FI100076B - Radio system for wireless subscriber connection - Google Patents

Description

100076

The present invention relates to a radio system implementing a wireless subscriber connection, comprising base stations and a subscriber network element connecting wireless terminals to a fixed network telephone exchange. In particular, the system has an air interface between a wireless terminal and a base station, essentially according to a TDMA mobile communication system in which at least one time slot of a signaling frequency (cO) coil is a control channel time slot at which the base station transmits control information for wireless terminals.

When building a telephone network, the installation of subscriber lines between the exchange and the subscriber equipment is not only a considerable cost item but also a time-consuming operation. Typically, a network of subscriber lines is formed by passing pair cables from a plurality of subscriber devices to a cross-connect device, connecting cables from a plurality of cross-connect devices to another cross-connect device from which the outgoing cable is routed to a exchange. The signaling interface to the exchange's subscriber lines is standardized and is either a 2-wire analogue subscriber line interface, a multiplexer interface according to CCITT's V2 recommendation or a message-based multiplexer interface according to ETSI's V5.1 recommendation. Making changes to leased lines is laborious and maintenance costs can be significant, especially in an area where local loops are overhead lines. One solution to these problems is to replace the fixed connection between the exchange and the subscriber-30 equipment with a radio connection. The solution is known as the Wireless Local Loop System (WLL).

The principle of the WLL system is shown in Figure 1. The wireless fixed terminal T comprises a radio unit 4 provided with an antenna and a telephone adapter which connects 2 a subscriber device 5 according to the 100076 standard to the terminal. The subscriber device can be a regular telephone, a fax machine or a modem. It is connected to the terminal by inserting a standard plug into the adapter connector on the terminal.

5 The user uses the subscriber unit 5 as in a normal fixed network, although the subscriber line connection consists of a radio connection between the terminal T and the base station BS 2 or 3. The base station can serve numerous subscriber devices. The base station is connected to a special subscriber network-10 element 1, which in turn is connected to a standard telephone exchange. Several base stations can be connected to one subscriber network element 1.

The WLL system can be constructed by applying the components of an existing mobile telephone system. The mobile telephone system can be, for example, an analog NMT system or a digital GSM system. In this case, the signaling of the WLL system is in accordance with that system; the standard base stations of the system and the radio part of the terminal similar to the radio part or terminal of the mobile-20 communication device may be a mobile station of the mobile telephone system. An important component of the WLL system is the subscriber network element, which connects subscribers to a standard local exchange. The subscriber network element converts the signaling of the WLL network, e.g. NMT or GSM signaling, into signaling suitable for a fixed network (e.g. PSTN) and correspondingly adapts the signaling of the fixed network to the interface of the WLL network.

The network element is connected to the local exchange by an open multiplexer connection of the V2 or V5.1 type using a 2 Mbit / s PCM system. If the local exchange only supports a 2-wire interface, the network element is connected to the exchange by converting the V2 signaling by means of a multiplexer to an analog 2-wire subscriber interface. The signaling between the network element and the base stations connected to it is the signaling of the applied mobile telephone network, I! 3 0007 6 modified, however, so that functions typical of a cellular network, such as cell handover and roaming, are inhibited. The subscriber must therefore remain within the coverage area of the base station assigned to it. The routing of the incoming and outgoing call 5 is based on the subscriber file of the network element. The operation of the network element corresponds to the operation of the hub: the call is forwarded from the subscriber interface to the exchange and the number analysis, calculation, etc. functions are performed in the exchange.

10 As mentioned above, the WLL network may be based on a known GSM system. GSM is a digital cellular system based on time division multiple access. Its channels are described below.

The logical channels are divided into traffic channels TCH transmitting speech and data, and control channels transmitting signaling and synchronization data. The control channels are broadcast channels, common channels and dedicated channels. The broadcast channels BCH are channels from the base station to the mobile station (downlink) and are (i) a frequency correction channel FCCH, the information of which the mobile station uses for its frequency correction, (ii) a synchronization channel SCH, which transmits frame synchronization information and support information to the mobile station. a broadcast channel BCCH 25 (Broadcast Control Channel) which transmits information about the public base station. The Common Control Channel (CCCH) includes a downlink paging channel PCH (Paging Channel) for sending paging calls to mobile stations, an uplink random access channel RACH (Random 30 Access Channel) for mobile stations to request a channel from the network, and a downlink paging acknowledgment channel AGCH (Access Grant). Channel) by which the network acknowledges requests sent by mobile stations. The dedicated channels include the Stand Alone Dedi-35 cated Control Channel (SDCCH) and the slow 100076 4 associated with the traffic channels, and the Fast Associated Control Channel (SACCH) and FCCH (Fast Associated Control Channel).

The logical channels are described as the physical 5 channels of the radio path. The physical channel is known to consist of successive "windows" defined by a time slot and a frequency. A particular physical channel always uses the same time slot number in each TDMA frame of eight time slots, but the frequency may change when using frequency-10 hopping.

The 51 TDMA frames form a control channel-like frame, which is shown schematically in Figure 2. Both the FCCH and the SCH have the same structure: the SCH time slot follows the FCCH time slot at one frame end and the superframe has five time slots allocated to each channel. Both channels use the frame time slot TNO. The broadcast channel BCCH uses four time slots from the superframe and the time slots are in consecutive frames. The remaining 36 time slots (each in its own TDMA frame) are reserved for the common channel CCCH, i.e. for the downlink frame for the paging and paging acknowledgment channels PCH and AGCH and for the uplink frame for the RACH. For practical reasons, both the BCCH and the CCCH also use the time slot TNO. The repetition length of the control channel superframe is thus 51 TDMA frame durations. 25 In the time slot, the TNO is not allowed to perform frequency hopping, but the above-mentioned channels using this time slot must use the received fixed frequency. However, according to the specification, the BCCH carrier must be transmitted continuously in all time slots at a constant power and a constant frequency, usually at the maximum allowed power. The frequency of the carrier at which the control information is transmitted is denoted cO. In time slots where no information is transmitted, a burst, the so-called dummy burst. The BCCH carrier cO transmitted continuously at constant power and constant frequency is utilized so that Matti 100076 periodically measures the signal strength of the carrier of the BCCH channels of neighboring cells and also determines the connection quality by means of a bit error rate, whereby the measurement results are used in handover.

5 In the GSM system, error-correcting coding is used on the traffic channel in order to achieve the highest possible radio transmission accuracy. Coding is a two-stage channel coding in which light block coding is mainly used only for error detection and convolutional coding for error correction. In convolutional coding, errors must be randomly distributed in time before the code can be used effectively. Therefore, interleaving is used in which the channel coded block is transmitted in at least four time slots, the bi-15s of the channel coded block are scrambled before transmission, whereby transmission errors are randomized, and adjacent blocks are interleaved to be transmitted in the same time slot. Interleaving per se is sufficient if the mobile station is in motion, but in addition, frequency hopping is used, which in the case of a slow-moving or stationary mobile station contributes to the random distribution of errors. A further advantage of frequency hopping is that the co-channel interference is averaged, which interference occurs when another user in another cell is on the same frequency.

25 The background of coding and frequency hopping is that the received signal is the sum of Rayleigh fading signals with different delays, resulting in strong variation in signal level and quality. Frequency hopping and hopping speed reduce and, at best, eliminate the correlation between Fri-30 successive bursts, resulting in randomization of errors due to interleaving and bit rearrangement.

However, a strong continuous transmitting carrier is a problem in some cases. It causes the interference level in the network 35 to rise, i.e., when the carrier of the traffic channel 6 100076 in the cell is the same as the BCCH carrier of the nearby cell, it causes interference in reception. This reduces the capacity of the network. In low capacity cells that use only one transmit and receive frequency on a traffic channel, i.e., one TRX, when using frequency hopping, an additional transmitter / receiver must be added to the base station that transmits the BCCH carrier in time at the same time as the actual transmitter / receiver. the receiver generates the frequency of the traffic channel.

10 This is because a large number of frequencies can be synthesized with one transmitter / receiver, but only one frequency at a time.

These interference problems also apply to the WLL system described at the beginning when applying the GSM system as such to WLL use. Especially when applying the WLL system in a sparsely populated area, it is very likely that the base station uses only one transmission and reception frequency (one TRX frequency) and that the coverage area of the base station is wide. In this case, not only is the transmission power of the BCCH carrier high, which increases the interference of the sauna channel in telephone traffic and decreases the network capacity, an additional transmitter / receiver must also be added to the base station for frequency hopping to form the BCCH carrier c0. Frequency hopping is also necessary or at least undesirable in the case of a single TRX, as its absence causes the efficiency of convolutional coding to be degraded, because in the case of stationary or only slow-moving subscriber equipment, interleaving alone is not sufficient to produce random error distribution. The capacity of the network is also reduced due to the fact that since the interference of the same channel is not averaged by the frequency diversity, a higher frequency repetition has to be used, i.e. with the Selma frequency allocation fewer channels are obtained.

It is an object of the present invention to provide a line based on a TDMA system, in particular a GSM system. 100076 7 a radio system implementing a lifeless subscriber interface / in which the BCCH carrier cO does not produce the above-mentioned problems that would be caused by the use of a standard BCCH carrier in the GSM system.

The set object is achieved as set out in claim 1.

In the WLL radio system according to the invention, the BCCH carrier cO is transmitted at constant power only in the time slot in which control information is transmitted and in other time slots the carrier is not transmitted at all or other time slots are used as necessary for traffic channels during which the BCCH carrier power is adjusted in the normal way. Thus, there is no transmission in these other time slots if there is no traffic. If there is traffic, power control, 15 frequency hopping, and discontinuous transmission DTX (Discontinuous

Transmission). DTX means that the transmission is interrupted during speech pauses. This results in less reception interference, good frequency diversity and better code performance.

The invention will now be described in more detail with reference to the accompanying figures, in which Figure 1 shows the principle of a WLL system, Figure 2 illustrates a control channel superframe and Figure 3 is an example of a BCCH carrier-25 arrangement according to the invention.

In WLL applications, the subscriber can move in his home cell but handover between cells is prevented. Therefore, it is unnecessary to measure the BCCH carrier strength of neighboring stations. Therefore, in the WLL system according to the invention, the BCCH carrier does not have to be transmitted continuously in all time slots of the frame using this carrier frequency cO, but is transmitted at maximum power only in the time slots forming the BCCH and the CCCH. This time slot is the time slot TNO of the RF channel cO in Fig. 3. Other time slots TN1, ..., TN7 8 of the frame using the carrier-frequency 35 cO can be used as traffic channel time slots as needed. In these time slots, power control is used for frequency cO in the normal way. Since the carrier is not transmitted at maximum power or is not transmitted at all, the common channel interference 5 is significantly reduced.

When only one TRX is allocated to the cell, the frequency cO in Fig. 3, a frequency jump can still be performed between the BCCH frequency cO and the synthesized frequency c1 using the same transmitter / receiver. One possible channel using frequency hopping-10 play is indicated by arrows connecting the time slots of these RF channels. The channel consists of the fourth time slot of the frame. One frequency synthesizer explains because when transmitting a burst in the fourth time slot of frequency C1, it is not necessary to transmit a BCCH carrier at all in the fourth time slot of frequency c0.

When multiple TRX frequencies are allocated to a cell, frequency hopping can be performed between these frequencies and the BCCH carrier cO can be kept off at time slots TN1, ..., TN7. Said time slots can, of course, be utilized for traffic channel use, whereby normal power control is performed on the carrier and in which case these time slots are also available for frequency hopping.

When using the arrangement according to the invention with respect to the BCCH-25 carrier, certain features can be omitted from the standard GSM system: the hand-over algorithm does not need to be performed and therefore the base station does not have to send a list of neighboring cells and the subscriber device does not have to monitor the BCCH carrier 30 and perform related measurements and do not report measurement results to the network. If frequency hopping is not performed, the BCCH carrier does not need to be transmitted continuously but only in time slot ΤΝ0.

In addition to the above, the invention has the advantage that, especially in the case of 35 low-capacity cells, the reuse of frequencies in the network t1 100076 9 is more efficient because the strong carrier is not constantly on. In addition, the power consumption of the terminal is reduced because there is no need to monitor the BCCH carrier of neighboring base stations. This is an important point, as in many cases the terminals are battery-powered.

It is to be understood that the foregoing description and the accompanying figures are intended only to illustrate the present invention. Various variations of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention as set forth in the appended claims.

Claims (7)

10 100076 Claims p A radio system implementing a wireless subscriber interface, comprising base stations and a subscriber network element connecting wireless terminals (T) to a fixed network telephone exchange, the air interface between the wireless terminal and the base station (2, 3) being substantially in accordance with a TDMA mobile communication system, whose carrier is the basic signaling carrier 10 (cO), at least one time slot (e.g. TNO) is a time slot of the control channel (FCCH, SCH, BCCH, CCCH) at which the base station transmits control information for wireless terminals at a constant power, characterized in that in other time slots of e.g. . TN1, ..., TN7) a burst is transmitted only at the required time intervals (e.g. TN2). A radio system according to claim 1, characterized in that the time slot (e.g. TN2) at which the burst is transmitted is a traffic channel time slot and in that the transmission power of the signaling carrier (cO) 20 acting as a burst carrier is adjusted as required by the mobile communication system. A radio system according to claim 1, characterized in that when the time slot of the frame is other than the time slot of the traffic channel or the time slot of the control channel, no signaling carrier (cO) is transmitted at all. Radio system according to Claim 1, characterized in that frequency hopping is permitted in the time slot of the traffic channel of the frame. Radio system according to Claim 2 or 4, characterized in that discontinuous transmission (DTX) is used on the traffic channel. Radio system according to Claim 1 or 4, characterized in that the system has one TRX frequency assigned to the base station, which is the carrier frequency (cO) of the signaling 35 and at least one other frequency (cl) II and that the frequency hop on the traffic channel is TRX- between the frequency and the other frequency (cl), both frequencies being generated by the same frequency synthesizer. Radio system according to Claim 1, characterized in that the wireless terminal (T) listens only to the information transmitted on the control channel by the base station assigned to it. 12 100076