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Title:
METHOD FOR THE COMBINATION AND SEPARATION OF BASEBAND SIGNALS
Document Type and Number:
WIPO Patent Application WO/2010/000338
Kind Code:
A1
Abstract:
The present invention relates to a sample point adjustment unit adapted to be used for the merging of a plurality of asynchronous baseband unit signals, and to a radio base system comprising such a unit. The invention further relates to a method for the combination of a plurality of baseband signals, when the different baseband units are not synchronised, and to a method for the separation of a plurality of baseband signals, when the different baseband units are not synchronised.

Inventors:
BJOERK VIMAR (SE)
Application Number:
EP2008/058725
Publication Date:
January 07, 2010
Filing Date:
July 04, 2008
Export Citation:
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Assignee:
ERICSSON TELEFON AB L M (SE)
BJOERK VIMAR (SE)
International Classes:
H04L7/00; H03H17/06; H04B1/40; H04W88/10
Domestic Patent References:
WO2005002073A12005-01-06
WO2001073947A12001-10-04
WO1994019869A11994-09-01
WO2000051376A12000-08-31
WO2003049395A12003-06-12
WO1993005598A11993-03-18
Other References:
FAHEEM SHEIKH ET AL: "Improved Factorization for Sample Rate Conversion In Software Defined Radios", CIRCUITS AND SYSTEMS, 2007. ISCAS 2007. IEEE INTERNATIONAL SYMPOSIUM O N, IEEE, PI, 1 May 2007 (2007-05-01), pages 2526 - 2529, XP031181817, ISBN: 978-1-4244-0920-4
ANA RUSU ET AL: "Flexible ADCs for wireless mobile radios", CIRCUIT THEORY AND DESIGN, 2007. ECCTD 2007. 18TH EUROPEAN CONFERENCE ON, IEEE, PISCATAWAY, NJ, USA, 27 August 2007 (2007-08-27), pages 172 - 175, XP031257715, ISBN: 978-1-4244-1341-6
YONGHONG GAO ET AL: "A fifth-order comb decimation filter for multi-standard transceiver applications", CIRCUITS AND SYSTEMS, 2000. PROCEEDINGS. ISCAS 2000 GENEVA. THE 2000 I EEE INTERNATIONAL SYMPOSIUM ON MAY 28-31, 2000, PISCATAWAY, NJ, USA,IEEE, vol. 3, 28 May 2000 (2000-05-28), pages 89 - 92, XP010502474, ISBN: 978-0-7803-5482-1
Attorney, Agent or Firm:
ALBIHNS AB (Göteborg, SE)
Download PDF:
Claims:
CLAIMS

1. Sample point adjustment unit (30) adapted to be used for the merging of a plurality of asynchronous baseband unit signals (BB1, BB2), comprising a phase measuring module (70) adapted to measure the phase difference between a first and a second clock reference (clk1, clk2), a conversion module (71, 80, 81) adapted to convert the phase difference between the clock references to a phase difference signal, characterized in that the sample point adjustment unit further comprises a Farrow filter (72) adapted to re-sample a second baseband signal (BB2) to the time domain of a first baseband signal

(BB1) by using the phase difference signal, and further comprising an interleaver adapted to merge the baseband signals using the first clock reference (clk1).

2. Unit according to claim 1, characterized in that the unit further comprises a multiplier module (76) adapted to multiply the phase difference signal with a frequency adjustment factor (N), and a phase adjusting module (77) adapted to phase adjust the re-sampled second baseband signal using the signal from the multiplier (76) before the first baseband signal is merged with the second re-sampled baseband signal.

3. Unit according to claim 2, characterized in that the second re-sampled baseband signal is phase locked to an RF frequency carrier.

4. Radio base station, comprising at least one sample point adjustment unit (30) adapted to be used for the merging of a plurality of asynchronous baseband unit signals (BB1, BB2), comprising a phase measuring module (70) adapted to measure the phase difference between a first and a second clock reference (clk1 , clk2), a conversion module (71, 80, 81) adapted to convert the phase difference between the clock references to a phase difference signal, characterized in that the sample point adjustment unit further comprises a Farrow filter (72) adapted to re-sample a second baseband signal (BB2) to the time domain of a first baseband signal

(BB1) by using the phase difference signal, and further comprising an interleaver adapted to merge the baseband signals using the first clock reference (clk1).

5. Radio base station according to claim 4, characterized in that the station further comprises a multiplier module (76) adapted to multiply the phase difference signal with a frequency adjustment factor (N), and a phase adjusting module (77) adapted to phase adjust the re-sampled second baseband signal using the signal from the multiplier (76) before the first baseband signal is merged with the second re-sampled baseband signal.

6. Radio base station, according to claim 5, characterized in that the second re-sampled baseband signal is phase locked to an RF frequency carrier.

7. Communication system, comprising at least one radio base station having a sample point adjustment unit (30) adapted to be used for the merging of a plurality of asynchronous baseband unit signals (BB1, BB2), comprising a phase measuring module (70) adapted to measure the phase difference between a first and a second clock reference (clk1, clk2), a conversion module (71, 80, 81) adapted to convert the phase difference between the clock references to a phase difference signal, characterized in that the sample point adjustment unit further comprises a Farrow filter (72) adapted to re-sample a second baseband signal (BB2) to the time domain of a first baseband signal (BB1) by using the phase difference signal, and further comprising an interleaver adapted to merge the baseband signals using the first clock reference (clk1).

8. System according to claim 7, characterized in that the system further comprises a multiplier module (76) adapted to multiply the phase difference signal with a frequency adjustment factor (N), and a phase adjusting module (77) adapted to phase adjust the re- sampled second baseband signal using the signal from the multiplier (76) before the first baseband signal is merged with the second re- sampled baseband signal.

9. System according to claim 8, characterized in that the second re-sampled baseband signal is phase locked to an RF frequency carrier.

10. Method for the combination of a plurality of baseband signals, when the different baseband units are not synchronised, comprising the steps of:

measuring the phase difference between a first clock reference and a second clock reference,

converting the phase difference to a phase difference signal,

characterized in that the method further comprises the steps of:

- calculating the signal level for a new sample point of a second baseband signal with reference to the first clock reference using a Farrow filter with the phase difference signal, and

merging the first baseband signal with the second, re-sampled baseband signal using the first clock reference.

11. Method according to claim 10, wherein the method further comprises the steps of: separating, in a received signal, a first baseband signal from a second baseband signal using a first clock reference, and

calculating the signal level for a new sample point of the second baseband signal with reference to the second clock reference using a Farrow filter with the phase difference signal.

12. Method according to claim 10 or 11 , wherein the method further comprises the steps of:

multiplying the phase difference signal with a frequency adjustment factor derived from the relation between the RF carrier and the reference signal intended for the BB2 carrier, and

phase adjusting the re-sampled second baseband signal using the multiplied phase difference signal - frequency adjustment factor before the first baseband signal is merged with the second baseband signal, such that the second re-sampled baseband signal is phase locked to the RF carrier.

13. Method according to claim 10 to 12, wherein an n:th baseband signal is also re-sampled with reference to the first clock reference using a Farrow filter with the delay difference between the first and the n:th clock reference, and where the n:th re-sampled baseband signal is also merged to the first baseband signal using the first clock reference.

14. Method according to claim 10 to 12, wherein two baseband signals are merged.

15. Method for the separation of a plurality of baseband signals, when the different baseband units are not synchronised, comprising the steps of: separating a first baseband signal from a second baseband signal using a first clock reference,

measuring the phase difference between a first clock reference and a second clock reference,

- converting the phase difference to a phase difference signal,

characterized in that the method further comprises the step of:

calculating the signal level for a new sample point of the second baseband signal with reference to the second clock reference using a Farrow filter with the phase difference signal.

Description:
TITLE

METHOD FOR THE COMBINATION AND SEPARATION OF BASEBAND SIGNALS

TECHNICAL FIELD

The invention relates to a method for the combination and separation of baseband signals, when different baseband units are not synchronised. The purpose is to be able to use Multi Standard Radio (MSR) equipment with different standards and at the same time fulfilling the 3GPP standard.

BACKGROUND

Today, there is a rapid development of new mobile network standards. Different standards are evolving for different, separate frequency bands but at the same time, different standards may also be used in the same frequency band. When different standards are to be used in the same frequency band with the same radio unit or communicated over the same communication link in a communication system, such as CPRI (Common Public Radio Interface), some problems may arise.

One possibility to allow different standards in the same frequency band is to assign the different standards to different Base Bands (BB) in the same radio base station. A radio base station that is provided with different BB units and that can handle different standards is often referred to as a Multi Standard Radio (MSR).

There is a demand for having a smooth migration from one standard to another when using an MSR. When different standards are handled by the same MSR, the data streams from different BB units needs on one hand to be combined when transmitting or separated when receiving and on the other hand the data streams must be synchronised. The synchronisation may either be done in an external unit or in the MSR radio unit itself. The synchronisation of the different BB units is very important. When the data streams from two BB units are clocked with different clock rates or when there is a variation in the clock rate of the BB units, i.e. the BB units are not synchronised, there will be a small frequency drift between the two BB units. This frequency drift will cause a bit slip between the two BB units that in turn will affect the radio performance. Data may be lost or may have to be resent or there may be an interruption in voice communication. When such a bit slip is present, the 3GPP standard can not be complied with.

There are different reasons why the synchronisation of the BB units may be difficult. One reason is that the clock reference of a BB unit will inevitably show some frequency drift. Another reason is that different BB units will show different types of drift. Another reason may be that the BB units are supplied by different suppliers and thus have different clock references from the beginning.

One possibility to solve the synchronising of different BB units would be to use an external clock reference that sends a clock reference to both BB units. Since the present BB units already exist and were built before there was a need for synchronising different BB units, they are not equipped with a hardware possibility to synchronise the clocks. This solution therefore necessitates the replacement of all BB units and/or the complete radio base station. Thus, this is not a cost-effective solution.

Since the infrastructure with several radio base stations having BB units already exists, it is advantageous if the radio base stations can be upgraded in such a way that they can implement both the old and the new standard simultaneously. Therefore, a different approach is required.

SUMMARY

It is an object of the invention to provide a sample point adjustment unit adapted to be used for the combination and separation of baseband signals. It is also an object of the invention to provide an improved method for the combination and separation of baseband signals.

More specific, an object of the invention is to provide a sample point adjustment unit adapted to be used for the merging of a plurality of asynchronous baseband unit signals. Such a unit can be used to upgrade existing radio base stations comprising a plurality of baseband units, where it is not possible to synchronize the baseband units. The unit can also be used in radio base stations comprising both synchronized and unsynchronized baseband units. Another object of the invention is to provide a method that allows a combination, when the different baseband units are not synchronised, and also a method for the separation of a plurality of unsynchronized baseband signals.

According to a first aspect of the invention, a sample point adjustment unit adapted to be used for the merging of a plurality of asynchronous baseband units comprises a phase measuring module that will measure the phase difference between a first and a second clock reference. The unit also comprises a conversion module that will convert the phase difference to a phase difference signal, and a Farrow filter that uses the phase difference signal to re-sample a second baseband signal to the time domain of a first baseband signal. The re-sampled second baseband signal is merged to the first baseband signal by an interleaver using the first clock reference.

According to a second aspect of the invention, the method for the combination of a plurality of unsynchronized baseband signals comprises the step of measuring the phase difference between a first clock reference and a second clock reference. The phase difference is then converted to a phase difference signal which is used by a Farrow filter to calculate the signal level for a new sample point of a second baseband signal with reference to the first clock reference. The first baseband signal is thereafter merged with the second, re-sampled baseband signal with an interleaver using the first clock reference. According to a third aspect of the invention, a method for the separation of a plurality of unsynchronized baseband signals comprises the step of separating a first baseband signal from a second baseband signal using a first clock reference. The phase difference between the first clock reference and a second clock reference is measured and the phase difference is converted to a phase difference signal. The phase difference signal is used by a Farrow filter to calculate the signal level for a new sample point of the second baseband signal with reference to the second clock reference.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail in the following, with reference to the embodiments that are shown in the attached drawings, in which

Fig. 1 shows a prior art system for the combination and separation of baseband signals,

Fig. 2 shows a graph of with a bit slip area originating from a system according to Fig. 1 ,

Fig. 3 shows a system for the combination and separation of baseband signals for a first embodiment according to the invention,

Fig. 4 shows a re-sample graph for the inventive system,

Fig. 5 shows a development of the system for the combination and separation of baseband signals according to the invention,

Fig. 6 shows a system with two separate radio units comprising two separate transmitters,

Fig. 7 shows a system with a Multi Standard Radio comprising two separate transmitters, Fig. 8 shows a flowchart for the method of interleaving two signals according to the invention, and

Fig. 9 shows a flowchart for the method of de-interleaving two signals according to the invention.

DETAILED DESCRIPTION

The embodiments of the invention with further developments described in the following text are to be regarded only as examples and are in no way to limit the scope of the invention. In the following description, a radio base unit comprised in a telecommunication system of the 3G-UMTS (3rd Generation - Universal Mobil Telecommunications System) type is used as an example of a radio base unit, and the abbreviations used are related to such a system using the 3GPP (3G Partnership Project) definitions. It should be understood that the invention can be applied to radio base stations of other types of communication systems such as a 4G system and the like, and that the definitions used should not limit the scope of the invention.

An example of a prior art system for the combination and separation of baseband signals is shown in fig. 1. This system comprises a first baseband unit 1 , a second baseband unit 2, a data merging/separation unit 3 and a radio-frequency unit 4 connected to an antenna 5. The first baseband unit, the second baseband unit, the data merging/separation unit and the radio- frequency unit corresponds to a multi standard radio unit, i.e. a radio base station. The radio base station is comprised in a communication system.

The first baseband unit 1 comprises a data signal input/output port 10 for communication with a provider. Through this port, data is transmitted to e.g. a subscriber from a provider is received, and data received from e.g. a subscriber is sent to the provider. An example of such a data signal is shown as signal 13. The first baseband unit further comprises a clock reference output port 11 which sends out an internal clock reference clk1. Further comprised is a data signal input/output port 12 that outputs the baseband signal BB1 to the radio-frequency unit. Through port 12, a data signal that is to be transmitted to e.g. a subscriber is sent, and data received from e.g. a subscriber is sent to the provider.

A baseband unit is used to pre-process the data to be transmitted from the radio base station. This includes modulation of the data on a so-called baseband signal. The type of modulation used for modulation in this case is dependent on the chosen mobile communication standard. Examples of communication standards such as these are WCDMA/UMTS, GSM, EDGE, Bluetooth etc.

The radio-frequency unit 4 which is connected to the port 12 of the baseband unit 1 modulates the signal to be transmitted onto a carrier signal at a desired output frequency. The signal is suitably amplified and is then emitted via antenna 5.

The second baseband unit 2 comprises a data signal input/output port 20 for communication with the same or another provider. Through this port, a data signal that is to be transmitted to e.g. a subscriber is received, and data received from e.g. a subscriber is sent to the provider. An example of such a data signal is shown as signal 23. The second baseband unit further comprises a clock reference output port 21 which sends out an internal clock reference clk2. Further comprised is a data signal input/output port 22 that outputs the baseband signal BB2 to the radio-frequency unit. Through port 22, data that is to be transmitted to e.g. a subscriber is sent, and data received from e.g. a subscriber is sent to the provider.

Since the radio base station should be able to handle both baseband units, a data merging/separation unit 3 is also comprised in the system. The data merging/separation unit comprises a first clock input 31 , a second clock input 32, a clock output 33, a first data signal input/output port 34, a second data signal input/output port 35 and a third data signal input/output port 36. Further comprised is a data module 37 that merges the data signals from the baseband 1 and baseband 2 and sends the combined signal 39 to the radio- frequency unit when in transmitting mode, and separates the received signal into two separate data signals when in reception mode. Also comprised is a clock selection module 38, which selects the clock reference that is to be used during transmission and also by the data merging/separation module. One clock reference is selected to be used as a master clock reference, and this clock reference is then used during the entire sequence.

Since only one of the clock references is used as a master clock reference, a problem arises if the two baseband units, i.e. the two clock references clk1 and clk2, are not synchronised. Even a small frequency drift between the two units will cause a bit slip between the data signals of the two units. This will affect the radio performance in a negative way.

Fig. 2 shows a graph in the time domain, in which a first signal 60 with amplitude A is sampled with the first clock reference clk1 , e.g. the signal and the clock reference of baseband unit 1. A second signal 61 , originally having a different clock reference, is likewise sampled with the first clock reference clk1. When the two clock references are not synchronised, a bit slip will occur, indicated as a bit slip area 62 with the time difference Δt between the two clock references. When the clock references have drifted one clock cycle, i.e. when Δt corresponds to one clock cycle, a bit will be lost.

This means that there will e.g. be a disruption in the data flow for the merged, transmitted data signal or data may be lost completely, which may lead to a request to resend or that the transmission is aborted and must be initiated again. The end result is that the prescribed standard for the radio system can not be complied with.

Thus, the inventive method incorporates a way for two asynchronous baseband signals to be interleaved and transmitted over the same bus to the same radio frequency unit. Further, two carriers can also be individually phase locked to two different data streams. The invention can be used both for the combination of two or more baseband signals as well as for the separation of two or more baseband signals.

In a first embodiment of the invention, the sampling position for one of the baseband signals is changed to the clock domain of the other baseband signal. One of the baseband signals will thus be re-sampled with the clock reference of the other baseband signal.

This means that one baseband signal, which uses one clock reference, will be re-sampled with the clock reference of the other baseband signal. In this way, both baseband signals will be using the same clock reference, and the effect will be the same as if the two basebands were synchronized. In the shown example, this means that the baseband BB2 would be re-sampled with the clock reference clk1 of baseband signal BB1. The re-sampling of the BB2 signal with another clock reference can not be performed in the existing system.

The transfer of the sampling position for one of the baseband signals to the clock domain of the other baseband signal is done using a Farrow poly phase filter. A Farrow filter is a multi-rate filter structure which offers the option of a continuously adjustable resample ratio. Such a filter can be used for fine- tuning the sampling positions of different signals, e.g. when an input signal has time-varying delays, and is well-known to the skilled person.

Such a re-sampling is possible for all over-sampled time domain transmitted signals where the over-sampling is larger than the frequency offset between the systems. Normally, an OFDM (Orthogonal Frequency Division Multiplex) signal time domain transmission is slightly over-sampled. This is the case in for example LTE (Long Term Evolution) and Wimax (Worldwide Interoperability for Microwave Access). Typical, a WCDMA TX BB signal is not over-sampled and needs to be up-sampled before a re-sampling with another clock reference can be done. Phase locking of the transmitted data to the carrier is also possible to obtain, which as an example is a requirement of the 3GPP WCDMA radio specification.

Fig. 3 shows base radio station to be used in a telecommunication system comprising an inventive sample point adjustment unit 30 replacing the data merging/separation unit 3 of Fig. 1. The sample point adjustment unit is used when signals from two different baseband units are to be merged in order to use the same radio transmitter. An existing radio base station can be upgraded by using an inventive sample point adjustment unit in order to allow the radio base station to handle different communication standards.

As described above, the first baseband unit 1 comprises a data signal input/output port 10, a first clock reference output port 11 and a data signal input/output port 12. Through port 12, a data signal that is to be transmitted to e.g. a subscriber is sent. The second baseband unit 2 likewise comprises a data signal input/output port 20, a second clock reference output port 21 and a data signal input/output port 22. Through port 22, data that is to be transmitted to e.g. a subscriber is sent.

The sample point adjustment unit 30 comprises a phase measurement module 70 that measures the phase difference between the first clock reference and the second clock reference. The result is filtered in a filter module 71 in order to minimize phase noise contribution from the measurement. The filtered phase difference between the clock references is used to control the frequency of a numerically controlled oscillator (NCO) 80. In this way, the NCO will follow the phase and the frequency of the BB2 clock reference. The signal from the NCO is forwarded to a subtract function 81 , in which the BB1 clock reference frequency is subtracted from the frequency signal from the NCO. The resulting signal represents the phase difference between the two clock reference signals, and will be referenced to as the phase difference signal. The phase difference signal is used to set a delay value for the Farrow filter 72. The Farrow filter will, based on the phase difference, calculate the signal level for the new sample points for the data from baseband unit 2. The re-sampled data signal from the baseband unit 2 is temporarily stored in the clock domain buffer 73 after the signal is re- sampled by the Farrow filter using the first clock reference clk1 and the calculated phase difference signal.

The re-sampled data signal sent from the Farrow filter module, i.e. the data signal from baseband unit 2, is now in the same clock domain as the data signal from baseband unit 1. The re-sampled data signal is transferred from the buffer to the interleaver module with a write strobe signal (dashed line) controlled from the Farrow filter module. Since both signals are now related to the first clock reference, they can be merged in the interleaver module 74 without any problems with bit slip. The resulting signal is forwarded to the radio frequency amplifier and transmitter. The signals from the two baseband units can now be transmitted in a synchronized manner.

The inventive unit can also be used for de-interleaving of received data. When a received data signal is to be separated into two data signals for two baseband units having unsynchronized, i.e. different, clock references, one of the signals is re-sampled to the clock domain of that baseband unit, when the received signal is in the clock domain of the other baseband unit. In the example shown in Fig. 3, one of the received signals is to be re-sampled into the time domain of the second baseband unit.

The received signal is in this example received by using the clock reference clk1 of the first baseband unit. The received signal is separated in the interleaver module 74 into a first baseband unit signal and a second baseband unit signal. Since the interleaver uses the clock reference clk1 of the first baseband unit, the data signal of the first baseband unit 1 is sent directly to the data signal input/output port 12. The data signal of the second baseband unit is sent to a receiving Farrow filter module 79, where the second baseband unit signal, which still is related to the first clock reference, is re-sampled into a signal in the time domain of the second baseband unit using the second clock reference. The re-sampling uses the phase difference signal from the subtract function 81 to set the delay in the Farrow filter module, which will calculate the signal level for the new sample points for the second baseband unit signal. The re-sampled signal is temporarily stored in the second clock domain buffer 78 before it is sent to the data signal input/output port 22 using a write strobe. The data signal is now in the time domain of the second clock reference. In this way, no information will be lost due to bit slip.

Fig. 4 shows a graph of a re-sampling of the data signal BB2 from baseband unit 2 with the clock reference clk1 of baseband unit 1 using a Farrow filter. The signal has the amplitude A. The crosses in the graph show the re- sampled amplitude values representing the signal BB2 from baseband unit 2 in the first clock reference clk1 time domain.

In some cases, there may be requirements that the radio base station transmit frequency and the assigned frequency for the transmitted data should originate from the same source, e.g. as required by the 3GPP WCDMA radio specification. This means that the same source shall be used for the radio frequency and the data clock generation.

If this is the case, the transmission radio frequency should be phase locked to the original data clock reference. The sample point adjustment unit of Fig. 3 with an optional phase locking module 75 is shown in Fig. 5.

The phase difference between the first clock reference clk1 and the second clock reference clk2 that originates from the subtract function 81 , i.e. the phase difference signal, is in this embodiment multiplied in a multiplier 76 with a factor N. N is a factor that corresponds to the relation between the RF carrier and the reference signal intended for the BB2 carrier. N is normally the quotient between to integers, but also irrational numbers are possible.

The frequency adjustment factor N is derived as described below. In Fig. 6, an example with two separate radio units comprising two separate transmitters is shown. The system comprises a first de-interleaver 90 having a first digital data input signal, here BB1 is used as an example of an input signal, and a second de-interleaver 91 having a second digital data input 5 signal, here BB2 is used as an example of an input signal. Both signals are synchronized and use the same clock reference clk1. The signals are phase locked using one PLL each and are modulated in a first RF modulator 92 and a second RF modulator 93, respectively.

In the shown system, the carrier frequency of a transmitter is locked to the 10 individual data stream of that transmitter. Both the data signals and the RF signals of the system use the same clock reference clk1. In the system, two separate RF signals are generated, where the clock reference of each individual data signal is used as a reference for the RF frequency of that transmitter. In this example, the clock reference used is the same for both 15 data signals, i.e. the different BB units are synchronised. From this follows that the relative frequency accuracy is the same for both the data and the RF signal since the RF frequency is generated from the data frequency reference, i.e. the clock reference.

The relative RF frequency accuracy F2 rf acc for carrier 2 is given by:

Of) p"2 — N2 * J fl err _ J fl ^ err rf _ acc _ * , ,

J nom J nom

The relative data frequency accuracy F2 data acc for carrier 2 is given by:

fl pro data _ acc = r err

J nom

The relative frequency accuracy is thus the same for both the data and the RF signal. In this example, both the data and the RF signal is using the same 25 clock reference which is required in many radio standards. In Fig. 7, a radio unit having a Multi Standard Radio (MSR) with two separate transmitters, where the carrier frequency is locked to the clock reference of one of the data streams, is shown.

The system comprises a single de-interleaver 95 having a digital data input signal, which is a combination of two digital data input signals. In the shown example, theses signals correspond to the baseband signals of BB1 and

BB2. Both signals are at this stage synchronized using the clock reference clk1. The de-interleaved signals are phase locked using one PLL each and are modulated in a first RF modulator 92 and a second RF modulator 93, respectively.

In this system, the baseband units are unsynchronised and the incoming data signals use different clock reference signals. As seen in the system, the signal of BB2 is re-sampled to the time domain of BB1 , i.e. the BB2 signal uses the clk1 clock reference, and are interleaved in an earlier stage. The resulting signal is the input signal of de-interleaver 94.

In this example, the relative RF frequency accuracy F2 rf l acc for carrier 2 is given by:

P"y N τ 2 * J f\ err J f\ err rf _l _ acc ~ N2 % , - —.

J nom J nom

The relative data frequency accuracy F2 data acc for carrier 2 is, as above, given by:

pro data acc — err

J nom

The relative frequency accuracy is thus not the same for the data and the RF signal.

The difference in RF frequency is: RF err = F2 rfJ - F2 rf = N2 * (/_ + fl err ) - N2 * (/_ + f\ err )

= N2 * (f2 err - fl err )

By extracting N2 from the above equation, a measure of the relation between the RF carrier and the reference signal intended for the BB2 carrier is obtained. This value can be used to phase-lock the BB2 signal to the RF carrier.

The phase difference signal multiplied with N is now used to frequency adjust the re-sampled BB2 data signal. The resulting signal from the multiplier 76 is used to frequency adjust the re-sampled data signal BB2 from baseband 2 in a frequency adjust module 77. This frequency adjustment will allow the transmitted data signal to be phase locked to the RF carrier which in turn means that the transmitted signal will comply with the 3GPP standard.

The inventive system makes it possible to combine different baseband units from different vendors in a multi standard telecommunication system. At the same time, it is possible to comply with required standards.

An example of the sample point adjustment procedure for a transmitting system will now follow, with reference to Fig. 8.

In a first step 101 , the phase difference between the first clock reference clk1 and the second clock reference clk2 is measured. The resulting phase difference value is filtered in a second step 102 in order to minimize phase noise contribution from the measurement.

In a third step 103, the phase relation between clk1 and clk2 is converted to a phase difference signal using an NCO and a subtraction function, which subtracts the clock reference from the NCO signal. The resulting phase difference signal is used to set the delay in a Farrow filter.

In a fourth step 104, the Farrow filter calculates the signal levels for the new sample points of the BB2 signal using the phase difference signal and the BB2 signal. The resulting signal, i.e. the BB2 signal having the clock reference clk1 , is temporarily stored in a buffer.

In a fifth step 105, the two signals BB1 and BB2, which are now in the same clock domain of clk1 , are merged together in an interleaver.

The resulting signal is forwarded to a radio frequency unit, where it is amplified and transmitted in a sixth step 106.

Alternatively, the phase difference signal calculated in the fourth step 104, is not sent directly to the interleaver, but is first phase adjusted in a step 104a. In step 104a, the re-sampled BB2 data signal is phase adjusted. To do this, the phase difference signal of step 103 is multiplied in a step 103a with a frequency adjustment factor N, which is a measure of the relation between the RF carrier and the reference signal intended for the BB2 carrier. The resulting signal, i.e. the phase difference signal multiplied with N, is used in step 104a to phase adjust the re-sampled BB2 data signal from step 104. The resulting signal will be phase locked to the BB2 carrier. This will allow the signal to follow the 3GPP standard. The resulting signal can then be sent to the interleaver in step 105 in order to be merged with signal BB1.

It should be understood that the described method can be used when more than two unsynchronised baseband signals are to be merged or separated. When e.g. a third baseband signal should also be merged, the phase difference between the first clock reference and the third clock reference is measured and transferred into a delay difference. The third baseband signal is then re-sampled with reference to the first clock reference using a Farrow filter with the delay difference between the first and the third clock reference. Thereafter, the third re-sampled baseband signal is also merged to the first baseband signal, together with the second re-sampled baseband signal, using the first clock reference. The same method can be used with any chosen number n of basebands units. When separating signals for more than two different basebands, every n:th sample of the received signal will be re- sampled and forwarded to each baseband unit. For a system having three baseband units, every third sample will be re-sampled in each of the baseband clock references.

An example of the sample point adjustment procedure for a receiving system will now follow, with reference to Fig. 9.

In a first step 201 , the phase difference between the first clock reference clk1 and the second clock reference clk2 is measured. The resulting phase difference value is filtered in a second step 202 in order to minimize phase noise contribution from the measurement.

In a third step 203, the phase relation between clk1 and clk2 is converted to a phase difference signal using an NCO and a subtraction function, which subtracts the clock reference from the NCO signal. The resulting phase difference signal is used to set the delay in a Farrow filter.

In a fourth step 204, the radio signal is received in a receiver and forwarded to the de-interleaver. In the de-interleaver, the signal is divided onto a first signal and a second signal. The de-interleaver uses the clock reference clk1 of the first signal. The second signal must thus be transformed to the time domain of the second baseband unit BB2.

In a fifth step 205, the Farrow filter calculates the signal levels for the new sample points of the BB2 signal using the phase difference signal of step 203 and the clock reference clk1. The resulting signal, i.e. the BB2 signal having the clock reference clk2, is temporarily stored in a buffer.

In a sixth step 206, the resulting signal is forwarded to the second baseband unit and from there to a user. The two signals are now divided into to different signals having different clock references.

The invention is not to be regarded as being limited to the embodiments described above, a number of additional variants and modifications being possible within the scope of the invention. The references to a specific communication standard used in the description should not limit the scope, but it should be understood that the principle will apply to different communication standards.