FI117613B - Network element for frequency synchronization of base station in cellular radio network, generates speed correction factor based on deviation of local clock time from reference time - Google Patents

Abstract

Network element (300) with reference clock (302) is fixed to base station (102) with local clock (330) via asynchronous data transmission connection (312). Reference clock sends time stamp signal (308) over connection. Station (102) receives time stamp signal and calculates deviation of local clock time from reference time. Based on deviation, speed correction factor is output and applied to local clock. A frequency synthesizer (212) generates frequency based on the corrected local clock. An independent claim is also included for the method of performing frequency synchronization of base station.

Description

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Base station frequency synchronization

Field of the Invention

The invention relates to frequency synchronization of a base station in the network part 5 of a cellular radio network.

Background of the Invention

The base station of a cellular radio network requires an accurate clock signal to ensure good frequency stability and accurate timing at the air interface. GSM specifications require a relative accuracy of 5 * 10'8 at the air interface, but this can be relaxed to 10'7 at base stations available in picocell environments. This good accuracy is achieved by transmitting the clock signal as a pulse current through national telephone network and GSM infrastructure, such as a mobile switching center or base station controller, to base stations.

The relative frequency stability of the National Reference Clock is 10'11 f 15 over a 24 hour period. However, the long transmission chain to the base station causes the clock signal to fluctuate and creep. The base station relies on 1.5 * 10 "8 accuracy with its 2 Mbit / s PCM (Pulse Code Modulation) Abis interface. The base station's internal transcoder has a 16 MHz clock (divided into 2 MHz) that is powered by helicopters. For PCM clock pulses, the flutter and creep are filtered off and the * * * 20 signals are averaged over a period of about 15 minutes, thus improving the accuracy of the "cleaned" 2 * * * *; [. * MHz clock signal and serving as a reference clock * · ·· "26 MHz clock in Base Station Controller * .. · * Function, BCF. All frequencies and timing on the radio interface are derived by • ·: trees from this 26 MHz clock.

: ... ί 25 The above-described known method for providing a base station with a precision clock relies on an existing transmission chain from a fixed network to the base station. This becomes a problem if part of this transport chain passes through a · · un-clocked network, as is the case with new cellular radio networks. These networks usually do not have a base station controller, but their functionality is * * · 30 distributed over an IP (Internet Protocol) network or intranet. IP networks are not clocked because they operate asynchronously and transmission times are highly variable and unpredictable.

• *. ·. A solution to the problem is to provide the network element with a very accurate clock, and the clock signal is distributed to the base station via a synchronous line, e.g., 117613 2 ISDN (Integrated Services Data Network) or HDSL (High Bit Rate Digital Subscriber Line).

However, for indoor cellular radio networks, the goal is to utilize existing network cabling in office environments by connecting base stations directly to the asynchronous network.

Installing additional cables to carry the clock signal is contrary to the basic reason for using an intranet: to make better use of an existing network. The additional cables do not need to connect the access points to the Local Area Network (LAN) at all. Base stations can then be connected directly to: 10 networks on an HDSL synchronous network that requires only a simple twisted pair cable.

There are many types of watches that can be used on base stations. Highly expensive watches require a constant (oven-maintained) temperature environment and provide good accuracy that even approximates the national reference 15 watches. To improve the cost-effectiveness of the system, expensive oven-based clocks should be avoided wherever possible, especially in base stations.

Brief Description of the Invention

It is an object of the invention to provide an apparatus which allows the above problems to be solved. This is achieved by the devices described below: ·. 20 replicas, which is a base station in a cellular radio network including ♦ * · a local clock, which is connected by an asynchronous data link to a network element comprising a reference clock, and includes a means for receiving a timestamp signal * and means for calculating a received timestamp signal based on: how much the base station's local clock time deviates from the reference clock * * · :. The base station further comprises means for generating a rate correction factor for the local clock based on at least one calculated offset, means for continuously correcting the clock clock operation based on the rate correction factor, · · ·. * ··. and a frequency synthesizer to generate the frequencies required at the base station using a local clock corrected by a rate correction factor.

The invention also relates to a processor for a cellular radio network base station, which processor is configured to receive a time signal: generated by a reference clock element of a network element of a cellular radio network, to calculate a received time stamp on the base station based on the received time stamp signal. is different from the reference clock.

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The processor is configured to generate a rate correction factor for the local clock based on at least one calculated offset, to continuously correct the local clock running based on the rate correction factor, and to develop the necessary frequencies at the base station using a rate correction factor localized clock.

The invention also relates to a method for performing frequency synchronization at a cellular radio network base station, the method comprising the steps of: receiving a time stamp signal generated in a reference cell clock of a cellular radio network network element over an asynchronous data transmission link based on an The method comprises generating a rate correction factor for the local clock based on at least one calculated deviation, continuously correcting the local clock running based on the rate correction factor, and generating the necessary frequencies 15 at the base station using the localized rate corrected clock.

The invention also relates to a computer software product for performing frequency synchronization at a cellular radio network base station comprising software routines for performing the steps of: receiving a time stamp signal generated in a reference clock of a cellular network network element, A computer software product includes software -. * ··. routines to perform the following steps: developing a speed correction factor. for the local clock, based on at least one calculated deviation, continuously correcting the local clock running based on the rate correction factor, and developing the frequencies required for the * * · * '.V base station using the * ··· ”localized rate correction clock.

Preferred embodiments of the invention will be apparent from the dependent claims.

··· 30 The basic idea of the invention is to control the operation of the base station clock based on the time produced by the time stamps.

The method and apparatus of the invention provide several advantages. The base station does not require an expensive clock, which lowers the base station's • ♦ \ * · cost. In addition, a synchronous data link is not needed to transmit time-V ·: 35 signals, but an asynchronous data link is sufficient. In the operation of the method, it is not the duration of the transmission delay that is essential, but the stability of the variation of the delay.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to the preferred embodiments, with reference to the accompanying drawings, in which:

Figure 1 shows an example of the structure of a cellular radio network;

Figure 2 shows the structure of a transceiver;

Figure 3 shows an example of a cellular radio network according to the invention; and FIG. 4 is a flowchart illustrating operations of the method of the invention. i

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figure 1, a typical structure of a cellular radio network will be described. Figure 1 contains only the blocks essential for explaining the invention, although it will be apparent to those skilled in the art that the conventional cellular radio network includes other functions and structures which need not be further considered herein. The example illustrates, without limiting the invention, a cellular GSM radio network (Global System for Mobile Communication) utilizing TDMA (Time Division Multiple Access).

The cellular radio network typically comprises a fixed network infrastructure, i.e., a network part and subscriber terminals 150, such as fixed, in-vehicle, or portable terminals. For example, the subscriber terminal 150 may be a conventional cellular telephone which may be connected to an additional card, e.g. 25 moving packets to order and process packets.

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The network part comprises base stations 100. A plurality of base stations 100. . ·. the base station controller 102 communicating with them centrally controls them. Base station 100 comprises transceivers 114. Base station 100 typically comprises 1 - '· "* 16 transceivers 114. One transceiver 114 provides radio capacity · · · · 30 for one TDMA frame, typically eight time slots.

The base station 100 comprises a control unit 118 which controls the transceiver. operation of the samplers 114 and the multiplexer 116. Multiplexer 116 arranges • · ·; ; The traffic and control channels used by the plurality of transceivers 114 for one-to-one transmission link 160. The transceivers 114 of base station 100 are connected to an antenna unit 112 that provides bidirectional radio connection 170 to a subscriber terminal 150. The structure of the connection is called the air interface.

Figure 2 shows in greater detail the structure of the transceiver. The receiver 200 comprises a filter which blocks frequencies outside the desired frequency band. The signal is then converted to an intermediate frequency or directly to a fundamental frequency, and in this form, the signal is sampled and quantized in an analog-to-digital converter 202. The equalizer 204 compensates, for example, interference caused by 10 multipaths. From the matched signal, demodulator 206 takes a bit stream which is transmitted to demultiplexer 208. Demultiplexer 208 separates the bit stream from different time slots into separate logical channels. Channel codec 216 decodes the bit streams of separate logical channels, i.e., decides whether the bit stream is signaling data transmitted to control unit 214 or whether the bit stream is speech transmitted to speech codec 122 of base station controller 102. Channel codec 216 also performs error correction. The control unit 214 implements the internal control functions by controlling the various units. Burst generator 228 adds the training sequence and tail to the data coming from the speech codec 216. Multiplexer 226 assigns a time slot to each of the 20 bursts. Modulator 224 modulates digital signals on a radio frequency carrier.

* This is an analog function, and therefore to perform it. a digital to analog converter 222 is required. The transmitter 220 comprises a bandwidth limiting filter. In addition, transmitter 220 controls the transmission output power. Synthesizer 212 provides the necessary frequencies for different units. Synthesizer 212 comprises a clock which is controlled by the second network element in the invention, for example, from base station controller 102. Synthesizer 212 generates the necessary frequency using, for example, a voltage controlled oscillator.

As shown in Figure 2, the structure of the transceiver may

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30 is further divided into radio frequency sections 230 and digital signal processor ·. The radio frequency parts 230 include a receiver, a transmitter 220, a transmitter 220, and a synthesizer 212. The digital signal processor and software includes a balancer 204, a demodulator 206, a demultiplexer 208, a channel coder 216, a control unit 214, burst generator 228, multiplexer 226 and modulator 224. An analog-to-digital converter 202 is needed to convert an analog radio signal to a digital signal, and a digital-to-analog converter 222 is needed to convert a digital signal to an analog signal.

The base station controller 102 comprises a switching field 120 and a control unit 5 124. The switching field 120 is used to switch speech and data and to switch signaling circuits. The base station 100 and the base station controller 102 form a base station system further comprising a transcoder 122. The transcoder 122 converts various digital speech coding formats used between a public switched telephone network and a radio network to make them centrally compatible, e.g., 64 kbit / s / s) and vice versa. The transcoder is usually located as close as possible to the mobile switching center 132, since this allows the transmission of speech in the form of a cellular radio network, which saves transmission capacity. The control unit 124 implements 15 voice control, mobility management, statistical data collection and signaling.

FIG. 1 illustrates how a circuit-switched transmission connection is established between a subscriber terminal 150 and a general optional telephone network terminal 136. In the figures, the line illustrates how information travels across the system through air boundary surface 170, from antenna 112 to transceiver 114, and multiplexed therefrom in multiplexer 116 over transmission link 160 to switching field 120 for connection to transcoder 122 output. thence by means of a connection established in the mobile switching center 132 to a terminal 136. connected to the optional telephone network 134. The base station control unit 118 controls the transmission multiplexer · · · 116 and the base station controller 102 controls the switching field 120. to ensure connection.

The invention is particularly suitable for use in office cellular radio networks. In this case, the base stations 100 may be called office base stations. One important advantage of office-based cellular networks is that they allow the use of a * * ♦ communication network in the building, free of charge, to provide a transmission link 160 between base stations 100 and '*: ** base station controller 102. The communication network may be, for example, an IP network. * ♦ · a network (Internet Protocol) or an ATM (Asynchronous Transfer Mode). For example, when using an IP network, each network element may have a separate IP address to which data packets are addressed. A telecommunication network can also be a large intranet, which connects geographically separate offices of a company.

As noted above, indoor cellular radio networks may not have a network element called a base station controller. Instead, the functionality provided by the base station controller can be distributed to network elements interconnected via an asynchronous data link, whereby the controller consists of, for example, two computers that together provide the normal base station controller functionality in a communication network and the necessary communications management.

According to the invention, the network element of the cellular radio network, for example the base station controller 102, transmits the time stamps and is received directly by the base stations and each base station develops its own reference frequency.

Another solution is that a local area network (LAN) node element, such as a hub, bridge, router or switch modified to be used in an indoor cellular radio network, transmits timestamps and is received directly by base stations and each base station develops its own reference frequency. In particular, the hub has the advantage of reducing the number of collisions 20 in traffic, and therefore provides more reliable transmission of time stamps.

· * · .. In turn, the base station can be equipped with a cheaper clock.

Units sending time stamps may require more expensive clocks, but. ···. in any case, less is needed.

A network element transmitting timestamps can receive an exact refe. * "25 rents from several alternative sources: • · ·: It can receive a clock generated by a National Reference Clock • · · * ··· * which the cellular radio network" sees "its A-interface, or MSC Through its interface to 132, the incoming clock pulses are then averaged a:. · .Ί, as well as in a known base station, so the clock of the unit sending the time stamps can at least reach an accuracy of 1 * 10'8.

• · ·, ···, The National Reference Clock may also be displayed through another telecommunication line, such as the E1 / T1 connection to the ISP. '"** The timestamp transmitting unit may have an internal independent clock, · ** · j especially if the accuracy loss over the IP network is greater than assumed here, or if a national reference clock is not available (for example, an 117613 8-cell indoor cellular radio network without A You may need a clock with a resolution of 1 * 10 to 9, such as an atomic clock or GPS (Global Positioning System), in which case the antenna of the GPS receiver would have to be located outside the building.

5 One independent clock may be sufficient for each indoor cellular radio network or building provided that the timestamp transmitting units can be easily connected to an independent clock by wire, for example if these units are housed in a single device room.

Another possibility is a solution in which the clock signal transmitted by the external base station via the air boundary surface acts directly as a reference clock. Here, the timestamp transmitter unit can synchronize its clock frequency over the air by this method.

The above solutions have the advantage that a single network element transmitting time stamps can serve multiple base stations, since the base stations do not need to be located close to the network element.

Figure 3 shows an example of the structure of a network part of a cellular radio network according to the invention. The dashed rectangle shown below shows the structures of the base stations of interest to the invention. Base station 102 comprises a local clock 330.

The dotted rectangle on the left represents a network element 300 connected to base station 102 via an asynchronous data link] ·: ·. The network element 300 comprises a reference clock 302. The asynchronous data transmission link 312 is in fact the same as the data transmission link 160 shown in FIG.

The reference clock 302 comprises means for generating a timestamp signal 306 and means for transmitting the timestamp signal 308 through an asynchronous data transmission link 312 from the network element 300 to the base station 102.

The timestamp receiving unit, or base station 102, periodically requests 336 timestamps 312 which indicate a time difference between two consecutive points 30 when the timestamp was generated. The request decision is thus decentralized. Another possibility is that the timestamp transmission unit, or network element 300, automatically transmits timestamps 312 without a specific timestamp request 336.

The base station 102 comprises means for receiving 314 a timestamp signal 308 transmitted over an asynchronous data link 312, and means for calculating 316 on the basis of the received timestamp signal 308 a local clock time 332 of the base station 102 deviates from the reference clock. 302 of the time.

Of course, the time stamp signal 308 is also used to change the local clock time 332 at the base station 102 to match the time of the reference clock 302.

This is necessary for the operation of the method, otherwise it would not be necessary to synchronize clocks, since transmissions of different base stations in a cellular radio network are generally asynchronous with each other.

The base station 102 further comprises means 322 for generating a rate correction factor 324 for the local clock 330 based on at least one calculated offset 10; means 326 for correcting the operation of local clock 330 based on rate correction factor 324; and a frequency synthesizer 212 generating the frequencies required by base station 102 using a local clock 330 corrected by rate correction factor 324.

Thus, the rate correction factor 324 describes how the 15 times the local clock 330 must be corrected in the future to make it more accurate. in other words, the time of the local clock 330 is not changed momentarily, but the clock speed is constantly monitored. The local clock 330 maintains the required accuracy for at least 50 to 100 hours. The local clock 330 uses a differential voltage control mechanism to change the clock speed.

The rate correction factor 324 is derived from the difference between the actual speed of the local clock 330 and the reference clock 302 according to the received timestamps.

«· ·. ··. According to a preferred embodiment, the rate correction factor 324 indicates that the supply voltage of the local clock 330 changes so that the actual correction of the clock rate /? 25d is performed by a differential control voltage.

• · *: The use of this differential voltage is approximately 80% accurate since the relative change in voltage is very small and the ratio of voltage to clock speed is not completely linear. Iteration provides means for accurately correcting the clock; 5 to 10 iterations should be sufficient. The total time for one iteration -: *] [: 30 per tiprocedure should be much shorter than the typical time scale that changes the characteristics of the differential voltage control circuit (which *:! Is about a year). Therefore, rate correction factor 324 is preferably adjusted more than · · · ··· * min than once a year.

You can request a time stamp once every 24 hours in low traffic: * ·, · 35 episodes (eg at night). This is just an example used in the calculations below m m .i 10 117613 -. Staff assigns a value to the network element 300 based on intranet-specific delay variation properties.

Time stamps are delayed in an IP network, but the variable relevant to this method is the variation of these delays. For a target accuracy of 2 * 10'8, the fluctuation in the vii-5 must be less than 2 ms.

According to a preferred embodiment of the invention, the base station 102 comprises means 320 for calculating a variation in the transmission delay of the time stamp signal 324 transmitted over the data transmission link 312; comparing the deviation to a predetermined threshold; and to conclude that the local clock's accuracy does not meet the required level if the deviation exceeds a predetermined threshold. The threshold may be, for example, the above 2 ms. If the clock accuracy is within the required accuracy, no action is required, but if the accuracy is not sufficient, for example, an alarm can be generated in the control system controlling the 338 cellular radio system.

An established protocol such as Network Time Protocol (NTP) can be used to send the time stamp signal. The timestamp sending units act as an NTP server and the timestamp receiving units act as NTC clients. NTP measures the transmission delay and corrects the timestamp as an iterative process that can take a few seconds. The protocol does not need to be based on 20 IPs, such as NTP, but can be built directly on the Ethernet link layer.

/; ·, Higher timestamp frequency does not improve the accuracy of this method. However, for example, a frequency of one time stamp per hour allows the units receiving the time stamps to estimate the variability of the delay and ensure that the above limits are met and otherwise: generates an alarm.

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The invention makes it possible to significantly reduce the number of expensive clocks in an office system, since a plurality of base stations can receive a reference frequency from the same timestamp transmission unit. One example of such a diagram is that in one building there is one network element that uses a floating network as a reference clock (which can be just a k · · * *; * / kappa base station). This reference network element sends time stamps • · * "· * to other base stations inside the building. Within a single building, the variability of the IP delay is typically quite small. For wide intranet networks, the delay variability may be too large, which would require more than one timestamp transmission unit.

Some parts of the network part according to the invention are preferably implemented by software executed in the processor. Some sections of the network part 5 according to the invention may also be implemented as a hardware solution, for example, using ASIC (Application Specific Integrated Circuit) or stand-alone logic.

The invention may also be described as the method described in Figure 4. The method begins at step 400. In block 402, a network clock reference clock is maintained in the cellular radio network. In block 404, a time-10 stamp signal is generated in the reference clock. In block 406, the timestamp signal is transmitted over an asynchronous data link from the network element to the base station. In block 408, based on the transmitted time stamp signal, how much the local clock of the base station differs from the time of the reference clock. In block 410, a velocity decay factor is generated for the local clock based on at least one calculated offset. In block 412, the local clock visits are corrected based on the rate-correction factor. In block 414, the required frequencies are generated at the base station using a local clock corrected for rate correction factor. The method ends at block 416.

Although the invention has been described above with reference to the example of the accompanying drawings, it is obvious that the invention is not limited thereto but may vary in many ways within the scope of the inventive idea set forth in the appended claims.

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Claims (12)

A base station in a cellular radio network comprising: - a local clock (330), - which base station is coupled with an asynchronous data transmission connection (312) to a network element (300) comprising a reference clock (302); - which base station (102) comprises means (314) for receiving a timestamp signal (308) transmitted via the asynchronous data transmission connection (308) and means (316) for calculating on the basis of the received timestamp signal (308) how much of the base station (102) local time (332) time deviates from the reference clock (302) time; characterized in that the base station (102) further comprises: - means (322) for generating a speed correction factor (324) for the local clock (330) on the basis of at least one calculated deviation; - means (326) for continuous correction of the thread of the local clock (330) on the basis of the speed correction factor (324); and - a frequency synthesizer (212) for generating the frequencies needed in the base station (102) using the local clock (330) corrected by the speed correction factor (324). Base station according to claim 1, characterized in that the speed correction factor (323) indicates when the local clock (330) supply voltage changes. 3. Base station according to claim 2, characterized by having- ···. the density correction factor (324) compensates for changes that occur slowly in ♦ · V. '.' the properties of the local clock (330). 4. Base station according to claim 3, characterized in that the speed correction factor (324) is controlled more than one gang per year. The base station according to claim 1, characterized in that the base station (102) comprises means (320) for calculating the deviation in the transmission delay for the time stamp signal (324) which has been transmitted: [[[: 30 via the data transfer connection ( 312), for comparison of the deviation with a predetermined limit value; and to conclude that the accuracy of the local clock does not correspond to the level required if the deviation exceeds' *: ** the predetermined limit value. /: 6. Processor for a base station in a cellular radio network, which processor is configured: · receiving a timestamp signal generated in the reference element of the cellular radio network's network clock; - calculating on the basis of the received timestamp signal how much time the local clock time in the base station deviates from the reference clock time 5; characterized in that the processor is configured: - to generate a speed correction factor for the local clock on the basis of at least one calculated deviation; - continuously correcting the local clock operation on the basis of the speed correction factor; and - generating the frequencies needed in the base station using the local clock corrected by the speed correction factor. A method of performing frequency synchronization at a base station of a cellular radio network, which method comprises the following steps: - (406) the timing stamp signal generated in the cellular radio network element reference clock is received at the base station over an asynchronous data transmission connection; - (408) on the basis of the transmitted timestamp signal, it is calculated how much the local clock time in the base station deviates from the reference clock time; ? Characterized by: - (410) a speed correction factor is generated for the local · *: * ·· clock on the basis of at least one calculated deviation; : T: - the clock of the local clock (412) is continuously corrected on the basis of the speed correction factor; and * * · 25. the frequencies needed in the base station are generated (414) by:. to use the local clock corrected by the speed correction factor. • · · * ··· [8. Method according to claim 7, characterized in that the has-. The frequency correction factor (323) indicates when the local clock (330) is feeding. voltage changes. • · · * · | · * 30 9. A method according to claim 8, characterized in that the speed correction factor (324) compensates for changes that occur slowly in the properties of the local clock (330). *. · * ·. Method according to claim 9, characterized in that the speed correction factor (324) is controlled more than one thread per each. *. **: 35 11. A method according to claim 7, characterized in that the deviation in the transmission delay for the time stamp signal transmitted via the data transmission connection is calculated, the deviation is compared with a predetermined limit value; and it is concluded that the accuracy of the local clock does not match the level required if the deviation exceeds the predetermined limit value. A computer software product for performing frequency synchronization in a base station of a cellular radio network, said computer software product comprising software routines for performing the following steps: - a timestamp signal generated in the cellular radio network's network element reference clock is received; 10. on the basis of the received timestamp signal, it is calculated how much time the local clock time in the base station deviates from the reference clock time; characterized in that the computer software product comprises software routines for performing the following steps: 15. a speed correction factor is generated for the local clock on the basis of at least one calculated deviation; - the thread of the local clock is continuously corrected on the basis of the speed correction factor; and - the frequencies needed in the base station are generated by using the local clock corrected by the speed correction factor. · * • * • 44 4 • · · »» · • * * • · • 4 «· · j 444 • · • 4 4 4 · • 4 • · 4 4 4 · * 44 ·. 4 4 4. • 4 • 4 • 44. '. 4 4 4 4 4 · 4 • 44; • · · 4 · • 4 • 4 4 * ·· 4 4 4 '*** • 4 4 4 4 4 4 · * * * • • 4 • · 4 4 · · 4 «• 4' • 44. '' 4 4 4 '.-' 4 4