WO2017162275A1

WO2017162275A1 - A reference signal system for noise reduction.

Info

Publication number: WO2017162275A1
Application number: PCT/EP2016/056264
Authority: WO
Inventors: Christian Elgaard; Anders Carlsson; Henrik Sjöland
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2016-03-22
Filing date: 2016-03-22
Publication date: 2017-09-28

Abstract

A reference signal system (100) for generating multiple reference signals (101, 102) for multiple transceivers comprised in a wireless communication device is disclosed. The reference signal system (100) comprises a reference clock signal generator (110) configured to generate a first reference signal (101) for providing to a first transceiver (121) or a first group of transceivers (131). The reference signal system (100) further comprises a delay element (140) configured to delay the first reference signal (101) with a time delay to generate a second reference (102) signal for providing to a second transceiver (122) or a second group of transceivers (132).

Description

A REFERENCE SIGNAL SYSTEM FOR NOISE REDUCTION

TECHNICAL FIELD

Embodiments herein relate to a reference signal system and method therein. In particular, they relate to noise reduction in a wireless communication device using the reference signal system for generating multiple reference signals for multiple transceivers.

BACKGROUND

Wireless communication devices, e.g. base stations, wireless terminals, mobile stations, usually comprise transceivers which comprise receivers and transmitters for receiving and transmitting radio signals. A transceiver usually requires a reference clock signal for operating. In mobile base stations targeting Multiple Input Multiple Output (MIMO) systems, antenna arrays with multiple antenna elements will be used. Each antenna element is, in many wireless communication systems, e.g. the 5th generation (5G) wireless communications system, likely to have its own transceiver or several antenna elements share a transceiver.

When digitally processing a combined signal from independent transceivers, for instance a transceiver array with 8 x 8 or more transceivers, noise that is uncorrelated between the transceivers can be reduced by a factor of N, where N is the number of receivers. This is because for given uncorrelated noise, the noise power in the combined signal is N times the noise power from each individual receiver, while the combined signal power is N*N times the signal power from each individual receiver, resulting in a relative noise reduction by a factor of N. However, correlated noise will be added and will not be reduced by adding more transceivers. Therefore it is beneficial if noise that is produced in each transceiver is uncorrelated to the noise of the other transceivers. However noise originating from the reference clock signal will be correlated if the same reference clock signal is distributed to all transceivers. A work around could be to produce individual reference clock signal for each transceiver to enjoy the benefit of having uncorrelated noise. However, implementing independent reference clock signals would not be an ideal solution, since then the various transceivers would not be locked to one common reference clock, resulting in problems with phase and frequency synchronization. Further individually generating all reference clock signals can be cumbersome, complex and power consuming. In many cases it is important that all the transceivers in a wireless communication device use the same reference clock signal for various reasons such as phase and frequency synchronization mentioned above.

Another solution is to simply distribute the same reference clock signal to the various transceivers, but then the reference clock signal must have very low phase noise for not becoming a bottleneck for the whole system. This requirement is likely to increase in the future.

There are lots of known methods for reducing or cancelling jitter or phase noise of a reference clock signal. For example, in JP2009200983, a jitter mitigation circuit is disclosed which reduces jitter by delaying an oscillator signal by short time delays equal to an integer number of the oscillator signal periods and then adding together. The target is to cancel periodic jitter from the reference clock signal itself, and the jitter is at high frequencies offset from a carrier frequency. However, in a cellular system evolving more and more antenna elements with corresponding transceivers, the noise at low frequencies offset from the carrier frequency is of importance. The method in JP2009200983 will not help the noise reduction at low frequency offsets, and is therefore not applicable for the wireless communication device with multiple transceivers.

SUMMARY

Therefore it is an object of embodiments herein to provide an improved reference signal system and method therein for generating multiple reference signals for multiple transceivers in a wireless communication device.

According to one aspect of embodiments herein, the object is achieved by a reference signal system for generating multiple reference signals for multiple transceivers in a wireless communication device. The reference signal system comprises a reference clock signal generator configured to generate a first reference signal for a first transceiver or a first group of transceivers. The reference signal system further comprises a delay element configured to delay the first reference signal with a time delay to generate a second reference signal for a second transceiver or a second group of transceivers.

According to another aspect of embodiments herein, the object is achieved by a method in a reference signal system for generating multiple reference signals for multiple transceivers in a wireless communication device. The reference signal system generates a first reference signal from a reference clock signal generator for a first transceiver or a first group of transceivers. The reference signal system further generates an i-th reference signal by delaying one of the first to an (i-1)-th reference signals with a time delay for an i-th transceiver or an i-th group of transceivers, wherein i=2, 3... N, and N is the number of generated reference signals.

According to the embodiments herein, a first reference signal is generated from a single reference clock signal generator. Then other reference signals are generated from the first reference signal using delay elements and distributed to multiple transceivers. In this way, all transceivers are locked to the same reference clock, but still benefit from having uncorrelated noise from the multiple reference signals due to time shifts between the reference signals. Thus, when signals from the transceivers are combined, the effective of phase noise of the reference signals on the transceiver signals may be reduced. Further, by carefully selecting a delay time it may be possible to reduce the effective of phase noise of the reference signals in a desired offset frequency range. Moreover, with this reference signal system, lower requirement on phase noise may be put on the reference clock signal generator and the reference signals, and consequently the reference signal system may not become the bottleneck for the whole system.

Thus, the embodiments herein provide an improved reference signal system and method therein for generating multiple reference signals for multiple transceivers in the wireless communication device, where the effective phase noise from the reference signals can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of embodiments herein are described in more detail with reference to attached drawings in which:

Figure 1 is a general block view of a reference signal system according to embodiments herein.

Figure 2 is an example of a reference signal system according to embodiments herein. Figure 3 is an example of a reference signal system according to embodiments herein. Figure 4 is an example of a reference signal system according to embodiments herein Figure 5 is a block diagram illustrating a wireless communication device in which

embodiments herein may be implemented. Figure 6 is a flow chart showing a method in a reference signal system according to embodiments herein.

Figure 7 is a diagram showing simulation results on phase noise of an original signal and a combined signal according to embodiments herein.

Figure 8 is a diagram showing simulation results on phase noise of an original signal and different combined signals according to embodiments herein.

Figure 9 is a diagram showing simulation results on phase noise of an original signal and different combined signals according to embodiments herein. DETAILED DESCRIPTION

Phase noise in a reference clock signal is generated through stochastic processes and varies with respect to time. It is also possible to see this in the frequency domain; typically the phase noise is measured versus frequencies offset from the carrier frequency. Adding a signal with a delayed version of itself will create nulls in this phase noise spectrum. Delaying a signal by T1 will cause a noise component with a period of 2 x T1 to be in perfect antiphase in the original and delayed signal. The addition of the original and delayed signal then results in cancellation of this noise. Same thing happens for a noise period of 2/3 x T1 , 2/5 x T1 , etc. On the other hand, for noise components with an integer number of periods in T1 , i.e. with periods equal to T1 , ½ x T1 , 1/3 x T1 , etc. the noise adds in phase so that no phase noise improvement occurs. The result, provided that the carrier of the original and delayed signals add in phase, is that at all offset frequencies the effective phase noise resulting from the delay and summation, is less than or equal to the phase noise of the original signal, and at some offset frequencies the noise is cancelled. Therefore, according to the embodiment herein, a reference signal system for generating multiple reference signals for multiple transceivers in a wireless communication device is implemented and shown in Figure 1. Figure 1 is a general view of a reference signal system 100, the reference signal system 100 comprises a reference clock signal generator 110 configured to generate a first reference signal 101 for a first transceiver TRX 121 or a first group of transceivers 131.

The reference signal system 100 further comprises a delay element 140 configured to delay the first reference signal 101 with a time delay T1 to generate a second reference signal 102 for a second transceiver TRX 122 or a second group of transceivers TRX 132. According to some embodiments, the time delay T1 is determined based on an offset frequency range in which the effect of phase noise of the first 101 and second 102 reference signals is to be reduced. The term offset frequency range, when used herein, refers to a frequency range related to the carrier frequency, being located at an offset frequency distance from the carrier frequency.

Increasing the number of signals to be added with different delays will result in more possibilities to shape the phase noise spectrum and to suppress the noise over a desired offset frequency range.

According to some embodiments, when the multiple transceivers comprise a number of N transceivers, the first reference signal 101 is provided to the first transceiver TRX 121 , the second reference signal 102 is provided to the second transceiver TRX 122, and the reference signal system 100 further comprises one or more delay elements 141 , 142... , each delay element is configured to delay one of the first reference signal to the (i-1)-th reference signal with a time delay to generate an i-th reference signal for an i-th transceiver, wherein i=3,... N.

In order to save power and delay elements, a reference signal may be shared by a number of transceivers. So the multiple transceivers may be divided into a number of N groups, the first reference signal 101 is provided to a first group of transceivers TRXs 131 and the second reference signal 102 is provided to a second group of transceivers TRXs 132. The reference signal system 100 further comprises one or more delay elements 141 , 142... , each delay element is configured to delay one of the first reference signal 101 to the (i-1 )-th reference signal with a time delay to generate an i-th reference signal for an i-th group of transceivers, wherein i=3,... N.

An example embodiment, a reference signal system 200 is shown in Figure 2, where a first reference signal 201 is produced and distributed to a first transceiver TRX 221 , then delayed by T1 and distributed to a second transceiver TRX 222, then further delayed by T1 and distributed to a third transceiver TRX 223, and so on. The resulting delays would then be:

Transceiver Reference Clock Signal Delay

1st 0

2nd T1

3rd 2 x T1 4th 3 x T1

Nth (N-1 ) x

Another example embodiment, a reference signal system 300 is shown in Figure 3, where the first reference signal 301 is generated by the reference clock signal generator 1 10 for the first transceiver TRX 321 , the second, third, ... and N-th reference signals 302, 303...30N are produced by delaying the first reference signal 301 with time delays T1 , 2xT1 ,... (N-1 )xT1 for the second, third... and N-th transceiver TRX 322, 323, ...32N respectively.

One further example embodiment, a reference signal system 400 is shown in Figure 4, where the first reference signal 401 is generated by the reference clock signal generator 1 10 for the first transceiver TRX 421 , the second reference signal 402 is produced by delaying the first reference signal 401 with T1 for the second transceiver TRX 422, the third reference signal 403 is produced by delaying the first reference signal 401 with 2xT1 or delaying the second reference signal 402 with T1 for the third transceiver TRX 423, and so on.

There are many different ways to implement the delay elements and the delay time for each reference signal may be different as long as a desired total time delay between the first and last reference signals can be achieved. In the above examples, the delay elements have the same size and have the same delay time of T1 or an integer times T1. However the delay time for each reference signal may be different and may be implemented by one delay element or a different number of delay elements. Each delay element may have different size and therefore may have different delay time.

According to some embodiments, the total time delay between the first and the last reference signals may be determined based on an offset frequency range in which the effect of phase noise of the first to the last reference signals is to be reduced.

Often offset frequencies of interest where the effect of phase noise of reference signals is desired to be reduced are in MHz range, this corresponds to delays on an order of micro seconds (με). Achieving an analog delay long enough without ruining the signal to noise ratio is the tricky part. One option would be to use acoustic devices such as Surface Acoustic Wave (SAW) filters where the signal propagates with the speed of sound through the filter. With filter sizes of a few miliimetres (mm) this delay becomes in the order of micro seconds (MS).

Therefore, according to some embodiments, the delay elements may be acoustic wave devices.

Figure 5 shows a wireless communication device 500, in which a reference signal system 100, 200, 300, 400 according to the embodiments herein may be implemented. The wireless communication system 500 comprises multiple transceivers 510, 511 , 512. The reference signal system 100, 200, 300, 400 generates multiple reference signals for the multiple transceivers in the wireless communication device. The wireless communication system 500 may comprise other units, e.g. a memory 520 and a processing unit 530 for information storage and signal processing etc.

Each transceiver 510, 511 , 512 produces a complex baseband signal, with ln-phase and Quadrature components. The signals from the individual transceivers are typically linearly combined through a weighted sum. The complex weights may be selected from a limited, predefined set, or calculated from pilot symbols in the received signals.

Correspondingly, when transmitting, the individual baseband signals to the transmitters may be derived from a common baseband signal and a complex weight. The complex weights may be implemented through analogue phase shifters and/or variable gain amplifiers under control of a stored-program in the processing unit 530, or the weighting can be performed digitally in a stored-program in the processing unit 530.

Therefore according to some embodiments herein, in the wireless communication device 500, the processing unit 530 is configured to constructively combine a plurality of transceiver signals from the multiple transceivers. When a plurality of transceiver signals are combined constructively, due to the multiple reference signals having different time shifts, which prevents a constructive summation of the reference signal related phase noise of the plurality of transceiver signals, noise may be reduced.

The delay and addition of signals corresponds to a finite impulse response (FIR) filter which has notches at some frequencies. Without phase compensation, this filter may have notches at the receive or transmit signal frequencies, and the desired component of the signals may thus be suppressed. However, when used in a MIMO or beamforming system, a stored-program in the processing unit 530 or analogue phase shifters may adjust the phase shifts of the radio frequency (RF) carriers of the antenna elements or their corresponding transceivers so that the desired component of the signals will add up constructively. The adjustment of phase shifts, or pre-coder weights, beamformer weights etc. can lead to constructive summation regardless of the delay distribution even if/when the delays are not multiples of the period of the reference clock signal. Signal degradation is thereby

automatically counteracted by the system and does not need to be considered when designing the reference signal delays.

Therefore according to some embodiments, the wireless communication device 500 is configured to adjust phase shifts of radio frequency carriers of the multiple transceivers, either by the processing unit 530 or by analogue phase shifters.

Corresponding embodiments of a method in a reference signal system 100, 200, 300, 400 for generating multiple reference signals for multiple transceivers comprised in a wireless communication device will now be described with reference to Figure 6. As mentioned above, the reference signal system 100, 200, 300, 400 comprises a reference clock signal generator 1 10 and one or more delay elements. The method comprises the following actions.

Action 601

The reference signal system 100, 200, 300, 400 generates a first reference signal 101 ,

201 , 301 , 401 from the reference signal generator 1 10 for providing to a first transceiver 121 or a first group of transceivers 131 .

Action 602

The reference signal system 100, 200, 300, 400 generates an i-th reference signal by delaying one of the first to an (i-1 )-th reference signals with a time delay for providing to an i- th transceiver 12i or an i-th group of transceivers 13i, wherein i=2, 3... N, and N is the number of generated reference signals. According to some embodiments, a total time delay between the first and the last reference signals may be determined based on an offset frequency range in which the effect of phase noise of the first to the last reference signals is to be reduced.

According to some embodiments, the delaying may be achieved by an acoustic wave device, i.e. the delay elements may be acoustic wave devices, such as SAW filters. Reference clock signals are usually provided to a phase locked loop (PLL) of a frequency synthesizer in the transceivers. The reference signal noise is therefore amplified by a noise amplification factor of 20log(fout/fref ) inside the loop bandwidth, wherein fout is the output signal frequency from the frequency synthesizer and fref is the reference signal frequency. In RF hardware implementation for 5G system, the maximum RF frequency may be as high as about 28 GHz, i.e. fout=28 GHz. This noise amplification factor would be too large to use conventional reference signal frequencies, such as 26 MHz. Increasing crystal oscillator reference clock frequency from a typical 26 MHz up to about 0.5 GHz may be a way in order to reduce the reference clock signal noise contribution. However using the proposed method according to the embodiments herein, since the effective noise of the reference signal could be reduced, a reference clock signal with low frequency could still be used, i.e. the frequency of the reference clock signal generator 110 may still be 26 MHz.

The reference signal systems 100, 200, 300, 400 above are just some examples of how delays can be introduced to distribute the reference signals. By adding more delay elements and selecting their time delays one can select an offset frequency band in which to suppress noise. For instance in a CMOS radio circuit for 5G system, the PLL bandwidth may be chosen to be about 5MHz. Simulations have been performed to investigate the performance of the proposed technique. Figure 7 shows simulation results on phase noise of an original signal and a combined signal, where curve 701 shows the phase noise of the original signal, curve 702 shows the phase noise of the combined signal, where the original signal is combined with a delayed version of itself, and the original signal has been delayed using a transmission line implemented with 800 segments.

Figure 8 shows another simulation results on phase noise of an original signal and two different combined signals, wherein curve 801 shows the phase noise of the original signal, curve 802 shows the phase noise of the combined signal when using two delay elements of totally 3.12 us of delay, i.e. Tdelay=3.12 us, curve 803 shows the phase noise of the combined signal when using four delay elements but with the same total delay. Notice that with four equal delay elements one can significantly suppress phase noise over a bandwidth of about 2/Tdelay, i.e. at about 200 kHz to 800 kHz from the carrier frequency. Figure 9 shows another simulation results on phase noise of an original signal and two different combined signal, wherein curve 901 shows the phase noise of the original signal, curve 902 shows the phase noise of the combined signal when using 17 delay elements of totally 2.08 us of delay, curve 903 shows the phase noise of the combined signal when using also 17 delay elements but with a total delay of 4.17us. In curve 902, the phase noise has been reduced in the range of ~0.2MHz - ~3 MHz from the carrier frequency and in curve 903 it has been reduced in the range of ~0.3MHz— 7 MHz from the carrier frequency.

To summarise the discussions above, advantages of the reference signal system 100, 200, 300, 400 according to embodiments herein include:

By using delay elements when distributing a reference clock signal to multiple transceivers, all signals from the multiple transceivers are locked to the same reference clock signal, but still benefitting from suppressed effective reference noise;

The delay elements may for instance be implemented by using acoustic wave filters;

By carefully selecting delay element time delays it is possible to reduce the effect of reference signal phase noise in a desired offset frequency band.

The embodiments herein are not limited to the above described embodiments. Various alternatives, modifications and equivalents may be used. When using the word "comprise" or "comprising" it shall be interpreted as non- limiting, i.e. meaning "consist at least of.

Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims.

Claims

1. A reference signal system (100, 200, 300, 400) for generating multiple reference

signals for multiple transceivers in a wireless communication device (500), comprising: a reference clock signal generator (110) configured to generate a first reference signal (101 , 201 , 301 , 401) for a first transceiver (121 , 221 , 321 , 421) or a first group of transceivers (131); and

a delay element (140, 240, 340, 440) configured to delay the first reference signal (101 , 201 , 301 , 401) with a time delay, T1 , to generate a second reference signal (102, 202, 302, 402) for a second transceiver (122, 222, 322, 422) or a second group of transceivers (132).

The reference signal system (100, 200, 300, 400) according to claim 1 , wherein the time delay, T1 , is determined based on an offset frequency range in which an effect of phase noise of the first and second reference signals is to be reduced.

The reference signal system (100, 200, 300, 400) according to claims 1 or 2, wherein the multiple transceivers comprise a number of N transceivers, wherein the first reference signal (101) is provided to the first transceiver (121), the second reference signal (102) is provided to the second transceiver (122), and wherein the reference signal system (100, 200, 300, 400) further comprises one or more delay elements, each delay element is configured to delay one of the first reference signal (101) to the (i-1)-th reference signal with a time delay to generate an i-th reference signal for an i- th transceiver (12i), wherein i=3,... N. 4. The reference signal system (100, 200, 300, 400) according to claims 1 or 2, wherein the multiple transceivers are divided into a number of N groups, wherein the first reference signal (101) is provided to a first group of transceivers (131) and the second reference signal (102) is provided to a second group of transceivers (132), and wherein the reference signal system (100, 200, 300, 400) further comprises one or more delay elements (141 , 142), each delay element is configured to delay one of the first reference signal (101) to the (i-1)-th reference signal with a time delay to generate an i-th reference signal for an i-th group of transceivers (13i), wherein i=3,... N.

The reference signal system (100, 200, 300, 400) according to any one of claims 3-4, wherein a total time delay between the first and the last reference signals is determined based on an offset frequency range in which the effect of phase noise of the first to the last reference signals is to be reduced.

6. The reference signal system (100, 200, 300, 400) according to any one of claims 1 -5, wherein the delay elements are acoustic wave devices.

7. A wireless communication device (500) comprising a reference signal system (100, 200, 300, 400) according to any one of claims 1-6 for generating multiple reference signals for multiple transceivers (510, 51 1 , 512) in the wireless communication device (500).

8. The wireless communication device (500) according to claim 7, wherein the multiple transceivers are configured to combine a plurality of transceiver signals, thereby noise is reduced due to the multiple reference signals having different time shifts, which prevents a constructive summation of the reference signal related phase noise of the plurality of transceiver signals.

9. The wireless communication device (500) according to any one of claims 7-8, being configured to adjust phase shifts of radio frequency carriers of the multiple

transceivers.

10. A method in a reference signal system (100, 200, 300, 400) for generating multiple reference signals (101 , 102; 201 , 202, 203; 301 , 302, 303; 401 , 402, 403) for multiple transceivers in a wireless communication device (500), the method comprising:

generating (601 ) a first reference signal from a reference clock signal generator (1 10) for a first transceiver (121) or a first group of transceivers (131 );

generating (602) an i-th reference signal by delaying one of the first signal to an (i-1 )-th reference signal with a time delay for an i-th transceiver (12i) or an i-th group of transceivers (13i), wherein i=2, 3... N, and N is the number of generated reference signals.

1 1. The method according to claim 10, wherein a total time delay between the first and the last reference signals is determined based on an offset frequency range in which an effect of phase noise of the first to the last reference signals is to be reduced.

12. The method according to any one of claims 10-1 1 , wherein delaying one of the first to an (i-1)-th reference signals with a time delay is achieved by an acoustic wave device.