METHOD FOR CONNECTING MOBILE STATION TO TDMA-BASED NETWORK

Abstract: The invention relates to a method for connecting a portable terminal to a TDMA-based data transmission system, such as the DECT system. In the method portable terminals (PT) are connected to fixed parts of the system (FS) over a radio path (RP), via which successive TDMA frames are transmitted which are divided into successive time slots. To be able to easily and effectively combine the advantages of the fixed network and of the wireless network, a transmission path is arranged on the radio path from the two-wire line (WL) so that a) transmission in the two-wire line takes place in both directions simultaneously in frames the length of which essentially corresponds to that of the TDMA frame, and b) the time slots of the frames are formed of the data packets (PHL_P; MAC_P) that have been formed for the radio path in the portable terminal.

Description: METHOD FOR CONNECTING MOBILE STATION TO TDMA-BASED NETWORK The present invention relates to a method for connecting a portable terminal to a TDMA-based network as described in the preamble of appended claim 1. A portable terminal refers here to a telecommunication terminal used for mobile communication that can be used while moving about or in a location that is not known beforehand. The method of the invention is to be used especially in a DECT system for connecting a portable DECT terminal to the DECT system, but it can also be applied in any other TDMA-based system. A disadvantage of a conventional fixed telephone network is that when a subscriber station (e.g. a telephone) is moved to another location, a telephone number has to be specified for the new location at the telephone exchange. Such changes cause a lot of work to the operator and mean (additional) costs to the subscriber. Previously known mobile and cellular networks do not have the above mentioned disadvantage, but in them the telephone number is the same regardless of where the telephone is physically located. The telephones of these systems also have subscriber-specific cards (e.g. SIM cards), so that a telephone can be used as a subscriber station of a desired subscriber. The telecommunication terminals (e.g. telephones) of a fixed net¬ work have at least the following advantages over the telecommunication terr minals of cellular networks: - the functioning of the telecommunication terminals is reliable - the telecommunication terminals receive power from the network and in this respect they are more reliable and user friendly, - since the telecommunication terminals are fixedly located at cer¬ tain locations, the information concerning the subscriber line (e.g. the operator and subscriber to be charged) can be easily identified, - speech quality is always very uniform, and - there is no congestion caused by the radio path. The object of the present invention is to provide a method by which the advantages described above can be combined in the same subscriber sta¬ tion as simply and effectively as possible. This object is achieved by the method of the invention, which is characterized by what is disclosed in the characterizing part of appended claim 1. The basic idea of the invention is to branch a fixed point-to-point type connection where digital full duplex data transmission is used, the con¬ nection being parallel to the radio path in the portable terminal, and to imple¬ ment the branching in an interface where the data packets (bursts) appear es- sentially in the form wherein they are transmitted to the handling process re¬ quired by the radio path (in other words prior to possible channel coding and/ or interleaving or other similar measures required by the radio path). By the solution of the invention e.g. the following advantages are achieved: - - no need for separate infrastructure for wireless and wireline fixed access, - one and the same subscriber station can be used in the most suit¬ able (reliable) way in each respective case, - a subscriber station can be transferred from one fixed subscriber connection to another fixed subscriber connection without the operator having to make any changes, - the same telephone number can be used regardless of whether a connection to the network is established via the radio path or a fixed connec¬ tion, and - radio spectrum can be saved, since the subscribers can be con¬ nected to the network also via a fixed interface. In the following the invention and its preferred embodiments will be described in greater detail with reference to the examples shown in the ac¬ companying drawings, in which Figurel illustrates a conventional DECT system, Figure 2 illustrates a DECT system of the invention, Figure 3 illustrates a frame structure used on the radio path in the DECT system, Figure 4 illustrates a frame structure according to the system of Figure 2 where a two-wire line is used, Figure 5 illustrates those parts of a portable DECT terminal which are essential to the invention, and Figure 6 illustrates the implementation of a wireline connection of the invention. In Figure 1 a reference model of the DECT system is described. In the figure corresponding (English) terms have been used (for clarity) that are used in international standards (see e.g. ETSI, ETR 015, March 1991 : Radio Equipment and Systems, Digital European Cordless Telecommunications (DECT), Reference Document). The DECT system can be connected to two types of network: - a local network LN, like, for instance, a subscriber exchange, and - a global network GN that is typically a national or an international network, e.g. a public telephone network. In the DECT reference model described in the standards, the net¬ work components are divided into two kinds, portable terminals PT and fixed systems FS. A fixed system is a combination of all the components that repre¬ sent the fixed components of the DECT network. These network components (PT and FS) are interconnected via the radio path. The fixed system FS can contain many base stations (Radio Fixed Part), but only one common control fixed part CCFP. To the portable terminal PT an end system ES is connected, which system comprises the application processes (telecommunication services) used. Conceptually the DECT system comprises three parts: - cells, wherein the subscribers move and each one of which is pro- vided with a number of functions that are required for controlling and manag¬ ing the cell in question, - a Central System comprising the protocol and management func¬ tions necessary for maintaining all the DECT connections in the system, and - Interworking unit(s) that perform protocol, format and code con- version operations between the DECT system and the system connected to it (e.g. a public telephone network, an ISDN network or a GSM network). Since the DECT system is known per se, it will not be described further here. The DECT system is defined in the standard ETS 300 175 of ETSI, from which an interested reader can get a more detailed picture of the system. Figure 2 illustrates how the principle of the invention changes the reference model presented in Figure 1. In accordance with the invention, an¬ other, wireline transmission path WL is arranged parallel to the radio path RP between the portable terminal and the fixed system by employing a (copper) two-wire line, in other words by employing a similar transmission medium as in an ordinary telephone connected to a fixed telephone network. Consequently the portable DECT terminal can be connected to the fixed system either via the radio path or the wireline (subscriber loop). The portable terminal PT is provided with a part W1 for wireless access, which is implemented in the con¬ ventional way for a DECT radio connection, and with a parallel part W2 for wireline access, which takes care of the connection to the two-wire line. A frequency band of 20 MHz between 1880 MHz...1900 MHz is re¬ served for the DECT system. On this frequency band the interval between the RF channels (carrier waves) is 1.728 MHz. The average transmission power is only 10 mW, which allows for a long operating time while it on the other hand reduces the price of the telephone. In Figure 3 a frame structure used on the DECT radio path RP is il¬ lustrated. The length of one frame is 10 ms and comprises 24 successive time slots (0...23), of which the first 12 are reserved for transmission from the fixed system to the portable terminal (F→ P) and the last 12 for transmission from the portable terminal to the fixed system (P→ F). Consequently the physical channels of the DECT system are thus formed by using the so called MC - TDMA/ TDD principle (Multi Carrier Time Division Multiple Access / Time Divi¬ sion Duplex). The length of one time slot is 416.7 μs. The length of the burst transmitted in a time slot is 364.6 μs and there are a total of 420 bits in it. This burst constitutes a packet (PHL Packet) of the physical layer of the DECT system, and is marked with the reference mark PHL_P. The burst comprises a synchronization field SYNC of 32 bits and a subsequent data field DATA of 388 bits. The data field comprises a packet (MAC Packet) of the MAC layer (Media Access Control) of the DECT system and is marked with the reference mark MAC_P. In Figure 3 a data field is presented as it would be used e.g. during an ordinary call. The data field is divided into two parts, A and B. In field A, the information of the signalling channel ( C ), the broadcast channel (Q) and the paging channel (P) is transmitted. So the transmission capacity (4.8 kbit s) of field A is shared between these channels. The remaining part CRC (16 bits) of field A is reserved for error control of the data. In field B, user in¬ formation (l-channel) is transmitted as a 320 bit long burst. The remaining part X (4 bits) of field B is defined by some of the bits of the l-channel and they are used for detecting disturbances. In the two-wire line of the invention, a frame structure essentially similar to that of the radio path is used, but in such a way that the transmission takes place simultaneously in both directions of transmission. (The solution is based on an echo cancellation technique that is described in greater detail in connection with Figure 5.) Figure 4 shows a frame structure that is used in the two-wire line. The frame length is 10 ms in both directions of transmission and the frame preferably comprises 12 successive time slots, in each one of which a packet PHL_P of the physical layer of the DECT system is (preferably) transmitted. In this way a speed of ca 504 kbit/s is achieved in both directions of transmission. (Although in the figure safety time has been allotted at the ends of each packet PHL_P of the physical layer, such safety time is not nec¬ essary in transmission via a two-wire line.) By using full duplex transmission by means of an echo cancellation technique, the spectrum width of the two-wire line can be made as narrow as possible. In Figure 5 those parts of a portable DECT terminal PT that relate to the present invention are described. The starting point is that in a portable terminal there is always an interface through which the DECT packets (preferably packets of the physical layer) can be transmitted in both directions of transmission. This interface has been marked with the reference mark PI. The packetizing and depacketizing of the packets takes place in unit 51 , which depacketizes the arriving packets into bit streams corresponding to the differ¬ ent channels (I, P, C and Q) and packetizes these bit streams in the transmit- ting direction. In accordance with the invention a selecting unit 52 has been ar¬ ranged in the above mentioned interface, to select the mode by which the ter¬ minal will be connected to the fixed parts of the network, in other words, whether access will be made via the radio path or the two-wire line. The selec¬ tion signal of the access mode has been marked with the reference mark CTRL. The selecting unit connects the packetizing and depacketizing unit ei¬ ther to the RF subsystem W1 or to the subsystem W2 of the fixed system to which, in turn, one or more two-wire lines WL have been connected. In Figure 6 the structure of the subsystem W2 is illustrated. Of the packets PHL_P in the transmission direction frames according to Figure 4 are formed in unit 61 , the frames being fed into a modulator / demodulator part 62. The modulator / demodulator part uses a modulation method suitable for the (copper) two-wire line, e.g. QAM or 2B1Q modulation. From the modulator the transmission signal is fed both to an adaptive echo canceller unit 63 and via a hybrid 64 to the two-wire line WL. The hybrid performs the conversion from a 2-cable conductor to a 4-cable conductor in a manner known per se. The received signal RX is connected via the hybrid to the echo canceller unit 63 that cancels the component (echo) corresponding to the transmission signal from the received signal. The signal so obtained is de¬ modulated in unit 62 and the frames are depacketized in unit 61. At the end of the fixed system there is a subsystem W2 essentially similar to the one in the portable terminal. At the Fixed System FS end the two-wire line can be con¬ nected either to the Radio Fixed Part or a common control fixed part CCFP. The subsystem W1 is not described in greater detail here, since it is implemented in a manner known per se. Above a method has been described by which a packet of the physical layer of the DECT system is transmitted in the time slots of the two- wire line. This is a preferred embodiment in the respect that it is as compatible as it possibly can be with the existing DECT system. It is also possible to transmit packets of the MAC layer in the time slots, whereby the synchroniza¬ tion pattern can be placed e.g. in front of the frame. A solution like that re- quires greater changes in the existing technique, but it is a somewhat more advantageous solution with regard to transmission capacity. Even though the invention has been explained above with reference to examples illustrated in the accompanying drawings, it is obvious that the in¬ vention is not to be so restricted but can be modified within the scope of the inventive idea described above and in the appended claims. Even though the DECT system has been used as an example above, the method can be ap¬ plied to any similar TDMA-based system.

Claims: CLAIMS 1. A method for connecting a portable terminal to a TDMA-based transmission system, such as a DECT system, in which method - portable terminals (PT) are connected to fixed parts of the system (FS) via a radio path (RP), over which successive TDMA frames, divided into successive time slots, are transmitted, characterized in - arranging a transmission path from a two-wire line (WL) parallel with the radio path so that - transmission in the two-wire line takes place simultaneously in both directions in frames the length of which essentially corresponds to that of the TDM A-frame, and - the time slots of the frames are formed of those data packets (PHL_P; MAC_P) which have been formed for the radio path in the portable terminal. 2. A method as claimed in claim ^characterized in that the number of time slots in the frame corresponds to the number of time slots in the TDMA frame available in one direction of transmission. 3. A method as claimed in claim 1 for connecting a portable DECT terminal to the DECT system, characterized in that the time slots of the frames are formed of data packets of a physical layer formed for the radio path. 4. A method as claimed in claim 2 for connecting a portable DECT terminal to the DECT system, characterized in that the two-wire line is connected from the portable terminal to a common control fixed part CCFP of the DECT system.

Inventors: NYMAN KAI, NYMAN, KAI
Applicants: NOKIA TELECOMMUNICATIONS OY, NYMAN KAI, NYMAN, KAI
Application year: 1997
Main classification: H04B7
IPC codes: H04B7