WO2010001320A1

WO2010001320A1 - Interference management

Info

Publication number: WO2010001320A1
Application number: PCT/IB2009/052768
Authority: WO
Inventors: Javier Espina Perez; Thomas Falck; Lars Schmitt
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property And Standards Gmbh
Priority date: 2008-07-03
Filing date: 2009-06-26
Publication date: 2010-01-07
Also published as: TW201010465A

Abstract

The invention addresses the problem that under certain conditions short range radios (such as IEEE 802.15.4 / ZigBee) do not stand a chance of getting access to the medium in the presence of WLAN radios transmitting with order of magnitude higher power. The concept of interference zonesis usedto classify the possible mutual interference circumstances between two radios with different transmission power. By detecting the interference zone a short range radio is in, it can adapt itsinterference mitigation strategy. The invention can be applied for instance for patient monitoring to maintain operation of wireless body sensor networks in the presence of WLAN.

Description

INTERFERENCE MANAGEMENT

TECHNICAL FIELD

The present invention relates to a method of managing interference in radio communication networks. In one embodiment the radio communication network comprises devices operating in accordance with different radio network standards, such as IEEE 802. Hx and IEEE 802.15.4. The invention also relates to a corresponding computer program product comprising instructions for implementing the steps of said method and to a radio communication device.

BACKGROUND OF THE INVENTION Many widespread wireless technologies operate in unlicensed frequency bands such as the industrial scientific medical (ISM) bands. Although devices enabled with such wireless technologies can operate with no licensing costs they use the same frequencies with minimal or no coordination with each other's transmissions. This results in interferences between near-by devices that disturb their wireless communications. Examples of disturbance are: increase in packet error rate and increase in missed packet rate, eventually reaching total disruption of the communication channel. There exist two main kinds of mechanisms that reduce the detrimental effects of interferences between wireless devices: (1) sensing-based medium sharing to avoid potential interference and (2) interference mitigation techniques to reduce actual interference.

Sensing-based medium sharing has a twofold purpose: (i) to coordinate the communication between the devices within a wireless network while the devices use the same wireless technology and (ii) to allow for coexistence, to some extent, with devices that use different wireless technologies. The most common scheme for sensing-based medium sharing is carrier sensing multiple access / collision avoidance (CSMA/CA), which is utilised by wireless technologies such as WiFi including IEEE 802.11a, IEEE 802.11b, IEEE 802. Hg, IEEE 802.1 In, and ZigBee including IEEE 802.15.4 and IEEE 802.15.4a. CSMA/CA-enabled devices start a transmission only after having sensed that the channel, i.e. the shared medium, is free. However, CSMA/CA like all medium sharing techniques, cannot guarantee a fair coexistence between wireless devices enabled with different wireless technologies. Hence, depending on the performance requirements of the application it is often necessary to take interference mitigation measures in case actual interference is encountered.

Interference mitigation is usually achieved by detecting the presence of interferences and switching to an undisturbed frequency channel. The presence of interferences may be detected directly by periodically measuring the channel or indirectly after assessing a drop in the performance of the wireless communication channel. After interferences have been detected a device may switch to an interference- free frequency channel to better communicate with the other devices within its network. Frequency channel management is the coordination process by which all communication partners switch to the same channel at roughly the same time. Considering ZigBee as an example, the process for switching the communication channel of a whole network, i.e. for all devices in the network, is initiated by a single device being responsible for network channel management. Based on devices reporting noticeable interference according to their location in the network, the network channel manager concludes to switch or maintain the current network channel. In case of the decision to switch the channel, a broadcast command to switch channels is sent to all devices in the network.

Although sensing-based medium sharing and in a greater extent interference mitigation techniques reduce the detrimental effects of radio frequency (RF) interferences, their resulting disturbance in the wireless communication is still unacceptable for many applications. This holds especially for demanding applications such as wireless patient monitoring and wireless device control, with tight requirements on reliability and latency. Those and other indoor applications are (or will soon be) often enabled by wireless technologies like WiFi (IEEE 802.11) and ZigBee (IEEE 802.15.4), which happen to share the same frequency band. This leads to a problem that has been often overseen and is not catered for by interference mitigation techniques. The problem may arise under the following conditions:

1. The wireless-enabled devices use wireless technologies that share the same frequency bands and implement coexistence mechanisms such as CSMA/CA. 2. The range within which a device causes interferences on other devices is different from the range within which the latter cause interferences on the first device. This happens in any combination of the following cases: a. The devices feature different wireless technologies (different transmit power, receiver sensitivity, spectrum spreading protection, error coding, packet length, etc.). b. The devices feature the same technology but use unequal transmit power levels. c. The devices feature the same technology, use equal transmit power levels but have slightly unequal receiver sensitivity (usual across different manufacturers) .

Generally the detrimental effects of interference continuously diminish with increasing distance between the interfered and interfering devices. Therefore, communications are usually only slightly or moderately disturbed as long as the interfering device is not located too close to the interfered device, e.g. devices located less than 5 meters apart for a typical ZigBee implementation. Nonetheless, the two aforementioned conditions introduce a discontinuity in the generic pattern of disturbance of an interferer: a physical area appears, in which the interference is highly detrimental. In some cases, for instance when a WiFi device acts as interferer and a ZigBee device (with much lower transmit power) as interfered, the disturbance is so high that ZigBee wireless communication is thoroughly inhibited. In such cases interference mitigation techniques will fail to coordinate a frequency channel switch of all network devices due to the missing communication possibility, unless an alternative channel was set beforehand.

Figs. Ia, Ib and Ic represent the three possible mutual interference situations between two devices with different interference ranges. Device A may be a ZigBee device and device B a WiFi device. The three mutual interference situations depicted in Figs. Ia, Ib and Ic define three interference zones around the interfering device B, depending on the position of device A, where "d" is the distance between A and B, "Γ_A" is the range within which A interferes a device of type B, and "Γ_B" is the range within which B interferes a device of type A. Fig.2 depicts the resulting interferences zones and Fig.3 shows the increased disturbance within "Zone 2". The zones show the following properties:

Zone 1 (d < VA_ < ΓR): both A and B interfere each other's communications.

Since both devices interfere each other's communications, they are both capable of noticing each other's presence. Hence, owing to CSMA/CA or a similar coexistence technique, they hear each other's transmissions and do usually not start sending until the channel is free. Given the limited coexistence properties of CSMA/CA and the extreme proximity of both devices disturbance in terms of reduced throughput or packet error rate can still be significant.

Zone 2 (rA_ < d < rn): B interferes A 's communications, but A does not interfere B 's communications.

Device A notices B's presence but not vice versa. On the one hand, thanks to

CSMA/CA or a similar coexistence technique, A avoids starting a transmission while

B is transmitting. On the other hand B transmits completely regardless of A. This creates, in comparison to "Zone 1", a dramatically unfair situation for A because (1) its transmissions are much more often interrupted by B's transmissions and (2) it drops the transmission of a high number of data packets after having assessed for a given number of successive times that B is occupying the medium. Therefore A suffers the highest disturbance and often loses any possibility to communicate by conventional means and, therefore, to coordinate a frequency channel switch with its network partners.

Zone 3 (YA_ < rn_< d): Neither A nor B interfere each other 's communications.

Both devices can communicate with their other network partners (not depicted) without suffering any kind of mutual disturbance.

It shall be noted that the interference zones are not only defined by the physical distance d between the device and the interferer but also by the distance Δf between the central frequencies of their respective channels. In the most general case, Zone 2 is a two-dimensional area in the (d, Δf)-plane, the shape of which depends on the shape of the spectrum emitted by device and interferer. This is illustrated in Fig.4.

Thus, there is a need for an improved method of interference management in a radio communication network.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a method of managing interference in a radio communication network comprising at least a first and second radio communication devices, said method comprising the following steps: - identifying mutual interference between the first and second devices; and changing normal transmission characteristics by sending a frequency realignment command to other devices in the network without detecting the current radio communication channel as being free for sending information. It is to be understood that when information is sent in accordance with normal transmission characteristics a mechanism for reducing interference is applied as explained in the background of the invention section. For instance, the medium may first have to be sensed free before the actual transmission can start.

Thus, the present invention provides a method that significantly improves and accelerates existing approaches for interference mitigation. The invention can be implemented in virtually any wireless-enabled device that is able to communicate in different frequency channels. This includes devices enabled with wireless technologies such as WiFi, such as IEEE 802.11a, IEEE 802.11b, IEEE 802.1 Ig, IEEE 802.1 In,

ZigBee (IEEE 802.15.4), and digital enhanced cordless telephone (DECT). Moreover, the invention does not require hardware add-ons and can therefore be easily implemented in wireless-enabled devices without a significant cost increase.

According to a second aspect of the invention, there is provided a computer program product comprising instructions for implementing the method according the first aspect of the invention when loaded and run on computer means of a radio communication device. According to a third aspect of the invention, there is provided a radio communication device for managing interference in a radio communication network comprising at least another radio communication device, said measurement device comprising:

- means for identifying mutual interference between said radio communication device and said other radio communication device; and - means for changing normal transmission characteristics by generating a frequency realignment command to be sent to other devices in the network without detecting the current radio communication channel as being free for sending information.

Other aspects of the invention are recited in the dependent claims attached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent from the following description of non-limiting exemplary embodiments, with reference to the appended drawings, in which:

- Figs. Ia, Ib and Ic are schematic illustrations of different interference situations between two devices with different interference ranges in a radio communication network;

- Fig.2 is a schematic illustration of different interference zones in a radio communication network;

- Fig.3 is a diagram showing disturbance levels of three different interference zones of Fig.2;

- Fig.4 is a diagram showing three different interference zones of Fig.2 in (d, Δf)- p lane; - Fig.5 is a schematic illustration of different interference zones in a radio communication network in accordance with a specific example;

- Fig.6 is a flow chart depicting the interference management method in accordance with one embodiment of the present invention; and

- Fig.7 is a block diagram of the radio communication device in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

According to the present invention a device enabled with a wireless communication technology mitigates interferences on the basis of the interfering effect that, during their simultaneous operation, the device has on other devices and vice versa. The devices in the network may use different wireless technologies. In this way, for better mitigating interferences, a device defines its channel management steps depending on which devices are interfered by which devices. Advantageously the device senses the medium to (1) determine which devices are interfering its operation and (2) determine which devices are interfered by its operation, which can be deduced from their transmission patterns.

In the exemplary case of two generic devices A and B, where A is the device implementing the present invention, there are four possible mutual interference situations that introduce different degrees of disturbance in A's operation. These situations are ordered in descending disturbance degree below: 1. B interferes A's communications, but A does not interfere B's communications. This occurs only when Γ_A < rβ.

2. Both A and B interfere each other's communications.

3. A interferes B's communications, but B does not interfere A's communications. This occurs only when Γ_B < Γ_A. 4. Neither A nor B interfere each other's communications.

It is to be noted that in any specific case two given devices, with fixed interference ranges Γ_B and Γ_A, can only produce either situation number 1 (if Γ_A < Γ_B) or situation number 3 (if Γ_B < Γ_A) but not both. This fact reduces the number of possible mutual interference situations to three, which yields three interference zones as described in relation to Fig.2.

While a device that detects situation number 4, i.e. no mutual interference, does not need to perform any interference mitigation, the remaining situations do require interference mitigation via a proper frequency channel management. For instance: a device A that detects situation number 2, i.e. both devices interfere and "see" each other, conventionally sends one or more frequency realignment messages to inform the other devices within its network that all of them have to switch to an alternative channel.

However, in accordance with an embodiment of the present invention, if device A detects situation number 1, i.e. B interferes A, but not vice versa, A sends one or more frequency realignment messages without previously checking if the medium is free, i.e. without obtaining clear channel assessment (CCA). This proposed procedure raises the likelihood of successfully conveying the frequency realignment message because (1) the unfairness due to the CSMA/CA rules is suppressed and (2), despite having detected some interference, the effective channel quality may be good enough for the receiving device to correctly understand the message.

In general, the frequency channel management procedure may include any combination of the following or additional steps depending on the mutual interference situation detected:

1. Scanning for an interference- free frequency channel. 2. Switching to another frequency channel.

3. Preventive-wise notifying other devices within the network which frequency channel to switch to in case of disturbing interference. This step is only effective prior to an interference situation.

4. Instructing other devices within the network to switch to a certain frequency channel.

5. Suppressing CSMA/CA and instructing other devices within the network to switch to a certain frequency channel (e.g. high priority transmissions).

6. Receiving and storing the notifications and instructions from other devices within the network regarding which frequency channel to use. In the following description a non-limiting exemplary embodiment of the invention will be described in more detail with reference to Fig.5 and the flow chart of Fig.6. In this example the network comprises devices operating in accordance with the ZigBee (hereafter referred to as IEEE 802.15.4) and WiFi standards. In this example it is assumed that the IEEE 802.15.4 device suffers from interference caused by WiFi devices. The interference zone is defined based on the mutual interference situation between the IEEE 802.15.4 device and the WiFi device. Since the transmit power level of WiFi devices is much higher (typically 15 dB) than that of IEEE 802.15.4 devices, and also owing to the wider emission bandwidth of WiFi devices, the WiFi device has a larger interference range (r_wlfi) than the IEEE 802.15.4 device (r₁₅.₄). This means that WiFi interferences are more detrimental to IEEE 802.15.4 than the other way around and that WiFi interferes IEEE 802.15.4 from a longer distance than IEEE 802.15.4 interferes WiFi. This produces (step 601) three possible interference zones between both devices as depicted in Fig.5 and explained below, where the distance between the WiFi device and the IEEE 802.15.4 is depicted by d_wlfi_i₅.4.

Both the WiFi and the IEEE 802.15.4 device interfere each other.

Since both devices interfere each other's communications, they are both capable of noticing each other's presence. Hence, owing to their CSMA/CA behaviour, both hear each other's transmissions and do usually not start sending until the medium, i.e. the channel, is free. Given the limited coexistence properties of CSMA/CA and the extreme proximity of both devices disturbance can still be significant, especially for the IEEE 802.15.4 device. Nevertheless it is possible for the IEEE 802.15.4 device to find a silence interval between WiFi transmissions in which it can successfully send frequency realignment messages to the other devices within its personal area network (PAN). Once the IEEE 802.15.4 device has started transmitting it is not interrupted by a WiFi transmission.

Zone 2 frnj_< dw_jfu±± < Λ«gλ' The WiFi device interferes the IEEE 802.15.4 device, but the latter does not interfere the WiFi device. The IEEE 802.15.4 device always avoids starting a transmission while the

WiFi device is transmitting. On the other hand the WiFi device cannot hear the IEEE 802.15.4 transmission and always assesses the channel to be free. This yields a dramatically unfair situation for the IEEE 802.15.4 device for two reasons: (1) the WiFi device often interrupts eventual IEEE 802.15.4 transmissions and (2) the WiFi device is faster in occupying the channel (no waiting time after an unsuccessful CCA) and hence uses it most of the time. Within this highly disturbing interference zone, the IEEE 802.15.4 device can hardly succeed in sending frequency realignment messages to the other devices within its network.

Zone 3 (rjjj_ < fw,_< dw_jfujj): No device is interfered by the other. Both devices can communicate with their other network partners (not depicted) without suffering any kind of disturbance.

In step 603 the IEEE 802.15.4 device analyses the transmission behaviour of the WiFi device and then based on this behaviour it determines in step 605 the interference zone in which it is located. When no WiFi interference is detected, the IEEE 802.15.4 device decides it is located in "Zone 3". When WiFi interference is detected that often interrupts IEEE 802.15.4 transmissions, the device decides it is located in "Zone 2". When WiFi interference is detected that rarely interrupts IEEE 802.15.4 transmissions, the device decides it is located in "Zone 1". The IEEE 802.15.4 device can determine whether an interference often interrupts its transmissions or not by observing the average number of retransmissions required after successful CCAs. Such information is contained in the MAC layer of the IEEE 802.15.4 radio integrated in the IEEE 802.15.4 device. A high average of required retransmissions while the medium is supposed to be free (after successful CCA) indicates that the WiFi interferer is often interrupting IEEE 802.15.4 transmissions. Furthermore the IEEE 802.15.4 device may use the interfering signal strength as additional criterion to discern between "Zone 1" and "Zone 2". The received signal strength indicator can be used for this purpose.

If the IEEE 802.15.4 device is determined in step 607 to be in an especially disturbing interference situation, i.e. in "Zone 2", it uses in step 609 unconventional communication means to instruct other IEEE 802.15.4 devices within the network to switch to a certain frequency channel. Such means are based on transmissions without previous CCA, which have higher chances of being actually transmitted and received. To this purpose the IEEE 802.15.4 device that detects a "Zone 2" may use any of the following unconventional communication means:

• Include the frequency realignment information in the beacon payload. • Allocate a guaranteed time slot (GTS) in order to use it for sending the frequency realignment information (applicable to IEEE 802.15.4 devices that do not implement ZigBee).

• Simply suppress CSMA/CA to send the frequency realignment information. Optionally, interference identification may be used as additional step for interference mitigation. A more or less trivial method to detect and identify interferences is to provide a wireless-enabled device with additional wireless transceivers, one for each interfering technology to be detected. Due to cost and size reasons such approach is not always feasible. The challenge lies in achieving the same by using only the transceiver already present in the communication device.

According to a further aspect of the invention, a wireless-enabled device detects and identifies interfering wireless technologies by performing signal strength measurements with its narrow-band transceiver and then analysing their frequency and time characteristics with respect to possible interfering technologies. The analysis is performed in parallel for all the wireless technologies under consideration and also adapts itself based on the information extracted from previous measurements.

The analysis relies on the observation of the following items to detect and identify interferences:

• The shape of the interference spectrum. This is determined by the interferer's typical transmission mask.

• The placement of the interference spectrum, i.e. the frequency on which the interference is centred within the frequency band analysed.

• The duration of the transmission bursts of the interference.

It is also possible to detect frequency hopping interferences. A wireless- enabled device detects and identifies the kind of interfering technology based on the speed with which the frequency hopping interference fills the time- frequency space, given a fixed scanning pattern. The scanning pattern consists in measuring the received signal strength on consecutive radio channels within a list of channels to scan and, based on the nature of the measurements, determining whether there is frequency- hopping-like activity on each channel. The time- frequency space is considered to be filled by a frequency hopping interference when frequency hopping activity is detected on a given set of channels.

The identification of the kind of interference helps a device to more efficiently mitigate interferences. There are two reasons for this: • When a device can identify the type of interferer it is exposed to, it also knows the expected behaviour of that interferer and will be able to better detect deviations from that behaviour caused by the device's transmissions. • Once known interferences are characterised, they can be mitigated better. In this way, an IEEE 802.15.4 device that becomes aware of WiFi interferences would avoid switching to channels that have a near-by central frequency because it knows that WiFi devices produce interferences in four contiguous IEEE 802.15.4 channels. So in this case the new communication channel should preferably be four channel hops away from the current channel.

Fig.7 shows a simplified block diagram of the communication device that is arranged to implement the method described above. An antenna 701 is arranged to receive radio signals and they are then fed to a transceiver unit 703. A signal processor

705 processes the received signal. Operations as demodulation and decoding are well known to a skilled person in the art. The processed signal is then fed to an interference detector 707 that detects the amount of interference and possibly its type. A central processing unit (CPU) 709 controls the overall operation of the device and is also arranged to change the normal transmission characteristics by generating a frequency realignment command as explained above. In one embodiment the signal processing and the interference detection run on the CPU.

The present invention can be applied to situations where mutual interference exists between two communication devices. The invention is especially advantageous in situations where coexistence of different wireless technologies is an issue. This is the normal case since most common wireless technologies such as IEEE 802.11 (WiFi) and IEEE 802.15.4 (ZigBee) operate in the same unlicensed 2.4 GHz ISM band and contend with each other for sharing the medium. Concretely the invention can be applied e.g. to wireless patient monitoring and wireless lighting control applications. It is also possible to use both IEEE 802.15.4 for wireless body-worn medical measurements and IEEE 802.11 for ambulatory monitoring side by side in hospitals. Customers expect that IEEE 802.15.4 / ZigBee- based patient monitoring and lighting control solutions are robust against interference and continue their operation also in the presence of WLAN. The invention also relates to a computer program product that is able to implement any of the method steps as described above when loaded and run on computer means of the communication device. The computer program may be stored/distributed on a suitable medium supplied together with or as a part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

The invention also relates to an integrated circuit that is arranged to perform any of the method steps in accordance with the embodiments of the invention.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, the invention being not restricted to the disclosed embodiment. Other variations to the disclosed embodiment can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used. Any reference signs in the claims should not be construed as limiting the scope of the invention.

Claims

1. A method of managing interference in a radio communication network comprising at least a first and second radio communication devices (A; B), said method comprising the following steps:

- identifying (603) mutual interference between the first and second devices; and changing (609) normal transmission characteristics by sending a frequency realignment command to other devices in the network without detecting the current radio communication channel as being free for sending information.

2. The method according to claim 1, further comprising dividing (601) the network into different interference zones characterised by the amount of radio channel interference.

3. The method according to claim 2, detecting the interference zone where the first device (A) is located and sending the frequency realignment command only when the first device (A) is located in a high interference zone.

4. The method according to any one of the preceding claims, further comprising scanning for an interference- free frequency channel, the indication of which is included in the frequency realignment command.

5. The method according to any one of the preceding claims, further comprising switching to the frequency channel indicated by the frequency realignment command.

6. The method according to any of the preceding claims further comprising detecting the type of the interference and using this information for sending the frequency realignment command.

7. The method according to any of the preceding claims, wherein the frequency realignment command is sent by suppressing collision avoidance algorithm.

8. The method according to any one of claims 1-6, wherein the frequency realignment command is sent including this command in a beacon payload.

9. The method according to any one of claims 1-6, wherein the frequency realignment command is sent in a guaranteed time slot.

10. The method according to any of the preceding claims, wherein the first and second communication devices operate in accordance with different communication standards.

11. A computer program product comprising instructions for implementing the steps of a method according to any one of claims 1 through 10 when loaded and run on computer means of a communication device.

12. A radio communication device (A) for managing interference in a radio communication network comprising at least another radio communication device (B), said measurement device comprising:

- means (707) for identifying mutual interference between said radio communication device (A) and said other radio communication device; and

- means (709) for changing normal transmission characteristics by generating a frequency realignment command to be sent to other devices in the network without detecting the current radio communication channel as being free for sending information.