Two-Cell Test System

Last updated: April 28, 2013

This section is only applicable to the lab application, unless otherwise stated.

Description

The range of transmission of a battery-powered mobile device is limited. Hence for good coverage, it is necessary for the mobile to efficiently move from one cell to another as it roams around the network. The mobile must transition to new cells as users cross cell boundaries and RF conditions in the current cell degrade. Some cell transitions can be performed autonomously by the mobile, as a result of the measurements it has made on its serving cell and the surrounding cells. Cell transitions may also be ordered by the network based on measurement reports from the mobile (note that in the two-cell test system, you must order the network-initiated cell transitions; it is not possible for the test sets to automatically order the transitions based on measurement reports from the mobile).

As a mobile moves around the network, it constantly updates the network by sending measurement reports regarding the serving cell as well as any neighbor cells specified in the serving cell's neighbor cell list. When the mobile is idle (not sending data), it can easily measure the RF conditions of the current cell and surrounding neighbor cells. While transmitting data, a GSM/GPRS/EGPRS mobile must make measurements in free timeslots, a W-CDMA/HSPA mobile must use compressed mode to make measurements on surrounding cells, or simply perform "blind" handovers (handover as directed by the network, without first measuring the target cell).

The two-cell test system provides a way to test a mobile's cell transition and cell selection procedures between cells of the same or different Radio Access Technologies (RATs). Cell selection and transition procedures are documented in 3GPP TS 51.010 (GSM/GPRS/EGPRS), and 3GPP TS 34.121 section 8 and 34.123 (W-CDMA/HSPA). GSM/GPRS/EGPRS mobile measurements for cell reselection, the criteria for cell selection and reselection, and cell reselection algorithms that determine when to initiate the cell transition process are defined in 3GPP TS 05.08. W-CDMA/HSPA cell reselection information is available in 3GPP TS 25.304.

Overview of Cell Transitions

This section describes various cell transitions that a mobile may perform, either autonomously (without being ordered by the network) or by network initiation. Cell transitions may occur while the mobile is in idle mode or actively transferring data.

Cell selection does not involve transitioning between cells; rather, it is the process by which the mobile selects a serving cell once it has completed its power-up sequence. When the mobile is powered on, it must determine which cell to camp to (register with and/or attach to).

Handover refers to a cell transition that occurs when a circuit-switched (CS) connection is in place (such as CS voice, CS data, or Dual Transfer Mode). Handovers can only be initiated by the network. During a handover, the network sends the mobile a Handover command, which provides information about the destination cell, including the traffic channel configuration (in the two-cell test system, this information is determined by the destination test set's current configuration; the originating test set retrieves this information from the destination test set before ordering the handover).

Handovers Available in the Two-Cell Test System
		Destination cell : GSM/GPRS/EGPRS		Destination cell : W-CDMA/HSPA
	Mobile State	Network-Initiated	Mobile-Initiated	Network-Initiated	Mobile-Initiated
Originating cell : GSM/GPRS/EGPRS	CS Voice	Yes³	N/A¹	Yes³	N/A¹
	CS Data	No		No
	Dual Transfer Mode	Yes		Yes²
Originating cell : W-CDMA/HSPA	CS Voice	Yes³		No
	CS Data	Yes		No
	Simultaneous Services	Yes		No
¹ Handovers can only be initiated by the network, as defined by 3GPP. ² A successful handover from Dual Transfer Mode to W-CDMA/HSPA requires a Packet Data RAB to be selected that supports Simultaneous Services. ³ This function is applicable to the lab application or feature-licensed test application.

Cell reselection is the process of changing the mobile's serving cell (either in idle mode or while actively transmitting data). Cell reselections can be initiated by the mobile or network. When the network initiates a cell reselection, it sends a Packet Cell Change Order (GPRS/EGPRS) or a Cell Change Order (W-CDMA/HSPA), which provides the parameters necessary for the mobile to find and synchronize to the destination cell. If the mobile was actively transferring data at the time of the cell reselection, any subsequent allocation of traffic channel resources to continue the packet data transfer are handled by signaling between the mobile and destination cell, and does not involve the origination cell.

Idle mode cell reselection refers to cell transitions that occur when the mobile is in idle mode.

In mobile-initiated idle mode cell reselection, the mobile decides on the basis of its own measurements to move from one cell to another to achieve improved RF quality. The network is not aware of the reselection until the mobile appears on the destination cell. The alternate cells a mobile may search for and transition to are determined by the serving cell's broadcast list of neighbors.

In network-initiated idle mode cell reselection, the network orders the mobile to transition to a new cell while in idle mode, using a Packet Cell Change Order (PCCO) or a Cell Change Order (CCO).

Idle Mode Cell Reselections Available in the Two-Cell Test System
		Destination cell : GSM/GPRS/EGPRS		Destination cell : W-CDMA/HSPA
	Mobile State	Network-Initiated	Mobile-Initiated	Network-Initiated	Mobile-Initiated
Originating cell : GSM/GPRS/EGPRS	Camped but not attached	N/A¹	Yes	N/A¹	Yes
	Attached	Yes	Yes	Yes	Yes
	PDP active	Yes	Yes	Yes	Yes
Originating cell : W-CDMA/HSPA	Camped but not attached	N/A²	Yes	N/A²	Yes
	Attached	N/A²	Yes	N/A²	Yes
	PDP active (but not actively transferring data)	Yes	N/A³	No	N/A³
¹ In a GSM/GPRS/EGPRS cell, network-initiated cell reselection is only possible when the mobile is attached or PDP active, as defined by 3GPP. ² In a W-CDMA/HSPA cell, for Release 99 mobiles, network-initiated cell reselection is only possible when the mobile is PDP active, as defined by 3GPP. ³ Mobile-initiated cell reselection is not possible from a W-CDMA/HSPA cell when the mobile is PDP active in CELL_DCH, as defined by 3GPP. Note that if you turn off the serving W-CDMA/HSPA test set's cell power, the mobile will find the other test set and establish radio resources with it to continue data transfer on the existing PDP context; however, this is not the same as a mobile-initiated reselection to the other test set.

PS data transfer mode cell reselection refers to cell transitions that occur when the mobile is PDP active and actively transferring PS data (such as GPRS, HSPA, or W-CDMA PS data). Whether the cell reselection is initiated by the mobile or by the network, the mobile is responsible for aborting the current data flow and restarting it upon arrival in the new cell. Any data packets routed to the old cell are lost and it is assumed that the higher layer protocol that was generating packets manages retransmission of any lost data to the mobile once it complete its transition to the new cell. A cell update procedure is performed by the mobile upon arrival to the new cell. The new cell is not responsible for re-establishing the PDP context, but rather the mobile or the network entity it was communicating with (for example, FTP server).

In network-initiated PS data transfer mode cell reselections, the current serving cell sends the mobile a PCCO or CCO to command it to change cells.

PS Data Transfer Mode Cell Reselections Available in the Two-Cell Test System
		Destination cell : GSM/GPRS/EGPRS		Destination cell : W-CDMA/HSPA
	Mobile State	Network-Initiated	Mobile-Initiated	Network-Initiated	Mobile-Initiated
Originating cell : GSM/GPRS/EGPRS	Actively transferring PS Data	Yes	Yes	Yes	Yes
Originating cell : W-CDMA/HSPA	Actively transferring PS Data	Yes	N/A¹	No	N/A¹
¹ Mobile-initiated cell reselection is not possible from a W-CDMA/HSPA cell when the mobile is PDP active in CELL_DCH, as defined by 3GPP. Note that if you turn off the serving W-CDMA/HSPA test set's cell power, the mobile will find the other test set and establish radio resources with it to continue data transfer on the existing PDP context; however, this is not the same as a mobile-initiated reselection to the other test set.

Two-Cell Test System Overview

You can connect two test sets via anExternal Device Connection to test your mobile's cell selection and cell transition behavior. The test sets can be configured to emulate two GSM/GPRS/EGPRS cells, or one GSM/GPRS/EGPRS cell and a W-CDMA/HSPA cell (the appropriate lab application must be installed on each test set). At this time you cannot emulate transitions between two W-CDMA/HSPA cells.

Once the system is configured (see Two-Cell Test System Physical Layout and Requirements ) and the mobile is powered on, the mobile performs the cell selection process by selecting one of the test sets in the system with which to communicate. After the mobile has camped on a test set, the mobile's cell transition procedures are ready to be tested.

To influence mobile-initiated cell transitions, you can change the properties of each cell (such as lowering, or turning offCell Power in one of the test sets, or changingCell Parameters ,Reselection Parameters andAccess Restriction Parameters ). Note however that to cause a network-initiated cell transition, you must manually order the transition using theExecute External 8960 Handover immediate action (CALL:HANDoff:EXTernal[:IMMediate] ). The test sets do not automatically order cell transitions based upon the mobile's measurement reports, you must initiate the transition. However, you can monitor the mobile's measurement reports on the test sets as you change the properties of the two-cell system.

Two-Cell Test System Physical Layout and Requirements

The figure below shows the recommended system layout of the two-cell test system.

The requirements for the physical layout of the two-cell test system are as follows:

Two Test Sets
The two-cell test system requires two test sets which emulate the two cells for the mobile to transition between. The inter-RAT two cell test system scenarios assume that one test set is running a GSM/GPRS/EGPRS application/format while the other is running a W-CDMA/HSPA application/format.
Ethernet (LAN) Connection
The two-cell test system relies on an ethernet connection (External Device Connection ) between the two test sets to allow them to exchange control messages during cell reselections and handovers. Before establishing theExternal Device Connection , you must set the originating test set'sExt 8960 IP Addr to theLAN IP Address 2 (Data+) of the destination test set (unless you've connected the two test sets using a hub and have connections to the test set's LAN 1 PORT and LAN 2 PORT ; in that case you can setExt 8960 IP Addr to the destination test set'sLAN IP Address orLAN IP Address 2 (Data+) .) The destination test set displays the originating test set'sLAN IP Address 2 (Data+) underConnected Device IP Address .

It is recommended that you connect the test sets via an ethernet switch or hub, rather than through a gateway or WAN link, to minimize the chance of data loss due to delayed packets. You can also connect the test sets directly via a cross-over cable, however it is then not possible to capture any protocol message logs from the Wireless Protocol Advisor (WPA), or to communicate with external servers.
RF Interface
The mobile must be able to communicate with both test sets. To accomplish this, it is recommended that you use an RF splitter to connect the mobile to both test sets simultaneously.
GPIB Connection
GPIB connection between both test sets and a controlling PC are necessary if remote control of the instruments in the system is required.
Ethernet Switch or Hub
An Ethernet switch or hub is needed in order for the mobile to connect to external servers as well as for connecting the PC running WPA to the test sets.

Operating Considerations

Important : DUT IP Address must be set to a different value in each test set in the two-cell test system.
The operating mode in both test sets must be set toActive Cell in order to perform cell transitions, as this is the only operating mode that provides signaling to the mobile.
While operating in the GSM/GPRS/EGPRS application/format, the connection type must be set to Auto in both test sets of the two-cell test system in order to maintain IP-based data connections during cell reselection. Any other connection type setting terminates the active data connection during cell reselection and does not restart the data connection once cell reselection is complete.
The DUT IP Address setting is inaccessible if the mobile is connected to the current test set but using an IP address originally assigned by the other test set. Refer to theExternal Device Connection documentation ("DUT IP Address Information") for additional information.
It is recommended that you configure the neighbor cell list in each test set to contain the other test set's information (using theSIB 11 Cell Info List in W-CDMA/HSPA or theCell Lists in GSM/GPRS/EGPRS). This enables the mobile to measure the other test set before transitioning to it (some mobiles will not perform a "blind" handover) and allows you to view the mobile's measurement reports for the test set. Ensure that you have properly configured the mobile measurement reports to match the destination test set's configuration (using theCompressed Mode settings in W-CDMA/HSPA or theMeasurement Reports settings in GSM/GPRS/EGPRS).
Network Control Order in the GSM/GPRS/EGPRS application determines how the mobile should perform cell reselections. IfNetwork Control Order is set toNC0 orNC1 in the test set to which the mobile is currently attached, it may not respond to network-initiated attempts at cell reselection. Also, if the test set to which the mobile is attached hasNetwork Control Order set toNC2 , the mobile may not perform autonomous cell reselection.