INTERFERENCE MITIGATION

FIELD OF THE INVENTION

The present invention generally relates to interference mitigation in a communication device. The invention relates particularly, though not exclusively, to mitigation of radio frequency interference which is produced by a transmitter using a first radio technology and which interferes reception in a receiver using a second radio technology.

BACKGROUND OF THE INVENTION

Many current and future generations of communication devices, such as mobile handsets, ultra mobile devices (UMD) and laptop PCs (Personal Computer) have wireless transmitters and/or receivers of different communication technologies integrated into a single or same host device. These communication technologies may involve, for example, cellular radio technologies, such as GSM (Global System for Mobile communication), PCS (Personal Communications Services) and 3 ^rd generation mobile radio technologies, as well as other communication technologies, such as WLAN (Wireless Local Area Network) and/or WiMAX (Worldwide Interoperability for Microwave Access) and/or DVB (Digital Video Broadcasting) radio technologies and/or Bluetooth technologies.

As a result of the international frequency assignments to which different communication devices must adhere, certain harmonics of certain cellular phone or Bluetooth channels, for example, can fall into the channel in use in another receiver, such as a WLAN or WiMAX or DVB receiver. Although the frequency bands of different technologies as such should ideally not overlap, it may be that at least one harmonic frequency used in one radio technology falls into the receiving band of another radio technology. For example, the third harmonic of a GSM specific transmission frequency currently falls into a certain WLAN channel.

The source of interference, that is the interfering transmitter, may reside either in the same device which comprises the interfered receiver or in a separate device located nearby. Even when the source of interference resides in the nearby located device, the level of interference may be high enough to block reception in the interfered receiver.

Receivers, such as WLAN, WiMAX and DVB wideband receivers, generally incorporate an automatic gain control (AGC) so that the output of an analog-to- digital converter (ADC) is optimized for efficient data recovery. Such an AGC is generally controlled by the total energy within the received channel and in the presence of interference greater than the wanted signal level it will reduce the gain so as to prevent saturation of the ADC. However, in the event that interference commences during packet reception only after the AGC has been set by the wanted signal level, the receiver will experience overload within the analog or ADC sections thereby preventing the effective operation of any interference mitigation technique within the digital signal processing section of the receiver.

SUMMARY

According to a first aspect of the invention there is provided an apparatus, comprising: an automatic gain control configured to control the gain of a received wireless signal, wherein the automatic gain control is configured to lock the gain of the received signal based on observed interference to a level which avoids clipping of a received signal containing both a wanted signal and interference during the time period of interference.

In an embodiment, the apparatus comprises a wideband receiver for wideband reception of a wideband signal. Said interference may be wideband or narrowband interference. In at least one embodiment, the apparatus may be a wireless terminal comprising a cellular transmitter producing narrowband interference to the wideband reception. In at least one embodiment, the apparatus may comprise a

short-range transmitter, such as a Bluetooth transmitter, which produces the interference.

In an embodiment, the apparatus comprises an interference detector configured for detecting or estimating the timing and/or level of an interference burst. In an embodiment, the interference detector comprises a set of non-overlapping rejection filters.

In an embodiment, the automatic gain control comprises input(s) for locking the gain based on packet detection or acquisition and based on detected or estimated interference, the locking based on detected or estimated interference overriding the locking based on packet detection or acquisition.

In another embodiment, the automatic gain control is configured to revert from a locking mode which is based on detected or estimated interference to normal operating mode in the event that after an interference burst no further interference burst is detected for a certain period of time.

According to a second aspect of the invention there is provided a method, comprising: controlling the gain of a received wireless signal by an automatic gain control, wherein the method further comprises: locking, based on observed interference, the gain of the received signal to a level which avoids clipping of a received signal containing both a wanted signal and interference during the time period of interference.

According to a third aspect of the invention there is provided a computer readable medium having stored thereon a computer program executable in an apparatus, the computer program comprising: program code for controlling an automatic gain control for controlling the gain of a received wireless signal; and program code for controlling locking, based on observed interference, the gain of the received signal on a level which avoids clipping of a received signal containing

both a wanted signal and interference during the time period of interference.

According to a fourth aspect of the invention there is provided an automatic gain control module, comprising: at least one output for controlling with a control signal the gain of a wireless signal received in a wideband receiver; and at least one input for receiving information about observed interference, wherein the automatic gain control module is configured for locking the gain of the received signal based on observed interference to a level which avoids clipping of a received signal containing both a wanted signal and interference during the time period of interference.

In an embodiment, the automatic gain control module is implemented as a physical hardware module comprising said input(s) and output(s). It may comprise an integrated logic for controlling its operation.

According to a fifth aspect of the invention there is provided an apparatus, comprising: means for controlling the gain of a received wireless signal, wherein said controlling means are configured to lock the gain of the received signal based on observed interference to a level which avoids clipping of a received signal containing both a wanted signal and interference during the time period of interference.

Various embodiments of the present invention have been illustrated only with reference to certain aspects of the invention. It should be appreciated that corresponding embodiments may apply to other aspects as well.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a wideband receiver in accordance with at least one embodiment of the invention;

Figure 2 shows interference coupling between a transmitter and a receiver; Figure 3 shows an example for implementing interference detection in according to at least one embodiment of the invention;

Figure 4 shows the basic operation of an automatic gain control; Figure 5 shows automatic gain control in accordance with an embodiment of the invention;

Figure 6 schematically shows an embodiment of the invention; Figure 7 yet schematically shows an embodiment of the invention; and

Figure 8 shows a block diagram of an apparatus in accordance with at least one embodiment of the invention.

DETAILED SPECIFICATION

Figure 1 shows a set of operational blocks in a wideband receiver 100 according to at least one embodiment of the invention. The wideband receiver may be a digital broadband broadcast receiver based on multi-carrier modulation or OFDM (Orthogonal Frequency Division Multiplexing). Examples of applicable wideband technologies include, inter alia, the following: WLAN, WiMAX, DVB technologies, such as, Digital Video Broadcast - Terrestrial (DVB-T) and Digital Video Broadcast - Handheld (DVB-H), Integrated Services Digital Broadcasting - Terrestrial (ISDB- T); 1seg, Digital Multimedia Broadcast-Terrestrial/Handheld (DMB-T/H), Terrestrial Digital Multimedia Broadcasting (T-DMB), Digital Audio Broadcasting (DAB), Digital Radio Mondiale (DRM), Forward Link Only (FLO), MediaFLO, Multimedia Broadcast Multicast Service (MBMS) of 3 ^rd generation partnership project (3GPP), Broadcast and Multicast Services (BCMCS) of 3 ^rd generation partnership project 2 (3GPP2), and data broadcast technologies in accordance with Advanced Television Systems Committee (ATSC) Data Broadcast Standard.

The blocks shown in Figure 1 represent functions performed in time domain. After radio frequency (RF) processing, the received wideband radio frequency signal is down converted in block 105. The down-converted signal is amplified in a variable

gain amplifier (VGA), for example, a voltage controlled amplifier 110. The gain of the amplifier 110 is controlled by an automatic gain control (AGC) signal generated by an automated gain control function 115. The amplified signal is converted from an analog signal into a digital signal in an analog-to-digital converter 120. The output of the analog-to-digital converter 120 is conveyed into a time domain interference detection unit 130. The interference detection unit 130 comprises a interference detection function which detects the presence of interference in time domain.

The interference is produced by an interfering transmitter. It may be, for example, a cellular transmitter, such as a GSM transmitter. Examples of other cellular technologies are, for example, Digital-Advanced Mobile Phone Service (D-AMPS), Personal Digital Cellular (PDC) cellular technologies, and many more. Alternatively, the interfering transmitter can be another that cellular transmitter, for example a Bluetooth transmitter. The interference may be periodical, such as time division multiple access (TDMA) based periodical interference. It may be broadband or narrowband interference.

After interference detection unit 130, packet detection and synchronization is carried out in one or more packet detection and synchronization units 135, and the detected interference (as detected in unit 130) is optionally cancelled from the digital signal in time domain by an interference cancellation function 140. After the optional time domain interference cancellation, the digital signal is processed

(demodulated and decoded) in frequency domain starting with a Fast Fourier Transform (FFT) or similar.

Figure 2 illustrates interference coupling. The interference produced by an interfering transmitter 200, for example a GSM transmitter, is coupled via the antenna 211 to the wideband reception antenna 212 (by antenna coupling), and therefrom to the wideband receiver 100, for example a WLAN receiver, resulting in interference in wideband reception.

In at least one embodiment of the invention, the interference is detected by using a

set of rejection filters. The number n of rejection filters used depends on the implementation. Each of the n rejection filters has typically a rejection bandwidth of 1/n of the used wideband channel bandwidth. In applicable embodiments, the number of the rejection filters is 2 or more.

Figure 3 shows an example for implementing the interference detection function 130 according to at least one embodiment of the invention. The received digital wideband signal from the analog-to-digital converter 120 is conveyed to each of a set of rejection filters 301-304. The number n of rejection filters depends on the implementation. In this example arrangement, the number n of the rejection filters is 4. Each rejection filter 301-304 has a rejection sub-band (or bandwidth) of 1/n, here 1/4, of the total bandwidth of the received channel. In this embodiment, the sub-bands are not overlapping. Each rejection filter 301-304 thus rejects 1/n, here 1/4, of the frequencies of the received spectrum of the wideband signal. In a WLAN receiver, for example, the channel bandwidth is typically 20 MHz, thus the rejection bandwidth becomes 5 MHz if four rejection filters are used. The rejection filters 301-304 may be implemented, for example, by infinite impulse response (NR) filters of order 6. The suppression level of each filter may be 40 dB, for example.

In the absence of interference, the received energy in each of the sub-bands is more or less equal (assuming the wanted signal produces a more or less "flat" spectrum). However, in the presence of interference (especially if the interference is narrowband interference) one or more of the sub-bands will receive a greater energy level. The presence of interference can therefore be detected by comparing the outputs or output energy (or power) of the rejection filters 301-304. The comparison can be implemented in various ways, one of those being shown in the example of Figure 3. In this implementation, the outputs of the rejection filters 301-304 are conveyed to a selector block 350, which selects the rejection filter output having a minimum energy. This output is compared with the total energy received from the analog-to-digital converter output in a comparison and interference detection block 360. If, based on the comparison, it is detected that the output energy level of the rejection filter which has the lowest level is

considerably lower than energy level of the n-1 other outputs, here n-1 = 3, this indicates the presence of narrowband interference in the rejection sub-band of the lowest output filter.

In other words, if the energy (or power) difference between the filtered signal 351 (i.e., the rejection filter output signal which had a minimum energy) and non- filtered signal 352 (i.e., the ADC output signal) is less than a certain threshold value, then a decision is taken that there is no interference. In an embodiment, this threshold value is a bit more than 1/4 of the energy of the non-filtered signal. If no interference is present, the non-filtered signal 352 is selected to enter the units 135 for packet detection and synchronization. Otherwise, the filtered signal 351 is selected to be used in packet detection and synchronization. It has been observed that although a portion of the signal spectrum (1/4 of the total spectrum, in this example) is filtered out, the packet detection and synchronization units 135 can still in many cases operate correctly.

The signal processing in a wideband receiver is understood to comprise a preamble processing phase and a subsequent data extraction phase. The preamble processing phase typically comprises time domain operations in which a preamble portion of a packet is used. Preamble processing may comprise operations, such as signal detection, automatic gain control, diversity selection, coarse frequency offset estimation, packet detection and timing synchronization in time domain. A packet preamble typically also contains information on the length of the packet and data rate used. The data extraction phase, on the other hand, typically comprises operations involving actual data extraction, for example, various demodulation and decoding operations performed in frequency domain.

In an embodiment, when interference is present, the filtered signal 351 is used for packet detection and timing synchronization, while the rest of the preamble processing and data extraction is performed based of the non-filtered signal 352. When narrowband interference is not present, all operations are performed based on the non-filtered signal 352.

Figure 4 shows the operation of conventional automatic gain control 115. Once a spectrum sensing part (not shown) of the receiver detects energy in the reception bandwidth, the automatic gain control 115 and an acquisition function 450 begin to operate. The acquisition function 450 can be implemented as a separate set of blocks or units, or it can be integrated into said packet detection and synchronization units 135. The automatic gain control 115 and acquisition function 450 are working concurrently. The automatic gain control 115 controls the gain of the variable gain amplifier 110. The gain is controlled with a gain address (amplification coefficient) so that the reception signal amplitude changes (increases or decreases) in time. One purpose of automatic gain control 115 is to control the reception signal so that the whole analog-to-digital conversion precision can be used without clippings in the signal. The acquisition function 450 calculates an autocorrelation response and makes a decision about the presence of the packet. After a packet is detected, the acquisition function 450 raises an "AGC Lock" flag. Upon receiving the flag, the automatic gain control 115 stops changing the signal amplitude, and the rest of the packet is received with the same amplification coefficient in the variable gain amplifier. This means that conventionally the automatic gain control is locked by the packet detection unit (or acquisition function 450). However, this kind of locking leads into signal clippings in the analog-to-digital converter in a situation in which interference begins to appear only during an ongoing packet reception (which is normally the case concerning periodical interference, for example).

Figure 5 shows an embodiment of the invention which is able to cope with interference appearing only during an ongoing packet reception. The operation in Figure 5 corresponds to the operation in Figure 4 in many parts. Accordingly, once a spectrum sensing part (not shown) of the receiver detects energy in the reception bandwidth, an automatic gain control 515 and an acquisition function 450 begin to operate. The acquisition function 450 can be implemented as a separate set of blocks or units, or it can be integrated into said packet detection and synchronization units 135 (Fig. 1 ). The automatic gain control 515 and acquisition function 450 are working concurrently. The automatic gain control 515 controls the gain of the variable gain amplifier 110. The gain is controlled with a

gain address (amplification coefficient) so that the reception signal amplitude changes (increases or decreases) in time. The acquisition function 450 calculates an autocorrelation response and makes a decision about the presence of the packet. After a packet is detected, the acquisition function 450 raises an "AGC Lock" flag. Upon receiving the flag, the automatic gain control 515 stops changing the signal amplitude, and the rest of the packet is received with the same amplification coefficient in the variable gain amplifier.

The preceding paragraph showed the "normal operation". However, in addition to the "normal operation", the automatic gain control 515 in accordance with at least one embodiment of the invention also takes into account the presence of interference, even if the interference (e.g., bursty periodic interference) would appear during reception of a packet. In an embodiment shown in Figure 5, the automatic gain control 515 is provided with an additional control. This can be named as an "External AGC Lock". The External AGC Lock locks the automatic gain control 515 based on the presence of the interference. For example, it may operate as follows. After the AGC lock is initiated by acquisition and an interference detection/estimation unit (for example, block 130 in Figs. 1 and 5) indicates that interference is present, the interference detection/estimation unit 130 raises an "External AGC Lock" flag. The "External AGC Lock" flag overrides the "AGC Lock" flag received from the acquisition function 450. Upon raising the "External AGC Lock" flag, the amplification coefficient is locked on the level of the combined reception signal (containing both interference and the wanted signal) so that the signal is not clipped in the analog-to-digital converter 120.

While the "External AGC Lock" flag is raised, this prevents the automatic gain control 515 to change the amplification coefficient of the variable gain amplifier 110 for the period of time that the "External AGC Lock" flag remains raised. This means that even after the reception of the ongoing packet the amplification coefficient is left the same in order to avoid clippings in a subsequent packet. When the signal (and interference) is not clipped it is possible to cancel or mitigate the interference in subsequent interference cancellation (e.g., block 140 in Fig. 1 ).

The "External AGC Lock" flag duration depends on the characteristics of the interference. In an embodiment, the automatic gain control 515 is preset to the level determined by a previous interference burst. In the event that no further interference burst is detected for a certain period of time, two burst intervals, for example, the presetting of the automatic gain control is discontinued and automatic gain control reverted to normal operation.

Figure 6 schematically shows an embodiment of the invention. In order to detect interference, the signal received from the analog-to-digital converter 120 is detected to sub-band (rejection) filters 301-304. Each sub-band filter 301-304 has a rejection sub-band as described in connection with Figure 3. The filtered outputs of the filters 301-304 are conveyed to a level comparator 660 which selects the rejection filter output(s) having a minimum energy. This output/these outputs is/are compared with the total energy received from the analog-to-digital converter and further compared with predefined threshold level(s) in order to detect the presence of interference in step 670. If interference is not detected, normal operation of the automatic gain control is carried out in step 681 to control the variable gain amplifier 110. If interference is detected, time domain interference behaviour is determined in step 671. Furthermore, if interference is detected, interference-free sub-bands can be selected in step 665 for packet detection and synchronization part 135 of the preamble processing, if desired.

The characteristics of the interface can be obtained directly from corresponding specifications, especially if the type of interference is known (for example cellular TDMA-based interference or Bluetooth-interference). Alternatively, or in addition, the characteristics can be determined by a suitable algorithm. This algorithm can make use of various interference detection or prediction functions, such as the one shown in Figure 3 and related description. Certain ways of recognizing periodic interference (interference characterization) in time domain have also been disclosed by the priority application US 60/940,304 (filed on 25 May 2007) from which the present application claims priority and the contents thereof being incorporated in the present application by reference. In more detail, various kinds of periodic interference, such as TDMA modulated signals, data bus originated

interference (e.g., display), signals with periodic frequency hopping sequence (e.g., Bluetooth) can be detected by using the interference detector 130 shown in Figures 1 and 3. It can be used to measure the start time and burst time of the interference. Based of these measurements, for example, the correct timing for the automatic gain control can be defined in step 672. This can contain, for example, the determination of the "External AGC Lock" flag duration. Further, in step 682, the "modified" automatic gain control of the variable gain amplifier 110 is performed based on the interference level (determined, for example, by unit 660) for the desired duration (as defined, for example, by the determined "External AGC Lock" flag duration).

Figure 7 yet schematically shows an embodiment of the invention. When a data packet arrives at the receiver, the receiver conventionally locks the automatic gain control (AGC lock) on a level which is based on the received energy in a packet preamble. The signal (at the area of interference) is clipped in the analog-to-digital converter. In order to avoid clipping in a later packet, the automatic gain control in locked by the external AGC lock on a raised level based on detected or estimated interference. This results in the following packet not being clipped. Accordingly, since the total combined signal (wanted signal + interference) is well received, this enables, among other things, that the interference can be subsequently mitigated or cancelled from the received signal by the interference cancellation function 140 or similar.

Figure 8 shows a simplified block diagram of an apparatus in accordance with at least one embodiment of the invention. The apparatus may be a wireless handheld user terminal. The apparatus 800 comprises a processing unit (or processor) 810, wideband reception hardware 820 coupled to the processing unit 810 and antenna, and a memory 830 coupled to the processing unit 810. The memory 830 comprises stored software (or software modules) 840 which is executable in the processing unit 810. The wideband reception hardware 820 may be arranged in a set of separate physical hardware blocks or modules. The automatic gain control, for example, may be arranged in one such block or module. The automatic gain control block or module may have one or more inputs for receiving locking signals

(such as a locking signal from packet detection and an external locking signal) and an output for sending a control signal to the variable gain amplifier mentioned in the preceding description. Said one or more inputs and outputs may be implemented, for example, by input or output pins or connectors. In addition to or instead of the software 840, the automatic gain control block or module may comprise some integrated logic for controlling its operation. The apparatus may optionally comprise cellular transmitter (or transceiver) or Bluetooth hardware 850 coupled to the processing unit 810 and an antenna.

Software 840 comprises wideband reception software, which comprises program code executable in the processing unit 810 for performing software operations relating to the data reception, such as interference detection control of interference which may be caused by the cellular or Bluetooth transmitter 850 or by another device. By software can be performed various calculations/algorithms relating, for example, to interference detection/estimation or cancellation, and other control of hardware modules, such as controlling the automatic gain control. If the apparatus comprises the optional cellular network or Bluetooth functionality, software 840 can comprise cellular or Bluetooth communications software, which performs software operation relating to the cellular or Bluetooth transmission (and reception). The apparatus 800 further comprises a user interface 860 enabling the user to use the apparatus 800. The user interface 860 is coupled to the processing unit 810 and typically comprises one or more input and output devices. These may contain, for example: a display and speaker(s), a keyboard, a microphone, a camera, and optionally a separate display and/or speaker for cellular voice call and other cellular operation.

In another embodiment, the software 840 may comprise firmware or a combination of software and firmware. The apparatus 800 may be a mobile phone capable of digital broadband broadcast reception. It may be, depending on the embodiment, a mobile or fixed device. It may be a digital television receiver or another electronic device (for example an electronic home appliance) which is able to receive wideband transmissions, such as WLAN transmissions, for example, in a personal or public network.

Various embodiments have been presented. It should be appreciated that in this document, words comprise, include and contain are each used as open-ended expressions with no intended exclusivity.

The foregoing description has provided by way of non-limiting examples of particular implementations and embodiments a full and informative description of the best mode presently contemplated by the inventors for carrying out the invention. It is however clear to a person skilled in the art that the invention is not restricted to details of the embodiments presented above, but that it can be implemented in other embodiments using equivalent means without deviating from the characteristics of the invention.

Furthermore, some of the features of the above-disclosed embodiments of this invention may be used to advantage without the corresponding use of other features. As such, the foregoing description should be considered as merely illustrative of the principles of the present invention, and not in limitation thereof. Hence, the scope of the invention is only restricted by the appended patent claims.