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PACKET DATA TRANSFER IN A CELLULAR
FIELD OF THE INVENTION
The invention relates to packet data transfer in a cellular radio system.
BACKGROUND OF THE INVENTION
Advanced digital cellular systems typically offer a circuitswitched data channel for data applications. As is wellknown, circuit-switched connections are very heavy to establish. In addition, at least one traffic channel of the system capacity has to be allocated to the circuit-switched connection until the connection is released. For these reasons, circuit-switched data connections are suitable mainly for applications which continuously require a transfer capacity of one transfer channel of the system for relatively long periods of time. Allocation of a circuit-switched channel for the transfer is a typical example of applications suitable for the system.
Many data applications involve a low, randomly occurring transfer demand and a need to keep the (virtual) transfer connection open continuously. A circuit-switched connection is not suitable for such purposes as the resources of the cellular system are reserved unreasonably, and the implementation of a service would therefore become too expensive.
Advanced digital cellular systems also comprise a short 30 message service. A short message may be used in data services for the transfer of small amounts of data. In the GSM system, for instance, sending a short message is similar to a conventional call set-up procedure, i.e., it is a very heavy signalling event. The applicability of short messages in the establishment of economical data connections is therefore limited.
There are prior art packet-switched radio systems implemented for data applications. Such systems require separate network investments for data connections. As the modern cellular systems as such comprise functions useful for data applications, it is advantageous to aim at utilizing these functions in providing economical data connections.
SUMMARY OF THE INVENTION 45
An object of the present invention is to solve the abovedescribed problem by providing a new packet transfer solution which is adapted for a digital cellular system and which offers data connections considerably more economi- 50 cal than the prior art cellular network services.
One concept of the invention is a method for the transfer of user data in packet format in a digital cellular radio system comprising base stations for communication with mobile radio stations, each one of the base stations com- 55 prising at least one organization channel shared by all of the mobile radio stations for sending pages from the base station to the mobile stations, for sending connection requests from the mobile stations to the base station, and for sending channel assignments to the mobile stations so as to establish 60 an actual connection on another channel. The method according to the invention is characterized by transferring the user data from the mobile stations to the base station interleaved with the connection request messages on at least one random-access-type organization channel in messages 65 which will not result in a normal connection establishment; sending a random-access-type packet data transfer request
on the random-access-type organization channel from the mobile station to the base station; detecting the time of sending the random-access-type transfer request on the basis of the request received at the base station; determining a timing advance corresponding to the distance between the base station and the mobile station on the basis of the detected time; sending the determined timing advance information to the mobile station; sending data packets on the random-access-type organization channel from the mobile station to the base station in messages longer than said transfer request and timing the transfer of the messages with respect to the operation of the base station by means of the timing advance.
Another concept of the invention is a mobile station for a digital cellular radio system comprising base stations for communication with mobile radio stations, each one of the base stations comprising at least one organization channel shared by all of the mobile radio stations for sending pages from the base station to the mobile stations, for sending connection requests from the mobile stations to the base station, and for sending channel assignments to the mobile stations so as to establish an actual connection on another channel. The mobile station according to the invention is characterized in that the mobile station comprises means for sending a random-access-type packet data transfer request on a random-access-type organization channel; means for receiving a timing advance corresponding to the distance between the mobile station and the base station from the base station so as to receive such timing advance information; means for transferring user data in packet format to the base station interleaved with connection request messages on at least one random-access-type organization channel in messages which are longer than the transfer request and will not result in a normal connection establishment by timing the transmission of the messages with respect to the operation of the base station by means of the timing advance information.
In the cellular radio system, the fixed network typically sends via the base station a connection establishment request provided with the identity of the mobile station on a special organization channel (also called a control channel) when a connection is to be established with a mobile station. The establishment of an actual speech or data connection is then attempted. Correspondingly, when a mobile station wants to establish a connection, it sends a connection establishment request on one of the organization channels of the cellular radio system, as a result of which the system allocates the mobile station a dedicated call-specific channel on which actual signalling takes place. All mobile stations within a specific base station site (cell) use and listen to the same common organization channel(s). Organization channels are never allocated as circuit-switched connections as only control signalling required to establish a circuit-switched connection takes place on the organization channels.
The transfer of user data in packet format, when practicing the invention, is based on existing connection establishment procedures, which are applied on the organization channel(s) reserved for the connection establishment requests of the cellular radio system. Data transfer thus needs no circuit-switched connection but the data to be transferred is interleaved with the standard messages.
Organization channels from a mobile station towards the system are usually so-called random access channels. A problem with data transfer on a random access channel is that the mobile station is able to send a random access message at any time within a time slot allocated for the organization channel. For this reason the random access
messages have to be very short in order to ensure that they match one time slot. A random access message is thus usually too short even for small amounts of data.
According to the present invention, a solution to the problem is that only a data transfer request sent by the 5 mobile station is of the pure random access type with unknown timing. On the basis of this random access message, the system determines a timing advance depending on the distance between the base station and the mobile station. The timing advance then enables the mobile station to time 10 the transmission of the burst to the beginning of the random access time slot and thereby utilize bursts having the length of the entire random access time slot in the subsequent transfer instead of short random access bursts. In this way, the amount of data transferred on the random access channel 15 will meet the practical requirements.
In the cellular network, the packet transfer according to the invention is transparent to mobile stations which do not use the packet transfer, that is, the cellular network operates otherwise as normal, but the data transfer on the organiza- 20 tion channels provides a new way of data transfer in parallel with the previous functions. Unlike the "circuit-switched" connection, the organization channel is not allocated to the data transfer of a specific mobile station, but several mobile stations may transfer data simultaneously on the channel. 25 Besides, all normal connection establishment signalling also takes place on the same organization channels.
The packet transfer from mobile stations to the base station (up-link transfer) is observed by other mobile stations as an increase in the connection request collision 30 intensity. This in turn increases the need of re-transmission of connection requests. In an overload situation, no connection requests of the mobile stations will pass through and therefore will not be acknowledged. The number of connection request attempts allowed to mobile stations is typically 35 limited in the cellular systems.
To ensure that the cellular system operates without interferences, the up-link packet transfer should not be allowed to load the above critical resource. Therefore, in one 4Q embodiment of the invention, the system is able to control the degree of load of the organization channels by allowing only a predetermined number of mobile stations to transfer data simultaneously. This is achieved by providing a packet transfer protocol by which the mobile station has to request 45 from the fixed network an authorization for a transfer intensity exceeding a preset limit. To facilitate the load control, the authorization is preferably valid for a limited time.
Packet transfer may also take place from the base station 50 to the mobile stations (down-link transfer), whereby the packets are interleaved with the ordinary control messages on the organization channel. This packet transfer is transparent to mobile stations which do not use the packet transfer as they follow only pages addressed to them. From 55 the viewpoint of the system, this means that the organization channel is loaded and there may not be capacity enough for control signalling itself, which results in queuing or rejection of pages. This can be avoided easily by giving priority to the control signalling over the packet transfer in the 60 down-link traffic.
To sum up, the present invention enables ordinary mobile stations within a cellular system to be provided with an additional packet transfer function realized on the organization channel of the cellular system by interleaving the data 65 packets with the ordinary connection requests sent on the organization channel. The packet transfer according to the
invention is non-transparent to the other mobile stations only in that the connection request collision intensity increases with the degree of load of the system. In other words, the same cellular system may support both mobile stations using the packet transfer according to the invention and mobile stations which are not aware of the packet transfer according to the invention. The packet transfer according to the invention can thus be added to a cellular network providing services without interfering with the operation of the existing mobile stations. The packet transfer according to the invention takes place on the organization channel so that the mobile station remains continuously accessible to the conventional cellular network services while it is all the time ready to receive packet data. The invention utilizes the conventional mobility management of the cellular network by sending packets addressed to the mobile stations only within the current location area of the addressed mobile station. The precise routing of the connections makes the use of the system more efficient (increases capacity).
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in greater detail by means of illustrating embodiments with reference to the attached drawings, in which
FIG. 1 illustrates schematically a portion of a cellular radio system in which the method according to the invention is applied;
FIGS. 2A, 2B, 2C and 2D illustrate TDMA channel organization in different superframe configurations of 51 TDMA frames in the GSM system;
FIG. 3 is a signalling diagram illustrating a normal connection establishment procedure in the GSM system;
FIG. 4 illustrates a GSM Paging Request message modified for the data transfer according to the invention;
FIG. 5 is a signalling diagram illustrating packet data transfer according to the invention from a mobile station MS to a base station BTS;
FIG. 6 illustrates a GSM Channel Request message modified into a message requesting the transfer of packet data according to the invention;
FIG. 7 illustrates a Channel Description information element in a GSM Immediate Assignment message modified for use as a packet acknowledgment message according to the invention;
FIG. 8 illustrates a message according to the invention
used in the packet transfer;
FIG. 9 shows a base station configuration; and
FIG. 10 shows a mobile station configuration.
DETAILED DESCRIPTION OF THE
The method according to the present invention is applicable on the control channels of the digital TDMA (Time Division Multiple Access) cellular radio systems. The method is particularly advantageous when applied in the Pan-European digital mobile radio system GSM and other similar digital systems, such as DCS 1800 and PCN (Personal Communication Network). In the following the preferred embodiment of the invention will be described as an application in the GSM system, to which the invention, however, is not limited.
FIG. 1 shows schematically a portion of a GSM cellular radio system. In the cellular radio system the area covered by the system is divided into radio cells. Two base station
controllers BSC1 and BSC2 are connected by digital transmission links 12 to a mobile exchange MSC. Furthermore, base stations BTS1, BTS2 and BTS3, BTS4 are connected by digital transmission links 11 to the base station controllers BSC1 and BSC2, respectively. Each BSC and the base 5 stations controlled by it form a base station system BSS. The radio coverage area of a single base station BTS typically forms a single radio cell. Each base station BTS1 to BTS4 has a predetermined fixed number of physical radio channels. The configuration and properties of the GSM system 10 are described in greater detail in the GSM specifications, which are hereby incorporated by reference. Only a few features essential to the invention will be described below.
In the TDMA (Time Division Multiple Access) systems a signal (physical channel) consists of TDMA frames each 15 comprising a number of time slots (eight time slots in GSM) in which logical channels are transferred. Logical channels comprise traffic channels for calls to be set up with mobile radio stations MS residing in the cell, and control channels for signalling with mobile radio stations MS residing in the 20 cell. In the GSM system, one superframe comprises 51 or 26 TDMA frames. FIGS. 2A, 2B, 2C and 2D illustrate different kinds of control channel superframes used over the radio path in the transmission direction (downlink) from the base station BTS to the mobile radio station MS (such as a mobile 25 telephone), FIGS 2C and 2D and in the reverse direction (uplink) from the mobile radio station MS to the base station BTS, FIGS 2C and 2D.
Logical channels essential for the invention include Common Control Channels CCCH. In the paging, the fixed 30 network pages a mobile station MS in order to alarm the MS to contact the network. To this end, the MSC commands the base station system BSS to send a page on a Paging channel PCH, which is one of the CCCH channels. When the mobile station MS receives a page or wants to initiate a call, it sends 35 a connection request on a Random Access Channel RACH, which is also one of the CCCH channels. In response to the connection request, the base station system BSS sends a channel assignment on an Access Grant Channel AGCH.
FIG. 3 illustrates messages transferred over the radio path when the base station system BSS pages a mobile subscriber station MS. The BSS pages the mobile station MS by sending a Paging Request message on the PCH. On receiving the paging request, the mobile station MS requests a 45 signalling channel by sending a Channel Request message on the RACH. Upon receiving the Channel Request message, the base station system BSS, which is not aware that the current connection request is associated with the previously transmitted Paging Request message, allocates a sig- 5Q nailing channel (SDCCH) for the mobile station MS by sending an Immediate Assignment message on the AGCH. When the SDCCH is allocated to the mobile station MS, the mobile station transmits on this channel a Paging Response message, which is forwarded to a VLR controlling the 5J paging procedure. On the basis of this message, the VLR establishes a radio connection and then transfers the call control to the MSC.
From the Channel Request message onward, the connection set-up initiated by the mobile station MS is such as go shown, the only difference being that the MS sends a page without the impulse given by the Paging Request message. The BSS is not able to distinguish between Channel Request messages sent for different reasons but always responds to them in the same way. 65
In the present invention the broadcast nature of the above-described paging messages and the random access
nature of the channel request messages are utilized by implementing a packet transfer solution in parallel therewith. The packet transfer according to the present invention does not require any call set-up and is compatible with the original service so that the mobile stations already existing in the cellular system will not be aware of this new facility. In other words, the invention embodies a substantially automatic user data transfer concept in which addressed data can be transferred in a single packet (TDMA time slot) without any related connection establishment.
The packet transfer according to the invention will be divided below into two cases: data transfer to the mobile station MS and data transfer from the mobile station.
MS terminating packet data transfer
In the data transfer to the mobile station the base station system BSS sends a Packet Data Paging Request message, which is a new message reserved for the packet transfer on the PCH. The format of the message is shown in FIG. 4. The Packet Data Paging Request message is of the same type as the Paging Request message, from which it differs mainly in two respects:
1) The message type indicates that the message is dedicated to the packet data transfer. This is done by selecting a bit pattern 0010011 dedicated to control messages but currently unused to indicate data transfer in the control message.
2) The Packet Data Paging Request message comprises an extra field of 141 octets which is utilized to transfer user data. The first octet indicates the message length.
When the mobile station MS receives a data packet, it acknowledges the successful receipt by sending a Packet Data Channel Request message to the base station BTS on the RACH. The format of this message may be of the same type as that of a message to be described below in connection with FIG. 6 except that the Establish Cause field gets the value 011.
MS originating packet data transfer
The mobile radio station MS sends packet-format data on the RACH in PacketDataChannelRequest messages. The base station system BSS acknowledges the successful receipt of the Packet Data Channel Request message by sending a Packet Data Paging Request message on the PCH.
As the Timing of the random access burst, in which form both the Packet Data Channel Request and the Channel Request message are sent, is not known, only one octet of information can be transferred in the burst. This is not sufficient for data applications, and it is therefore necessary that a standard burst can be used for the packet data transfer. The transmission of such a normal burst is timed so that when the burst is received in the base station system BSS, it is synchronized with the TDMA frame. This requires that the base station system BSS is controlled similarly as in the standard connection establishment.
In the standard connection, establishment the base station detects the time of reception of the random access burst Channel Request. On the basis of the time of reception, the base station selects a timing advance to be addressed to the mobile station in an Immediate Assignment channel assignment so that any later signalling on the SDCCH may take place with a proper timing so that a normal burst can be used.
This feature of the base station of the TDMA-based digital cellular network is utilized in the invention as will be described below with reference to FIG. 5. The mobile station MS which wants to transmit packet data sends a Packet Data