Broadcast Information Management | GPRS for Mobile Internet

5.2 Broadcast Information Management

In each cell, the BCCH and PBCCH, if present, continuously broadcast information on the serving cell and neighbor cells ' configuration.

The serving cell and neighbor cell parameters are broadcast within messages called SI messages on BCCH and packet system information (PSI) messages on PBCCH. Based on this information the MS is able to decide whether and how it may gain access to the system via the cell it is camping on.

The first subsection describes how the network schedules the different SI and PSI messages on BCCH and PBCCH, respectively. The next subsection details the types of parameters that can be found by the mobile within the different instances of these messages. Among them, cell reselection and frequency parameters are broadcast. Some details on the acquisition of cell reselection parameters are given, since their monitoring is dependent on whether they are sent on BCCH or PBCCH.

During a TBF establishment, the mobile is allocated frequency channels. It derives its frequency allocation from frequency parameters that are broadcast by the network. The last subsection explains how this is performed.

5.2.1 SI Message Scheduling

All information broadcast on the BCCH or PBCCH is carried in SI messages or PSI messages, respectively. As there are several SI and PSI message types having a different content, it is useful for the MS to know where these types of messages are located in a group of multiframes. A variable TC was introduced in the GSM standard to facilitate the MS operation. Each type of SI or PSI message is associated with a TC value.

For SI message types broadcast on the BCCH, TC is defined by the following formula:

(5.1)

This means that a given SI message type is located on a specific 51-multiframe identified by a 51- multiframe number equal to "FN DIV 51" every eight 51-multiframes (DIV means "divided by"). All SI message types are associated with a fixed TC value. For example, all the SI type 1 messages are sent when TC value is equal to 0. This means that all SI message types are scheduled in the same location within a group of 51-multiframes by any network manufacturer irrespective of operator.

On the PBCCH, the occurrence of PSI1 message type is defined by the following formula:

(5.2)

A PSI1 message is broadcast by the network every PSI1_REPEAT_PERIOD occurrence of the 52-multiframe when TC is equal to 0. Unlike with the SI message type, the other PSI message types are not associated with a defined TC value fixed by the standard. This means there is an acquisition phase for the MS to determine the association between the PSI message type and the TC value when the MS camps on a new cell. The PSI message types other than the PSI1 message are divided into two categories:

High-repetition-rate PSIs. These PSIs are broadcast on the PBCCH occurrence that is not used by the PSI1 in a sequence determined by the network. This sequence is repeated at each TC that is equal to 0 every PSI1_REPEAT_PERIOD of 52-multiframes. The PSI_COUNT_HR parameter broadcast by the network indicates the number of PSIs with high-repetition-rates.
Low-repetition-rate PSIs. These PSIs are broadcast on the PBCCH occurrences that are not used by the PSI1 and high-repetition-rate PSIs in a sequence determined by the network, and are repeated continuously. The sequence of these PSIs is repeated at the beginning of the hyperframe when the FN is equal to 0. The PSI_COUNT_LR parameter broadcast by the network indicates the number of PSIs with low-repetition-rates.

Note that as the PSI1_REPEAT_PERIOD, PSI_COUNT_HR, PSI_COUNT_LR parameters are broadcast in a PSI1 message, the MS needs to first read the PSI1 message to understand the mapping of the PSI message structure. Figure 5.1 gives an example of the PSI messages mapping in a given configuration.

Figure 5.1: Mapping of PSI messages.

5.2.2 MS Acquisition of Broadcast Information

5.2.2.1 Monitoring of BCCH Information

The BCCH is used to broadcast both GSM and GPRS network parameters. These parameters are the frequencies that are used in the cell, the neighbor cell frequencies, the GSM and GPRS logical channel description, and the access control parameters. The mobile uses the broadcast serving cell frequencies to derive its frequency allocation during resource assignment. It uses the neighbor cell frequencies for measurement and cell reselection purposes. The logical channel description indicates how the different logical channels are multiplexed on the time slots. The network broadcasts access control parameters and puts constraints on the access channels in order to avoid congestion.

These parameters are broadcast on the following SI messages:

SI type 1 messages, which contain the serving cell frequency parameters;
SI type 2, SI type 2bis, and SI type 2ter messages, which contain neighbor BCCH frequency list and access control parameters;
SI type 3 messages, which contain control channel descriptions, cell options, and cell selection parameters;
SI type 4 messages, which contain cell selection parameters and CBCH configuration;
SI type 13 messages, which contains GPRS cell options.

SI type 2, 3, and 4 messages are always broadcast. SI type 1, 2bis, 2ter, and 13 messages are optional. Other SI messages exist but are not directly linked to the GPRS service.

Broadcast of the SI type 13 message in the cell indicates the availability of the GPRS service. The position of the SI type 13 message is indicated in either the SI type 3 or SI type 4 message. If PBCCH is not present in the cell, the SI type 13 message gives the GPRS cell parameters such as cell reselection mode, power control parameters, and the AB format to be used on the GPRS channels.

If PBCCH is present in the cell, the SI type 13 message indicates the description of the PBCCH (position, radio channel description, training sequence code, and PR of transmission compared to BCCH). The MS attempts to decode the full BCCH data of the serving cell at least every 30 seconds in order to detect any change in the network configuration. It also decodes the neighbor BCCH data block that contains the parameters affecting the cell reselection for each of the six strongest neighbor cells at least every 5 minutes.

5.2.2.2 Monitoring of PBCCH Information

The PBCCH broadcasts parameters relevant to both GSM and GPRS. When the PBCCH is present in the cell, it will be monitored by MSs that want to get GPRS attached and access to GPRS services. A GPRS mobile that monitors the PBCCH does not have to monitor the BCCH, since the BCCH information is also broadcast on PBCCH.

On PBCCH, the following types of messages are broadcast:

PSI type 1 messages, which contain GPRS cell options, PCCCH description, and PRACH control parameters;
PSI type 2 messages, which contain frequency parameters;
PSI type 3 and PSI type 3bis messages, which contain the necessary cell reselection parameters of the serving cell and neighbor cells, and neighbor cell BCCH frequency information.

Other PSI messages are sent optionally by the network.

The mobile attempts to regularly decode the PSI type 1 messages of the serving cell (at least every 30 seconds). Within the PSI type 1 message, the network indicates whether a change took place within one or more types of broadcast PSI messages with the PBCCH_CHANGE_MARK parameter. The network may indicate which family of PSI messages has changed by using the PSI_CHANGE_FIELD parameter in order to avoid having the mobile rescan the complete PBCCH. Whenever a change in the PBCCH_CHANGE_MARK value is detected , the MS must reread the new PSI messages within the next 10 seconds.

Note that the MS may suspend its data transfer in order to decode the above information. However, in order to avoid data transfer interruption during packet transfer mode, the network can broadcast PSI on the PACCH. The mobile thus avoids leaving the data transfer for the monitoring of the PBCCH.

5.2.2.3 Cell Reselection Parameter Acquisition

For cell-reselection purposes, the mobile must acquire special parameters that are used for the evaluation of the cell-reselection criteria. Some are linked to the serving cell and others dedicated to the neighbor cells.

When there is no PBCCH in the serving cell, these parameters are broadcast by the network in the SI type 3 or SI type 4 messages. The serving cell parameters used for cell reselection are broadcast on the serving BCCH, whereas the neighbor cell parameters are broadcast on the neighbor BCCHs. The mobile must decode the SI3 and SI4 messages on the six strongest (in terms of RXLEV) neighbors' BCCH in order to acquire these parameters.

When the PBCCH is present in the serving cell, the network broadcasts both the neighbor cell and serving cell parameters that are used for cell reselection on this channel. They can be found in the PSI type 3 messages. The network may have to broadcast these parameters in several instances of the PSI3 message depending on the number of neighbor cells. Different encoding mechanisms have been introduced in order to provide a maximum number of parameters with a minimum number of PSI3 message instances. The acquisition time of neighbor cell parameters on PBCCH is much faster, since they are all transmitted on the same channel.

5.2.3 Frequency Parameters

When the network allocates radio resources to the mobile, it provides frequency parameters needed by the mobile to know its frequency channel allocation. The radio channel description used by the mobile consists of:

One frequency in case of a nonhopping radio frequency channel;
A list of frequencies, a MAIO, and a HSN (see Section 1.5.6.3) when frequency hopping is used.

The list of frequencies that is used by the mobile for GPRS transfer is called GPRS mobile allocation. The radio channel description is provided to the mobile during radio resource assignment.

When frequency hopping is used in the cell, the size of the mobile allocation can be very large. In order to avoid sending the list of frequencies directly during the assignment phase, special mechanisms have been defined.

5.2.3.1 Cell Allocation

The operator allocates a set of radio frequency channels to each cell. This set is defined as the cell allocation (CA). The CA defines the list of frequencies that can be assigned to the MS in the cell. In reality it defines the radio frequency channels that can be used during the period immediately following the very first resource assignment. Other radio frequency channels can be assigned to the mobile later, during the assignment phase. The CA is provided in the SI type 1 message or in the PSI type 2 messages when there is a PBCCH in the cell.

In the SI type 1 message, the CA is directly defined using special ARFCN encoding that depends on the number of frequencies provided.

In the PSI type 2 message, the network provides the mobile with reference frequency lists (RFLs). One RFL consists of a list of frequencies that are used in the cell. Up to four RFLs can be stored by the mobile. The CA is then defined in this case as the union of RFLs.

5.2.3.2 GPRS Mobile Allocation

The GPRS mobile allocation (GPRS MA) is defined as a subset of either the CA or RFLs. It is deduced from the CA or RFLs using a bitmap as illustrated in Figure 5.2.

Figure 5.2: Example of mobile allocation definition.

Different encoding mechanisms have been defined in order to provide frequency lists within RFLs in PSI type 2 messages or CA in SI type 1 messages. For GSM-900, the number of frequencies is limited to 124. The encoding mechanism resides in a bitmap (each bit of the bitmap standing for an ARFCN from 1 to 124) indicating the frequencies of the list.

The MS must store up to seven GPRS MAs that can be broadcast in PSI type 2 messages. A GPRS MA within the PSI type 2 message is defined using a bitmap referring to one or more RFLs.

During the assignment, the network can provide the radio channel description with either:

One frequency in case of a nonhopping radio frequency channel;
One GPRS MA number, one HSN, and one MAIO (referred to as "indirect encoding" in the GSM specifications);
A bitmap referring to one or more RFLs (broadcast on PBCCH) of the CA (broadcast on BCCH or PBCCH), one HSN, and one MAIO (referred to as "direct encoding 1" in the GSM specifications);
A list of frequencies, one MAIO, and one HSN (referred to as "direct encoding 2" in the GSM specifications).

Note that the drawback of the direct encodings 1 and 2 is the large amount of space required in the assignment message. If many frequencies are sent, it will be necessary to provide the assignment message in two radio blocks. This increases the delay and complexity of the establishment. The drawback of the indirect encoding is that more space is required in the PSI2 messages to broadcast the RFLs, leading to a lower repetition rate of some other PSI messages.