7.4 3G Handover Design


7.4.1  Introduction


As the mobile station (MS) gets out of the service cell and goes into another service cell, the link between the former base station and the MS will be substituted for the link between the new base station and the MS.
Handover, in mobility management, is mostly performed by RRC layer protocol in 3G.

1. Protocol state

The UE state can be classified into IDLE state and CONNECTED state. The IDLE state can be classified into UTRAN IDLE, GPRS IDLE and GSM IDLE, three of which has CONNECTED state. The UTRAN CONNECTED state can be classified into four states: URA-PCH, CELL-PCH, CELL-FACH and CELL-DCH. Handover, generally speaking, is that UE is transferred from one communication connection to another one in the CONNECTED state. In this text, handover refers to that UE in CELL-DCH state, unless otherwise specified.

2. Handover Classification

According to the setup and release of radio link between MS and network, handover can be classified into softer handover, soft handover and hard handover.
Soft handover refers to that a MS begins to connect with a new base station, the communication between the mobile station and the former base station is still on. Soft handover is only applying to the CDMA cells with the same frequency.
The difference between soft handover and softer handover is, softer handover is performed in the same NodeB where the maximum gain ratio combination of diversity signals are implemented, while soft handover is performed between two NodeBs, diversity signals selective combination in RNC.
Hard handover consists of intra-frequency handover, inter-frequency handover and inter-system handover. Note that soft handover refers to the intra-frequency handover, but not all intra-frequency handovers are soft handovers. If the target cell and the former cell have the intra-frequency but belong to different RNC, and there is no Iur interface between RNCs, the intra-frequency hard handover will occur. Besides, the internal code handover in the same cell is also hard handover.
Inter-system hard handover consists of the handover between FDD mode and TDD mode, the handover between WCDMA system and GSM system in R99, and the handover between WCDMA and cdma2000 in R2000.
The startup compress mode is required to measure inter-frequency and inter-system for inter-frequency hard handover and inter-system hard handover.
According to the purpose, handover can be classified into edge handover, urgent handover at poor quality, urgent handover at quick level decrease, interference handover, velocity sensitivity handover, charge handover, layered/leveled handover, etc.
The typical process of handover is measurement control → measurement report → handover decision→ handover implementation → new measurement control.
In the phase of measurement control, the network informs UE of the measurement parameters through the sent measurement control message. In the phase of measurement report, the measurement report message is sent to the network by UE. In the phase of handover decision, the network makes a handover decision according to the measurement report. In the phase of handover implementation, UE and the network go along the signalling procedure and make a response to signalling.

7.4.2  Measurement Procedures


In WCDMA system, there are intra-frequency measurements, inter-frequency measurements, inter-RAT measurements, traffic volume measurements and UE-internal measurements.
The same type of measurements may be adopted in different functions or processes of UTRAN, such as cell reselection, handover and power control. The UE shall support a number of measurements running in parallel. The UE shall also support that each measurement is controlled and reported independently of every other measurement.
Cells that the UE is monitoring (e.g. for handover measurements) are grouped in the UE into three different categories:
1.Cells, which belong to the active set. User information is sent from all these cells.  The cells in the active set are involved in soft handover or softer handover.
2.Cells, which are not included in the active set, but are monitored according to a neighbour list assigned by the UTRAN belong to the monitored set.
3.Cells detected by the UE, which are neither included in the active set nor in the monitored set belong to the detected set. Reporting of measurements of the detected set is only required for intra-frequency measurements made by UEs in CELL_DCH state.
In IDLE mode, the UE shall perform measurements according to the measurement control information included in System Information Block Type 11 on BCCH. In CELL-FACH, CELL-PCH or URA-PCH state, the UE shall perform measurements according to the measurement control information included in System Information Block Type 12 on BCCH, while in CELL-DCH state, according to the measurement control information transmitted by UTRAN.
The measurement results will pass two smoothness processing. The first processing is on the physical layer, with an aim to filter fast fading and report the measurement results from physical layer to higher layer; The second processing is done before event evaluation, when the higher layer takes weighted average on the measurement results reported from the physical layer, based on the time, to confirm the coefficient of filter.

1. UE Measurement

l   P-CCPCH RSCP
RSCP, Received Signal Code Power, is a measured received power of a code from P-CCPCH in TDD cell. The reference point of RSCP is the antenna connector at UE.
l   SIR
S/N is defined as (RSCP/ISCP) ´ (SF/2).Measuring SIR should be in DPCCH after the combination of wireless link. The reference point of SIR is the antenna connector at UE.
Where:
RSCP, Received Signal Code Power, is a received power of pilot bit in a code.
ISCP, Interference Signal Code Power, is a received signal interference measured in pilot bits. Only non-orthogonality of the interference is concerned in measurement.
SF=Spreading Factor.
l   P-CPICH RSCP
Received Signal Code Power is a code power measured in P-CPICH. The reference point of RSCP is the antenna connector at UE. If transmission diversity is adopted in , the received code power from each antenna should be measured separately, then added, and sequentially be the power of the whole received codes in P-CPICH.
l   UTRA carrier RSSI
RSSI = Received Signal Strength Indicator, a broadband received power within relative channel width. The measurement will be taken at the downlink of UTRAN. The reference point of RSSI is the antenna connector at UE.
l   GSM carrier RSSI
RSSI = Received Signal Strength Indicator, a broadband received power within relative channel width. The measurement will be taken at the BCCH carrier of GSM. The reference point of RSSI is antenna connector at UE.
l   CPICH Ec/No
Ec/No refers to ratio of the received energy of each code to noise power density in a channel. Ec/No has something in common with RSCP/RSSI. The measurement will be taken at basic CPICH. The reference point of Ec/No is the antenna connector at UE. If basic CPICH adopts transmission diversity, the received energy of each code (Ec) from each antenna should be measured separately. The value of adding the received energy of each code on basic CPICH will be Ec.
l   BLER of transmission channel c
It refers to the evaluation of Block Error Rate (BLER) of transmission channel. The evaluation of BLER is based on CRC of each transmission block after the combination of wireless link. Only the transmission channel with CRC requires the evaluation of BLER. In the connection mode, BLER can be measured in any transmission channel. In the idle mode, the BLER on the transmission channel PCH should firstly be measured if BLER needs measuring.
l   UE transmitting power
It refers to the transmitting power of the whole UE at a carrier. The reference point of UE transmitting power is the antenna connector at UE.
l   In UE, in addition to the measurements above mentioned, there are the measurements in the aspects of time and order, which would not be described here.

2. RNC Measurement

l   RSSI
RSSI, Received Signal Strength Indicator, refers to a broadband received power within UTRAN uplink carrier channel bandwidth at the access point of UTRAN. The reference point of RSSI measurement is the antenna connector.
l   SIR
S/N is defined as (RSCP/ISCP)´SF. Its measurement should be taken in DPCCH after the combination of wireless link at Node B. In the compression code, SIR should not be measured at transmitting interval. The reference point of SIR measurement is the antenna connector.
Where:
RSCP, Received Signal Code Power, refers to a received power in a code.
ISCP, Interference Signal Code Power, refers to a received signal interference. Only non-orthogonality of interference is concerned in measurement.
SF refers to the spreading factor used in DPCCH.
l   SIRerror
SIRerror = SIR – SIRtarget_ave, where:
SIR refers to the SIR measured at UTRAN in dB.
SIRtarget_ave = refers to the average value of SIRtarget during a period of time. This period of time is the same as that when counting the average value of SIRerror. The average value of SIRtarget is arithmetic average, SIRtarget_ave in dB.
l   Transmitting carrier power
The transmitting carrier power is the ratio (0…100%) of the whole transmitting power to the maximum transmitting power, where the whole transmitting power [W] is related to the average power [W] of a carrier at an access point of UTRAN. The maximum transmitting power is related to the average transmitting power [W] of a carrier at an access point of UTRAN under the condition that each cell is on the maximum power .The measurement may be taken at any transmitting carrier from the access point of UTRAN .The reference point of measuring transmitting carrier frequency is the antenna connector. The carrier frequency of each branch should be measured in case of transmitting diversity.
l   transmitting code power
Transmitting code power goes under the condition of given carrier, given scrambling and channel code. You can take measurements at DPCCH of any specific wireless link from the access point of UTRAN, to show the pilot bit power at DPCCH. All time slots should be involved in the measurement of transmitting power in the compression mode. For example, the time slot of transmitting interval should be involved. The reference point of measuring transmit code power is the antenna connector. The transmitting code power [W] of each branch should be measured and added in case of transmitting diversity.
l   BER of transmission channel
BER of transmission channel is to evaluate the mean bit error rate of DPDCH data after the combination of wireless link. The BER of transmission channel (TrCH) is as a result of measuring the puncture bits of channel coding input terminal at Node B. The evaluation of transmission channel BER may be reported at the end of each TTI at TrCH. The reported transmission channel BER should be a BER evaluation at the latest TTI of the current TrCH. Only the BER of transmission channel through channel coding are needed to be reported.
l   BER of physical channel
BER of physical channel is to evaluate the mean bit error rate of DPCCH data after the combination of wireless link at Node B. BER of physical channel may be reported at the end of each TTI of all sent transmission channels. The reported BER of physical channel should be a mean BER evaluation at the latest TTI of each transmission channel.
l   Other measurements: round trip time, transmission time delay, leading accesses, etc.

7.4.3  Co-frequency Handover


The WCDMA handover algorithm is briefed as follows. WCDMA soft handover algorithm adopts Ec/Io of pilot CPICH to be a measurement value of handover, which is reported to RNC through three-layer signaling.
The following terms are used for describing handover:
Active set: The cells in active set are connected to MS in the form of soft handover.
Neighboring set/Monitoring set: They both list the cells which are measured by MS continually, but the pilot Ec/Io in these cells are not mature enough to enter active set.


If during the period of ΔT, pilot _Ec/Io> optimum pilot _Ec/Io reports range hysteresis event 1A, and active set is not full, the cell enters active set, which is called event 1A or wireless link addition.
If during the period of ΔT, pilot _Ec/Io> optimum pilot _Ec/Io reports range hysteresis event 1B, the cell will be deleted from active set, which is called event 1B or wireless link deletion.
If during the period of ΔT, active set is full, optimum alternate pilot _Ec/Io> former worst pilot _Ec/Io + _Ec/Io + hysteresis event 1C, the strongest alternate cell (the strongest cell in monitoring set) will substitute for the weakest cell in active set. Such event is called event 1C or the combination of wireless link addition and deletion. Supposing there are at most two cells in active set as shown in Figure 5-20.
Where:
Reporting range indicates a threshold value of soft handover;
Hysteresis event 1A indicates adding hysteresis;
Hysteresis event 1B indicates removing hysteresis;
Hysteresis event 1C indicates replacing hysteresis;
ΔT indicates trigger time;
Optimum pilot _Ec/Io indicates the highest value of cell measurement in active set;
Former worst pilot _Ec/Io indicates the highest value of cell measurement in active set;
Optimum alternate pilot _Ec/Io indicates the highest value of cell measurement in monitor set;
Pilot _Ec/Io is a measured and filtered value.

7.4.4  Handover between WCDMA System and GSM System


The WCDMA criteria and GSM criteria support the bidirectional handover between WCDMA and GSM. As a result of coverage and load balancing, such handovers are used. In the early WCDMA configuration, it is necessary to hand off to GSM system for continuous coverage, and the handover from GSM to WCDMA can be used to reduce the load in GSM cell. Due to load, the bidirectional handover is significant as the service of WCDMA network grows. The inter-system handover is triggered by source RNC/BSC. From the point view of received system, the inter-system handover is similar to the inter-RNC handover or the inter-BSC handover.

1. Compression mode

If WCDMA adopts a way of continuous sending and receiving, but no WCDMA signal gap is generated, MS can not take inter-system measurement through one receiver. In this regard, the compression mode is significant for the inter-frequency measurements and the inter-system measurements.
The introduction of compression mode is to take alien frequency measurement or alien-system measurement at FDD. The reason for that is a set of transceiver can only work on a group of transceiver frequency at the same time. If you want to measure the signals with other frequencies, you should power off the transceiver and hand off the frequency to target frequency for measurement. To ensure the normal transmission of downlink signals, the former signals should be transmitted during the transmission time left, which is called downlink compression mode. As the measurement frequency is close to the uplink transmitting frequency, to ensure good measurement, the uplink signal transmit should be stopped at the same time, which is called uplink compression mode.

Fast power control can not be used during the period of compression mode gap, so some interleaved gain will be lost; In this regard, during the compression frame, higher Eb/No is demanded to decrease capacity. The process of typical inter-system handover is as follows:
The inter-system handover trigger is implemented at RNC, for example MS is out of WCDMA coverage range;
RNC commands MS to begin with the inter-system measurements in compression mode;
RNC chooses the target GSM cell based on the MS measurements;
RNC sends a handover command to MS.
The handover from GSM system to WCDMA system sources from the BSC of GSM. Due to discrete transmission and receiving, the compression mode is not required for the measurement value of WCDMA from GSM.

7.4.5  Inter-frequency Handover in WCDMA


Most of UMTS operators have 2 ~ 3 available FDD carrier, where: one frequency is enough to operate, while the other ones will be used to meet the demands of increasing capacity. We can adopt two different ways on how to use the frequencies: For the site with high capacities, several frequencies can be used at the same site, or, different frequencies can be used in macro-cell layer and micro-cell layer. These schemes should be supported for the inter-frequency handover at WCDMA carrier.
The same as the inter-system handover, inter-frequency handover requires the compression mode measurement.

7.4.6  Handover Design


The soft handover design consists of the configuration of the soft handover-related parameters and the control of soft handover rate. As WCDMA adopts soft handover-related threshold, it ensures a relatively stable configuration of the parameters such as the threshold. But the control of the handover rate is identical with IS-95, which is about 30% to 40%, because too much soft handover will not only increase the cost on the radio resources, but also reduce the capacity of the down link when the soft handover is increased to a certain degree.
On the down links, the system interference will be increased along with the increase of the soft handover link. In case that the system interference exceeds the diversity gain of the soft handover, the soft handover will bring no benefit to the system capacity. In this regards, a well-prepared design is demanded before performing the soft handover in WCDMA. We can keep the soft handover rate in a suitable range by providing enough diversity in the up/down link.
Parameters with regard to the network performance:
Reporting Range: It is used to set the events 1a and 1b, namely the parameter R in formulas 1a-1 and 1a-2, 1b-1 and 1b-2. The bigger the R value is, the wider the soft handover area is. That is because the bigger the R value is, the easier it is to access ACTIVE SET.
W, which is used to calculate the cell quality of the active set is the value adopted for different cells. You will use it when you calculate the formulas 1a-1, 1a-2, 1b-1 and1b-2.
Hystersis: The magnetic hysteresis value in the event report. Like in GSM, the purpose of introducing this value is to avoid the Ping-pong effect as possible. If the value is set too big will result in that the handover may occur difficultly, but if it is set too small, the Ping-pong effect may not be avoided.
Reporting deactivation threshold: The maximum number of cells in the active set when the event is effective, is less than the maximum number of cells in the active set by 1. It is actually used to confirm the maximum number of cells in the active set (only for 1A event). If the value is set too big, the system interference may exceed the diversity gain of the soft handover, otherwise. If it is set too small, it may fail to fully use the diversity gain of soft handover.
Reporting activation threshold: The minimum cell number of the active set when the event is effective (only for 1C event).
Time to trigger: Try to avoid the impact of fast fading. If this value is too big, the handover may be delayed while if it is too small, the handover may occur frequently.
Amount of reporting: The maximum amount of reporting after the event report changes to the cycle report. It is often used together with the Reporting interval.
Reporting interval: Reporting cycle after the event report changes to the cycle report. It is used together with the amount of reporting. In using it, we should try to avoid over-adding the signaling flow.

Reporting Cell Status: It is used to indicate the cell composition principle of the measured result, including the maximum number of reporting cells and the attributes of the reporting cell. 

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