MIN_DEF |
Default value of RXLEV_MIN, used to evaluate handover to undefined adjacent cells. |
|
HO_MARGIN(n) |
A parameter used in order to prevent repetitive handover between adjacent cells. It may be also used as a threshold in the power budget process. Range (0, 24 dB); step size 1 dB. |
|
HO_MARGIN_DEF |
Default value of HO_MARGIN, used to evaluate handover to undefined adjacent cells. |
|
N_CELL list |
List of allowable adjacent cells for handover. Range (0, 32). |
|
MS_TXPWR_MAX |
Maximum TX power a MS may use in the serving cell. Range (5, 39 dBm) for GSM and (0,36 dBm) for DCS 1 800; step size 2 dB. |
|
MS_TXPWR_MAX(n) |
Maximum TX power a MS may use in the adjacent cell "n". Range (5, 39 dBm) for GSM and (0,36 dBm) for DCS 1 800; step size 2 dB. |
|
MS_TXPWR_MAX_DEF |
Default value of MS_TXPWR_MAX, used to evaluate handover to undefined adjacent cells. |
|
BS_TXPWR_MAX |
Maximum TX power used by the BTS. |
|
O .X5 |
Boundary limits of five interference bands for the unassigned time slots. Typical range ‑115 to ‑85 dBm. (See 3GPP TS 48.008). |
|
Hreqave |
RXLEV, RXQUAL and MS_BTS Distance averaging periods defined in terms of number of SACCH multiframes. Range (1, 31); step size 1. |
|
Hreqt |
The number of averaged results that can be sent in a "handover required message" from BSS to MSC. Range (1, 31); step size 1. |
|
Intave |
Interference averaging period defined in terms of the number of SACCH multiframes. Range (1, 31); step size 1. |
|
N1..N8,P1..P8 |
The number of samples used in the threshold comparison processes. Range (1, 31); step size 1. |
|
P_Con_INTERVAL |
Minimum interval between changes in the RF power level. Range (0, 30 s) step size 0.96 s. |
|
T_Hand_RQD |
Minimum interval between handover required messages related to the same connection. Range (0, 30 s); step size 0.96 s. |
|
Pow_Incr_Step_Size |
Range 2, 4 or 6 dB. |
|
Pow_Red_Step_Size |
Range 2 or 4 dB. |
|
Number of Ranges (NR) |
Number of ranges in BA_RANGE indicating the number of ranges of ARFCNs containing BCCH carriers for use as stored BCCH information. |
|
RANGEi_LOWER |
Lowest ARFCN in the ith range of carriers containing BCCH carriers for use as stored BCCH information. |
|
RANGEi_HIGHER |
Highest ARFCN in the ith range of carriers containing BCCH carriers for use as stored BCCH information. |
All thresholds shall be able to take any value within the range of the parameter to which they apply. Typical operating ranges are given for some thresholds.
Annex B (informative):
Power Control Procedures
Power control is important for spectrum efficiency as well as for power consumption in a cellular system. For good spectrum efficiency quality based power control is required. Power control for a packet oriented connection is more complicated than for a circuit switched connection, since there is no continuos two-way connection.
The power control formula for the MS is specified in subclause 10.2.1 (formula 1):
P = 0 – CH – (C + 48) (all power calculations in dB)
This is a flexible tool that can be used for different power control algorithms. (Note that the constants 0 and 48 are included only for optimising the coding of CH).For the BTS, there is no need to specify any algorithm, but a similar formula can be used. The following are examples of possible algorithms for uplink power control:
– Open loop control.
With this method the output power is based on the received signal level assuming the same path loss in uplink and downlink. This is useful in the beginning of a packet transmission.
– Closed loop control.
With this method the output power is commanded by the network based on received signal level measurements made in the BTS in a similar way as for a circuit switched connection.
– Quality based control.
This method can be used in combination with any of the two methods above.