GB2352596A

GB2352596A - DC offset correction in radio receivers

Info

Publication number: GB2352596A
Application number: GB0012981A
Authority: GB
Inventors: Paul Mcmahon
Original assignee: Simoco International Ltd
Current assignee: Simoco International Ltd
Priority date: 1999-05-28
Filing date: 2000-05-26
Publication date: 2001-01-31
Also published as: GB0012981D0; WO2000074231A1; GB9912586D0; AU4939000A

Abstract

This invention relates to the removal of a DC offset from a received radio signal. A radio receiver has means for demodulating a received radio signal, means for converting it to digital data and digital signal processing means for estimating a DC offset in the digital data from the digital data and for applying a correction to the digital data on the basis of the estimated DC offset. The DC offset is therefore estimated and corrected using digital data, allowing more sophisticated methods of measuring and removing DC offset to be implemented more straightforwardly. The system may operate on bursts or packets of data, with the offset estimation taking the average signal level of each burst. A new offset may be calculated for each burst, or alternatively the same offset may be used for subsequent bursts or until, e.g., the received signal originates from a different source.

Description

2352596 RADIO RECEIVERS This invention relates to an apparatus for and a

method of processing a received radio signal and in particular to an apparatus for and a method of removing a DC component of the received signal.

Radio transmissions generally comprise a modulated radio frequency carrier signal. Such signals are demodulated at a receiver to remove the carrier signal and retrieve the modulating baseband signal from which the carried information can then be extracted. In an analogue system, for example, the modulated baseband signal can be translated by straightforward analogue signal processing to an acoustic signal. Where digital encoding is used, more complex signal processing may be required to decode the carried information.

When a radio signal is demodulated it can in practice include an erroneous residual DC (direct current) component, which component is often referred to as a DC offset. The DC offset typically manifests as a relatively long term average DC level in the demodulated signal (whereas in an ideal signal the long term average DC level should be zero).

This DC off set may be due to errors in the signal introduced by the demodulation process itself, e.g.

because the process is inaccurate or the components of the radio receiver used for demodulation of a received signal or for signal processing have degraded over time.

A DC of f set can also be introduced by errors in the content of the received signal itself, which errors could be due e.g. to interference and/or environmental conditions such as the weather. In practice, the absolute value of the DC offset is usually dependent on the strength of the radio signal such that a higher strength signal will tend to have a larger absolute value of DC offset.

2 It is desirable to avoid there being any DC offset in a demodulated radio signal, because any DC offset can adversely affect subsequent signal processing, the quality of the reproduced received signal and the sensitivity of the receiver, thereby degrading the overall performance of the radio receiver. Furthermore, in digital radio receivers, removal of DC offset can prevent unnecessary processing of redundant digital information caused by the erroneous DC signal component.

In a known method to minimise DC offset remaining in a demodulated radio signal, the mean signal level of the baseband received signal is calculated and subtracted from the demodulated received signal prior to any further signal processing steps (such as analogue to digital conversion, digital signal processing, and/or decryption of the signal). This method basically works on the assumption that the mean baseband signal level should be zero, so that calculated mean signal can give a reasonable approximation of the DC offset. However, existing techniques for doing this method are implemented in hardware and are relatively complex, costly and inflexible.

The Applicants have also recognised that this method can in some circumstances introduce further errors into the received signal. For example, where there is a relatively rapid change in received signal strength, and correspondingly DC offset, in a time period which is short with respect to the time period over which the mean signal strength is calculated, the calculated mean signal level will take a while to adjust to the new.mean signal strength. Thus where a small signal follows a large signal the calculated DC offset correction 'applied to the small signal will be too large in the period shortly after the drop in signal strength, thereby introducing errors into the signal. Likewise, the calculated mean signal level will not be sufficient to remove fully the DC offset for a short time after a large signal follows a small signal.

This problem cannot be solved by simply reducing the time period over which the mean signal level is calculated so that it changes more quickly, as the mean signal level must be calculated using a time period long enough for the calculated mean to be insensitive to variations in the signal caused solely by the information carried to avoid removing actual information from the signal.

Relatively rapid changes in signal strength can arise in particular in mobile radio communication systems, e.g. as mobile radio receivers move around, and/or switch to monitor or receive from other base stations. They can also arise where a radio receiver is receives on a TDMA (Time Division Multiple Access) multiplexed radio channel signals of different levels on adjacent time slots. This could be the case, for example, for a base station of a TDMA radio system such as the GSM (General System for Mobile communications) and TETRA (TErrestrial Trunked RAdio) systems which may continually receive signals from different mobile units in adjacent TDMA timeslots. The change in signal strength and thus DC offset between successive TDMA timeslots may therefore be virtually instantaneous.

Thus the problem of signal loss due to DC offset removal calculated by a long term moving average is particularly acute in mobile and TDMA radio systems..

According to a first aspect of the present invention there is provided a radio receiver having means for demodulating a received radio signal, means for converting it to digital data and digital signal processing means for estimating a DC offset in the digital data- from the digital data and for applying a correction to the digital data on the basis of the estimated DC offset.

According to a second aspect of the present invention, there is provided a method of processing a received radio signal, comprising demodulating the received radio signal, converting it to digital data, estimating a DC offset in the digital data based on the digital data and applying a correction to the digital data on the basis of the estimated DC offset.

In these aspects of the present invention, the received signal is demodulated and converted to digital data and the DC of f set in the signal is then estimated and corrected for by analysing the digital data. As a digital signal can be analysed in software, the need for relatively complex hardware is avoided. The present invention generally allows more sophisticated methods of measuring and removing DC offset to be implemented, more straightforwardly. The invention thus provides a more flexible technique for DC offset correction, which is better able to compensate for both short and long term variations in DC offset, such as changing reception conditions, long term DC drift (e.g. caused by ageing or environmental changes), and rapidly changing DC characteristics such as may occur when looking at signals from different sources in quick succession.

Thus where the present invention is used in digital radio receivers (in which the transmitted information is encoded in a digital form), instead of DC offset being removed before the signal is converted to digital data, DC offset is removed after conversion. This has the further advantage that any DC level introduced during analog to digital conversion, or by an analog to digital converter, will be taken account of by the digital signal processor.

The digital data may comprise any form suitable for the application for which it is to be used. However, particularly where the received signal contains successive identifiable discrete signal portions (e.g.

individual time slots on a TDMA radio channel, packets of data, or signal bursts (a burst being a continuously received signal from a single source)), it is preferable for the digital data to comprise discrete digital data portions. The digital data portions may be arbitrary, e.g. determined by the digital signal processor independently. However, it is preferable for the digital data portions to correspond to inherent identifiable discrete signal portions of the received signal.

For example, in a TDMA radio system, the digital data portions may correspond to individual or groups of timeslots of the TDMA signal. The digital data portions may each correspond to a signal burst from a given transmitter in the received signal i.e. a signal received continuously from the same source, with the end of the burst usually being defined by a change in the signal source. (In a TETRA system it is common for each timeslot received at a base station to also be a signal burst, i.e. each timeslot is a signal from a different radio unit.) In another example, the digital data portions may correspond to data packets in the received signal. In all of these examples it is between these inherent identifiable discrete signal portions of the received signal that it is common for DC offset to change significantly. Thus, by considering the received signal in these signal portions or such digital data portions, e.g. by estimating the DC offset in the digital data portions and applying a correction to the digital data portions, the changing DC offset between the signal portions of the received signal can be dealt with more effectively.

According to a third aspect of the present invention, there is-therefore provided a radio receiver for use in a radio communications system, comprising means for demodulating received radio signals, means for dividing a received signal into successive discrete signal portions, means for estimating the DC offset of a single signal portion and means for applying a DC offset correction to a single signal portion.

According to a f ourth aspect of the present invention, there is provided a method of processing received radio signals, comprising dividing a received signal into successive discrete signal portions, estimating a DC offset for each signal portion individually, and applying a DC offset correction to each signal portion individually.

The DC offset can be estimated and corrected as desired. For example, a component of the signal or data caused by DC offset can be estimated. The DC offset correction can also be derived and applied as desired. It is preferably applied so as to try to remove the estimated DC offset.

In particular, where the received signal is treated as a series of successive discrete signal or data portions, a separate DC offset is preferably estimated for each portion. The DC offset estimate for each portion could be determined as desired, e.g. by calculating the mean signal level of the portion. Only a small section of the portion need be analysed if desired, e.g. where the signal level is expected to be constant. However, preferably the DC offset estimate is derived using the entire portion, as this allows the most appropriate estimate to be made.

Where the signal is being analysed in discrete portions, the DC offset corrections are preferably also applied to each portion separately. Most preferably the same correction is applied to the entire portion. This means that the same correction is applied throughout the relevant wanted signal portion and helps to avoid, for example, applying the wrong correction to the start of the portion, thereby resulting in a better chance of decoding the received signal correctly.

Thus it is preferred that the signal portions are considered independently of each other. For example, adjacent data packets of received digital signals or time slots of received TDMA signals are preferably 7 treated separately.

In these arrangements because successive signal portions are considered independently of each other, changes in signal strength or DC offset between for example, consecutive or closely following packets or TDMA time slots can be responded to more rapidly and thus compensated for more straightforwardly and accurately, unlike when using a moving mean signal level which the Applicants have, as discussed above, recognised is not able to cope so well with such situations. Thus these arrangements are particularly advantageous where there is a rapid change in signal strength and thus DC offset, such as where a large signal on one TDMA time slot is immediately followed by a smaller signal from a different source on the next time slot.

A correction based on the DC offset estimated for a signal or data portion may simply be applied to a succeeding or later portion in a feed- back type arrangement. However, in a particularly preferred embodiment the DC offset for a signal or data portion is estimated and a correction based on that estimated DC offset applied to that same data or signal portion. Thus, where the received signal is converted into digital data portions, the digital signal processing means preferably estimates the DC offset of a digital data portion and applies a correction to that digital data portion on the basis of the estimated DC offset. Applying a correction to a signal or digital data portion based on the DC offset estimated for that same portion provides more accurate DC offset removal. It, for example, can overcome the above mentioned problem of a DC offset correction being unsuitable when it is based on (e.g. as an average of) a previous signal or data portion that has a particularly large or small DC offset in comparison to the DC of f set of the data portion to which the correction being is applied.

As discussed above, the discrete portions are preferably, or preferably correspond to, inherent identifiable discrete signal portions, such as TDMA timeslots, signal bursts or data packets, in the received signal. The Applicants have recognised that these, or other suitable, signal or data portions can be captured and then treated overall independently of each other.

To allow such independent treatment, each signal or data portion may, for example, be stored temporarily such that the DC offset can be estimated for the portion and a correction applied to the portion before the portion continues to be used or processed in the radio receiver. Thus, in a particularly preferred embodiment, the radio receiver further comprise means, such as a buffer for example, which captures data corresponding to portions of the received signal, and means for determining a DC offset for each portion and for removing a DC offset from each portion separately whilst it is in or as it passes out of the buf f er.

Whilst, as described above, it is preferable for the DC offset correction applied to a signal or data portion to be based on the DC offset estimated for that same data or signal portion, the DC offset for that portion of the signal may additionally or alternatively be based on the DC offset estimated for a preceding portion of the signal. Thus, the DC offset estimated for a portion of the signal may additionally or alternatively be stored and used e.g. to derive a DC offset correction for a succeeding portion of the signal (e.g. on its own or-with an offset estimate for that signal portion) This may, for example, be desirable when the signal level does not change significantly between the signal portions. The digital signal processor may therefore compare succeeding portions of the digital data to determine whether the signal level changes between succeeding signal portions and estimate a new DC offset whenever and/or only when the signal level changes by more than a predetermined amount.

As mentioned above, it is common for large changes in DC offset to occur when the transmitter or signal source changes. Thus, additionally or alternatively, the DC offset correction for a portion of the signal may be stored and used for one or more succeeding or later portions of the signal from the same signal source, and a new DC offset estimated whenever and/or only when the signal source changes.

Thus, in a particularly preferred embodiment, a new DC offset is only estimated whenever either the signal level changes by a predetermined amount or the signal source changes (i.e. the receiver is receiving from a different source).

In other words, the Applicants have recognised that individual signal sources often have the same DC offset for a period of time and that this can be used to reduce the number of calculations of DC offset in radio communications systems which use plural signal sources.

Thus, in another embodiment, the estimated DC offset for a signal received from a particular signal source, such as a base station or mobile unit of a mobile radio communication system, is used to derive a DC offset correction value which may be stored and then applied to the signal subsequently received from that signal source. This may be done for plural signal sources.

Thus, according to a fifth aspect of the present invention there is provided a radio receiver having means for estimating DC offset of a signal received from a signal source, means for deriving a correction value on the basis of the estimated DC off set of the signal received from the signal source; means for storing the correction value; and means for applying the stored correction value to signals subsequently received from the signal source.

According to a sixth aspect of the present invention there is provided a method of removing DC offset from a signal received from a radio signal source, comprising estimating the DC offset of a radio signal received from the signal source, deriving a correction value on the basis of the estimated DC offset of the signal received from the signal source; storing the correction value; and applying the stored correction value to signals subsequently received from the signal source.

There may be plural signal sources and therefore plural stored DC offset correction values each relating to a signal received from a particular signal source.

Thus, DC offset is calculated for each signal source separately and the calculated DC offset is used to derive a DC offset correction value that is stored in order that it can be used to remove DC offset from later signals received from the respective signal source. DC offset does not therefore need to be calculated every time a signal is received from a signal source. Thus, calculation effort for the estimation of DC offset is reduced.

Another alternative is for the DC offset estimated for one signal portion to be averaged with the DC offset estimated for a succeeding signal portion to determine the DC offset correction to apply to the succeeding portion. This dampens the change in DC offset between signal portions and can reduce errors particularly when only a part of each signal is used to calculate the DC offset.

Where the present invention is used in an arrangement where the received radio signals are processed at least partly as two separate simultaneous signal streams, such as I and Q components, it is preferable to estimate and preferably also correct for DC offset for each stream of the signal independently because the offset in each stream of the signal can differ.

Thus according to a seventh aspect of the present invention, there is provided a radio receiver which derives two or more simultaneous signal streams from received radio signals and comprises means for estimating separately a DC offset for each derived stream of the received signal.

According to a eighth aspect of the present invention, there is provided a method of processing a received radio signal comprising deriving two or more simultaneous signal streams from the received signal and estimating a DC offset for each derived stream of the received signal separately.

Preferably the DC offset corrections are applied to each stream of the signal independently as well.

The methods in accordance with the present invention may be implemented at least partially using software e.g. computer programs. It will thus be seen that when viewed from further aspects the present invention provides computer software specifically adapted to carry out the methods herein above described when installed on data processing means, and a computer program element comprising computer software code portions for performing the methods herein above described when the program element is run on a computer. The invention also extends to a computer software carrier comprising such software which when used to operate a radio system comprising a digital computer causes in conjunction with said computer said system to carry out the steps of the method of the present invention., Such a C omputer software carrier could be a physical storage medium such as a ROM chip, CD ROM or disk, or could be a signal such as an electronic signal over wires, an optical signal or a radio signal such as to a satellite or the like.

It will further be appreciated that not all steps of the method of the invention need be carried out by computer software and thus f rom a further broad aspect the present invention provides computer software, or such software installed on a computer software carrier, for carrying out at least one of the steps of the methods set out hereinabove.

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a schematic view of a radio receiver according to the invention; Figure 2 is a graphical illustration of a baseband TDMA radio signal; and Figure 3 is a larger scale view of the graph of Figure 2.

Referring to Figure 1, a mobile radio unit comprises receiver hardware 1 for receiving TDMA radio signals such as those broadcast over GSM and TETRA networks. The invention is applicable to a broader range of radio receivers, but this embodiment will be described in relation to this type of broadcast.

The receiver hardware 1 processes a received signal down to baseband and in this example produces two signal streams, I and Q, which are then passed to two analog to digital converters 2, 3 which convert the I and Q components of a demodulated signal to digital data. Each analog to digital converter 2, 3 has a DC off set correction circuit 4, 5 respectively provided before a decoding circuit 6. The DC offset correction circuits 4, 5 and decoding circuit 6 may be implemented in a digital signal processor.

Each DC offset-correction circuit comprises a buffer 7, 8, a DC offset determining or estimating means 9, 10 and a DC offset removal means 11, 12.

When a signal is received at the receiver 1, it is demodulated and converted into its I and Q components or streams. The I and Q components pass to the analog to digital converters 2 and 3 respectively.

Digital data produced by each digital to analog converter 2, 3 passes to a respective buffer 7, 8 in which the data is temporarily stored. The DC offset determining means 9, 10 identifies signal bursts (or TDMA slots or data packets) stored in the buffers 7, 8.

This can be conveniently done by means of an internal clock which is synchronised with the TDMA signal in receivers of this type. Alternatively, some other means may be used to identify the data packets, such as analysis of the data by dedicated software in the determining means 9, 10.

The DC offset determining means 9, 10 then estimate a DC offset for each identified signal burst individually by e.g. calculating the mean signal level is of each signal burst over the entire burst. The estimated DC offset is passed to the DC offset removal means 11, 12 along with the corresponding original data from the buffers 7, 8. The DC offset removal means then subtracts the calculated DC offset for each signal burst from the respective signal burst over the entire signal burst and passes DC offset compensated data to the decoder 6.

The DC offset determining means 9, 10 may, additionally, store the DC offset of an analysed signal burst for subsequent use. For example, the DC offset level may only be calculated when there is a large change in signal level. In other words, DC offset is calculated for a signal burst and the value calculated for DC offset is used for subsequent signal bursts until a suitable trigger initiates a further calculation, for example when signal'strength changes significantly or a new transm. itter is used (since DC offset can be caused by transmitters being out of alignment as well as inadequacies in the receiver).

A further example is to store the DC offset calculated in respect of signals from a particular signal source and use that value whenever a signal is received from that source. DC offsets may be calculated and stored for all signal sources in this way, or just for signal sources which cause significant DC offsets.

These signal sources may be the base stations of a mobile communications system or, in the case of a trunked radio system for example, both the base stations and the mobile radio units of a mobile communications system.

Alternatively, newly calculated DC offset levels may be combined with previously calculated DC offset levels as a way of damping the change of DC offset level between signal bursts.

Figures 2 and 3, show two consecutive signal bursts of a TDMA radio signal, e.g. a TETRA signal. In this example the first signal burst 13 is much stronger than the second signal burst 14 (such as could be the case where a base station of a TDMA mobile radio system receives a signal from different mobile radio units on adjacent time 6lots). Figure 3 shows that the mean level of the f irst signal burst (shown by line 15) is far larger than the highest level of the signal in the second signal burst 14. Thus, if the DC offset calculated in relation to the first signal burst 13 is used to correct the second signal burst, the data in the second signal burst will be swamped by the correction applied thereby probably making it impossible to decode the second signal burst correctly. This problem is overcome by the present invention as DC offset is calculated and removed from data packets corresponding to the signal bursts separately.

Claims

1. A radio receiver having means for demodulating a received radio signal, means for converting it to digital data and digital signal processing means for estimating a DC offset in the digital data from the digital data and for applying a correction to the digital data on the basis of the estimated DC offset.

2. The radio receiver of claim 1, wherein the means for converting the radio signal to digital data converts signal portions of the radio signal to digital data portions.

3. The radio receiver of claim 2, wherein the digital signal processing means estimates the DC offset of a digital data portion and applies a correction to that digital data portion on the basis of the estimated DC offset.

4. A radio receiver for use in a radio communications system, comprising means for demodulating received radio signals, means for dividing a received signal into successive discrete signal portions, means for estimating the DC offset of a single signal portion and means for applying a DC offset correction to a single signal portion.

5. The radio receiver of claim 2 or claim 3, wherein the digital signal processing means estimates a separate DC offset for each digital data portion.

6. The radio receiver of claim 2, 3 or 5, wherein the digital signal processing means estimates the DC offset for a digital data portion using the entire digital data portion.

7. The radio receiver of any one of claims 2, 3, S or 6, wherein the digital signal processing means applies the correction to each digital data portion separately.

8. The radio receiver of any one of claims 2, 3 or 5 to 7, wherein the digital signal processing means applies the same correction to the entire digital data portion.

9. The radio receiver of any one of the preceding claims further comprising a buffer which captures the digital data or signal portions.

10. The radio receiver of claim 9 wherein the DC offset is determined for each digital data or signal portion and removed from each portion separately by the digital signal processing means in or as it passes out of the buffer.

11. The radio receiver of any one of the preceding claims, further comprising means for storing the DC offset and/or derived DC offset correction for a data or signal portion and for using the stored DC offset and/or stored derived DC offset correction to derive a DC offset for a later or succeeding data or signal portion.

12. The radio receiver of any one of claims 2, 3 or 5 to 11 wherein the digital signal processing means compares succeeding digital data portions to determine whether the signal level changes between succeeding digital data portions and estimates a new DC offset whenever and/or only when the signal level changes by more than a predetermined amount.

13. The radio receiver of any one of claims 2, 3 or 5 to 12, further comprising means for storing the DC offset correction for a portion of the signal and wherein the digital processing means uses the stored DC offset correction for one or more later or succeeding portions of the signal from the same signal source, and estimates a new DC offset whenever and/or only when the signal source changes.

14. The radio receiver of any one of claims 10 to 13, wherein the DC offset estimated for one digital data portion is averaged with the DC offset estimated for a later or succeeding signal portion to determine the correction to apply to the succeeding portion.

15. The radio receiver of any one of claims 2, 3 or 5 to 14, wherein the digital signal processing means uses the estimated DC offset for a digital data portion corresponding to a signal portion of the signal received from a particular signal source to derive a correction which is applied to digital data portions corresponding to a signal portion of the signal subsequently received from that signal source.

16. The radio receiver of any one of the preceding claims, wherein the data or signal portions, correspond to data packets, timeslots or bursts of the received 2S signal.

17. The radio receiver of any one of the preceding claims, further comprising means for deriving two or more simultaneous signal streams from received radio signal and means for estimating separately DC offset for each derived stream'of the received signal.

18. A radio receiver which derives two or more simultaneous signal streams from received radio signals and comprises means for estimating separately a DC offset for each derived stream of the received signal.

19. A radio receiver having means for estimating DC offset of a signal received from a signal source, means for deriving a correction value on the basis of the estimated DC offset of the signal received from the signal source; means for storing the correction value; and means for applying the stored correction value to signals subsequently received from the signal source.

20. The radio receiver of claim 19, wherein the means for storing the correction value stores plural DC offset correction values, each relating to a signal received from a particular signal source.

21. A method of processing a received radio signal, is comprising demodulating the received radio signal, converting it to digital data, estimating a DC offset in the digital data based on the digital data and applying a correction to the digital data on the basis of the estimated DC offset.

22. The method of claim 21, wherein the digital data comprises digital data portions corresponding to signal portions of the received signal.

23. The method of claim 21 or 22, wherein the digital signal processing means estimates a DC offset for a digital data portion and applies a correction to that digital data portion on the basis of the estimated DC offset. 30

24. A method of processing received radio signals, comprising dividing a received signal into successive discrete signal portions, estimating a DC offset for each signal portion individually, and applying a DC 35 offset correction to each signal portion individually.

25. The method of any one of claims 21 to 24, wherein a separate DC offset is estimated for each digital data portion.

26. The method of any one of claims 21 to 25, wherein each signal or data portion is stored temporarily whilst the DC offset is estimated and the correction is applied to the portion.

27. The method of any one of claims 21 to 26, wherein succeeding digital data portions are compared to determine whether the signal level changes between succeeding digital data portions and a new DC offset is estimated whenever and/or only when the signal level changes by more than a predetermined amount.

28. The method of any one of claims 21 to 27, wherein the DC offset correction for a portion of the signal is stored and used for one or more later or succeeding portions of the signal from the same signal source, and a new DC offset is estimated whenever and/or only when the signal source changes.

29. The method of claim 21, wherein the radio signal carries digitally encoded data.

30. A method of removing DC offset from a radio signal received from a radio signal source, conprising estimating the DC offset of a radio signal received from the signal source, deriving a correction value on the basis of the estimated DC offset of the signal received from the signal source; storing the correction value; and applying the stored correction value to signals subsequently received from the signal source.

31. A method of processing a received radio signal comprising deriving two or more simultaneous signal streams from the received signal and estimating a DC offset for each derived stream of the received signal separately.

32. A computer program element comprising computer software code portions for performing the method of any one of claims 21 to 31 when the program element is run on a computer.

33. A radio receiver substantially as hereinbefore described with reference to the accompanying drawings.

34. A method of providing a received radio signal substantially as hereinbefore described with reference to the accompanying drawings.