FRAME BASED SYSTEM INFORMATION TRANSMISSION

Abstract: In a telecommunications system, system information such as System Frame Number (SFN) is truncated so that a system information field of a frame need carry only a portion of the system information (the LSB part), along with a subpart of a remainder (non-LSB part) of the system information. For system information having P number of subparts for its non-LSB (i.e., MSB) part, for each of P number of consecutive frames a differing subpart is included in the system information field. Recovery of the P number of the system information of P number of consecutive frames enables recovery of the entire MSB part of the system information. The invention thereby reduces overhead of transmission of the system information by requiring transmission of fewer bits for the system information, and thereby allowing the frame to have capacity to carry other types of information.

Description: FRAME BASED SYSTEM INFORMATION TRANSMISSION BACKGROUND This application claims the benefit and priority of United States Provisional Patent Application Serial Number 60/150,361, filed August 24, 1999, which is incorporated by reference herein in its entirety. 1. FIELD OF THE INVENTION The present invention pertains to telecommunications, and particularly to the communication of system information (such as system frame number) to constituent nodes of a telecommunications system. 2. RELATED ART AND OTHER CONSIDERATIONS Cellular telecommunications systems employ a wireless link (e.g., air interface) between a (mobile) user equipment unit and a base station (BS) node. The base station node has transmitters and receivers for radio connections with numerous user equipment units. One or more base station nodes are connected (e.g., by landlines or microwave) and managed by a radio network controller node (also known in some networks as a base station controller [BSC]). The radio network controller node is, in turn, connected through control nodes to a core communications network. Control nodes can take various forms, depending on the types of services or networks to which the control nodes are connected. For connection to connection-oriented, switched circuit networks such as PSTN and/or ISDN, the control node can be a mobile switching center (MSC). For connecting to packet switching data services such as the Internet (for example), the control node can be a gateway data support node through which connection is made to the wired data networks, and perhaps one or more serving nodes. A telecommunications connection between a mobile user equipment unit and another party (e.g., in the core communications network or another mobile user equipment unit) thus involves an uplink from the mobile unit through a base station and a radio network controller (RNC), and a downlink in the reverse direction. Most cellular systems employ some kind of frame format to transmit system and user data over the air interface, both on the uplink and downlink. In order to identify a frame, each frame is assigned a System Frame Number (SFN). In Wideband Code Division Multiple Access (WCDMA) systems, the SFN is transmitted as part of the Broadcast Channel (BCH). The length of the SFN is normally predetermined (e.g., typically 12 or 16 bits). The SFN is important for frame identification, which has particularly significant ramifications for operations such as those occurring in handover situations and during sleep mode, for example. While being important, the SFN nevertheless exacts system overhead by, e.g., reducing the available capacity for user data being transmitted over the air interface. With the entire SFN being transmitted in each frame, a considerable amount of capacity of the BCH is utilized. The BCH carries much more information than SFN, and it would be advantageous to maximize the BCH capacity by compacting the system information (such as SFN) to as few bits as possible. What is needed, therefore, and an object of the present invention, is a technique for reducing the number of bits per frame required to send an item of system information, such as SFN. BRIEF SUMMARY OF THE INVENTION In a telecommunications system, system information such as System Frame Number (SFN) is truncated so that a system information field of a frame need carry only a portion of the system information (the LSB part), along with a subpart of a remainder (non-LSB part) of the system information. For system information having P number of subparts for its non-LSB (i.e., MSB) part, for each of P number of consecutive frames a differing subpart is included in the system information field. Recovery of the P number of the system information of P number of consecutive frames enables recovery of the entire MSB part of the system information. The invention thereby reduces overhead of transmission of the system information by requiring transmission of fewer bits for the system information, and thereby allowing the frame to have capacity to carry other types of information. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Fig. 1 is a schematic view of an embodiment of a telecommunications system which utilizes the present invention. Fig. 2 is a diagrammatic view of a system information field showing a division thereof into parts and subparts in accordance with an example mode of the invention. Fig. 3 is a diagrammatic view showing transmission of a series of consecutive frames over an air interface using a technique of the present invention which includes only partial system information in a frame. DETAILED DESCRIPTION OF THE DRAWINGS In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. Fig. 1 shows a telecommunications network 18 in which a user equipment unit 20 communicates with one or more base stations 22 over air interface (e.g., radio interface) 23. Base stations 22 are connected by terrestrial lines (or microwave) to radio network controller (RNC) 24 [also known as a base station controller (BSC) in some networks]. The radio network controller (RNC) 24 is, in turn, connected through a control node known as the mobile switching center 26 to circuit-switched telephone networks (PSTN/ISDN) represented by cloud 28. In addition, radio network controller (RNC) 24 is connected to Serving GPRS Support Node (SGSN) 25 and through backbone network 27 to a Gateway GRPS support node (GGSN) 30, through which connection is made with packet-switched networks (e.g., the Internet, X.25 external networks) represented by cloud 32. As understood by those skilled in the art, when user equipment unit 20 participates in a mobile telephonic connection, signaling information and user information from user equipment unit 20 are transmitted over air interface 23 on designated radio channels to one or more of the base stations 22. The base stations have radio transceivers which transmit and receive radio signals involved in the connection or session. For information on the uplink from the user equipment unit 20 toward the other party involved in the connection, the base stations convert the radio-acquired information to digital signals which are forwarded to radio network controller (RNC) 24. The radio network controller (RNC) 24 orchestrates participation of the plural base stations 22 which may be involved in the connection or session, since user equipment unit 20 may be geographically moving and handover may be occurring relative to the base stations 22. On the uplink, radio network controller (RNC) 24 picks frames of user information from one or more base stations 22 to yield a connection between user equipment unit 20 and the other party, whether that party be in PSTN/IDSN 28 or on the packet-switched networks (e.g., the Internet) 32. The example embodiments illustrated herein happen to employ code division multiple access (CDMA), wherein the information transmitted between a base station and a particular mobile station is modulated by a mathematical code (such as spreading code) to distinguish it from information for other mobile stations which are utilizing the same radio frequency. Thus, in CDMA, the individual radio links are discriminated on the basis of codes. Various aspects of CDMA are set forth in Garg, Vijay K. et al., Applications of CDMA in Wireless/Personal Communications, Prentice Hall (1997). In view of the diversity aspects of CDMA, the user equipment unit 20 in Fig. 1 is depicted as being in contact with multiple base stations 22 (e.g., base station 22 1 and base station 22 2 ). The present invention particularly pertains to the communication of frames over the air interface 23, and more particularly to the transmission of system information within each frame. Although the techniques described herein are illustrated with the system information being System Frame Number (SFN), it should be understood that the principles of the invention are applicable to other types of system information and that the illustration of the SFN is provided as an example and not limitation. The present invention reduces the length of system information carried or transmitted per frame by splitting the system information into two or more parts. The two or more parts of system information are transmitted with differing frequencies (one of the parts is transmitted at a different frequency than another of the parts). Accordingly, the present invention reduces the system information overhead. The preparation of a system channel such as the BCH is known in the art, as well as the equipment included in user equipment unit 20 and in a base station 22 for inserting SFN into a system channel. Likewise, the operation and structure of equipment included in user equipment unit 20 and base station 22 for transmitting and receiving frames is well known, so that these operational and equipment aspects are not described herein. What is important, however, to the present invention, is the generation of information to be included for the SFN in the system channel of a frame. In accordance with the present invention, system information of L bits length is split into plural parts. In the example of Fig. 2, an SFN having L = 12 is shown as being split into two parts, in particular a least significant bit (LSB) part and a most significant bit (MSB) part. In Fig. 2, the LSB part is shown as having N number of bits and comprising the bit string 110001 ; the MSB part is shown as having K number of bits and comprising the bit string 011011. Thus, L = N+K. Further, Fig. 2 shows that MSB part of the SFN with its K number of bits is split into P number of subparts, illustrated as subparts PI, P2, and P3. Of these P number of subparts, P-l number of the subparts have a length of M bits, while a last of the subparts has a length of K- M*(P-1). As explained below, each of the P subparts is transmitted in a separate frame, so that after P number of frames all the K number of bits of the MSB part have been transmitted and can be assembled. Thus, in Fig. 2, N=6, K=6, M=2, and P=3. Fig. 3 shows how, in accordance with the present invention, SFN is transmitted over air interface 23. In the example shown in Fig. 3, frames are being transmitted on the downlink from a representative base station 22 to user equipment unit 20. It should be understood, however, that the transmission of system information such as SFN can occur in like manner in the reverse direction (i.e., on the uplink). Rather than show the entire frame, Fig. 3 only shows a portion of each of three frames FI, F2, and F3, and more particularly only a portion of Broadcast Channel (BCH) which is required for the transmission of SFN. Such portion of the frame is herein referred to as the system information field of the frame. It will be understood, with reference to the TIME axis of Fig. 3, that the frames FI, F2, and F3 are sent sequentially (e.g., frame FI precedes frame F2, frame F2 precedes frame F3, etc.). The first frame FI of Fig. 3 corresponds to the SFN shown in Fig. 2 which has the LSB field of 110001. But rather than the system information field of frame FI having to include all twelve bits of the SFN as is done in the prior art, in accordance with the present invention the system information field of frame FI need carry only eight bits. The eight bits carried by the system information field of frame FI are the six bits of the LSB field and two of the bits from the MSB field, i.e., one of the subparts of the MSB field. In particular, the system information field of frame FI carries subpart P3 of the MSB, i.e., the bit sequence 70. The system information field of the second frame F2 of Fig. 3 carries a portion of the next SFN, i.e., the SFN which follows the SFN depicted in Fig. 2. The SFN which follows the SFN shown in Fig. 2 and which is included in part in frame F2 is 011011110001. Again, as with frame FI , all six bits of the LSB field - the bit string 110010 - are carried in the system information of frame F2, and one of the subparts of the MSB field. For frame F2, subpart P2 of the MSB field is included in the system information field. The third frame F3 of Fig. 3 carries a portion of the next SFN, i.e., a portion of 011011110011. As with the preceding frames, all six bits of the LSB field - the bit string 110011 — are carried in the system information field of frame F3, and one of the subparts of the MSB field. For frame F3, subpart P3 of the MSB field is included. Thus, in the illustration described in Fig. 2 and Fig. 3, eight bits (6 bits from the LSB field and 2 bits from a subpart of the MSB field) are included in the system information field for transmission per frame. This contrasts with prior art practice of including all 12 or 16 bits (the full SFN) in each frame. The present invention thereby reduces the transmission overhead while still allowing complete determination of SFN with 6 bit resolution in every frame and full SFN resolution after P number of consecutive frames. When N LSB bits are transmitted in every frame, the frame can be identified with a resolution of 2 N . Furthermore, in accordance with the invention there is a cyclic rotation through the P number of subparts of the MSB field as to which subpart is included in a frame. It should be understood that the example of Fig. 2 and Fig. 3 (wherein N=6, K=6, M=2, and P=3) is just one scenario for system information formatting. Table 1 provides examples for other formatting configurations. It is also possible to split the system information which is to be transmitted into more than two parts and to transmit those parts at different repetition frequencies. The LSB part can normally be transmitted in every frame, while other parts can be distributed with different patterns and repetition frequencies over the frames. In order to identify which bits are transmitted in the current frame it can be useful if the number of LSB bits in the part transmitted in every frame are sufficient to encode the state of the transmission cycle. However, in the general case it would be sufficient if the bits in each part are enough to encode the relative transmission cycle of the part with the next lower repetition frequency. Thus, the scheme of the present invention can be utilized for two or more parts of system information in a hierarchical manner. As understood from the foregoing, it is the LSB field whose content changes more frequently than the MSB field, for which reason the LSB field needs to be sent more often than the entire MSB field. In this sense, it is more important that the LSB field of the system information field be available on an up-to-date basis at the receiver. Therefore, in accordance with the present invention, the LSB field is transmitted with greater frequency, since the LSB bits are the more important bits to know in view of their faster rate of content change. It was mentioned above that the person skilled in the art knows how to generate and include system information in a frame, and how to transmit the frame over an interface such as air interface 23. Therefore, Fig. 1 shows in representative fashion an system information field generator 100 which generates the SFN for application over the air interface as being included in the base stations 22 (e.g., system information field generator lOOi in base station 22ι and system information field generator 100 2 in base station 22 2 ). The system information field generator 100 prepares the system information field in accordance with the techniques herein discussed for inclusion in a frame for transmission over the interface. The user equipment unit 20, on the other hand, has a system information field decoder 102 which, as part of a frame decoder, decodes the system information field of a frame in accordance with the principles of the present invention. Although not illustrated as such in Fig. 1 , it should be understood that, for the uplink, the user equipment unit 20 has a system information field generator and each base station 22 has a system information field decoder. Although not illustrated as such, a radio network controller (RNC) 24 also needs to have a system information field generator for various purposes such as, e.g., knowing when to page a user equipment unit (UE). There are several possible techniques for synchronizing the system information field generator and the system information field generator 100 included in the base stations supervised by the radio network controller (RNC) 24. A first technique is to consider the system information field generator in the radio network controller (RNC) 24 as a master SFN generator, in which the radio network controller (RNC) 24 controls the underlying nodes (e.g., underlying base stations). In other words, the system information field generator 100 of the base station is "hard" synchronized to the master SFN generator in the radio network controller (RNC) 24. A second technique is to employ free-running system information field generators in the nodes and have the radio network controller (RNC) 24 estimate the (essentially constant) difference between its own SFN and that of each node. That is, the radio network controller (RNC) 24 knows the error but does not require the nodes to adjust the error to zero, thereby realizing a "soft synchronization". This second technique is particularly useful if an SFN adjustment during operation would slide the data streams. Thus, in accordance with the present invention, system information is truncated so that a system information field of a frame need carry only a portion of the system information (the LSB part), along with a subpart of a remainder (non-LSB part) of the system information. For system information having P number of subparts, for each of P number of consecutive frames a differing subpart is included in the system information field. Recovery of the P number of the system information of P number of consecutive frames enables recovery of the entire MSB part of the system information. Advantageously, the compaction afforded by the invention does not affect system performance since different parts of the SFN have different purposes. For example, handover requires fast provision of the LSB bits; paging needs the entire SFN but not as quickly. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. For example, while transmission of system information has been illustrated herein with respect to frame transmitted over an air interface, it should be understood that the truncated manner of expression of system information of the present invention is not limited to the air interface. Indeed, the techniques of the present invention are also applicable to other interfaces over which system information such as SFN is transmitted, such as (in some technologies) an interface between fixed nodes (e.g., base station 22 and radio network controller (RNC) 24, for example). TABLE 1 K M P LAST BITS/ PART FRAME 2 10 3 4 1 5 3 9 3 3 3 6 4 8 1 8 1 5 4 8 2 4 2 6 6 6 2 3 2 8 7 5 1 5 1 8 4 12 6 2 6 10 4 12 4 3 4 8 4 12 4 3 4 8 6 10 2 5 2 8

Claims: WHAT IS CLAIMED IS: L A telecommunications network (18) wherein frames are transmitted over an interface (23), characterized in that a node of the network has a system information field generator (100) which splits a string of system information associated with a frame into at least a first and a second part, and wherein the first part of the system information is transmitted over the interface at a different frequency than the second part of the system information. 2. The apparatus of claim 1, wherein the first part is a least significant bit part and the second part is a most significant bit part, and wherein the first part is transmitted at a greater frequency than the second part. 3. The apparatus of claim 1, wherein a bit content of the first part changes more frequently than a bit content of the second part. 4. The apparatus of claim 2, wherein the second part is divided into P number of subparts, and wherein the second part is cyclically distributed into P consecutive number of frames. 5. A telecommunications network (18) wherein frames are transmitted over an interface (23), characterized in that a node of the network has a system information field generator (100) which splits a string of system information associated with a frame into at least a first part and a second part, the node also splitting the second part into P number of subparts, and wherein in each frame the first part of the system information is included for transmission along with less than P number of subparts. 6. The apparatus of claim 5, wherein the second part is cyclically distributed into P consecutive number of frames. 7. The apparatus of claim 5, wherein a bit content of the first part changes more frequently than a bit content of the second part. 8. A method of operating a telecommunications network (18) wherein frames are transmitted over an interface (23), the method characterized by: splitting a string of system information associated with a frame into at least a first and a second part; and transmitting the first part of the system information over the interface at a different frequency than the second part of the system information. 9. The method of claim 8, wherein a bit content of the first part changes more frequently than a bit content of the second part. 10. The method of claim 8, wherein the first part is a least significant bit part and the second part is a most significant bit part, and the method comprises transmitting the first part at a greater frequency than the second part. 11. The method of claim 10, wherein the second part is divided into P number of subparts, and wherein the method further comprises cyclically distributing the second part into P consecutive number of frames. 12. A method of operating a telecommunications network (18) wherein frames are transmitted over an interface (23), the method characterized by: splitting a string of system information associated with a frame into at least a first part and a second part; splitting the second part into P number of subparts; including in each frame the first part of the system information for transmission along with less than P number of subparts. 13. The method of claim 12, wherein a bit content of the first part changes more frequently than a bit content of the second part. 14. The method of claim 12, further comprising cyclically distributing the second part into P consecutive number of frames.

Inventors: NYBERG BO, WILDE ANDREAS, NYBERG, BO, WILDE, ANDREAS
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 2000
Main classification: H04B7
IPC codes: H04B7