Patent Publication Number: US-2006013172-A1

Title: RSSI threshold selection for channel measurements based on RSSI of the received packets

Description:
FIELD OF THE INVENTION  
      Embodiments of the present invention relate to devices, networks, methods, computer programs and chipsets for use in communicating via a frequency-hopping radio communications channel. In particular, they relate to the selection of candidate hop frequencies for the communications channel that avoid interference.  
     BACKGROUND TO THE INVENTION  
      Bluetooth (trademark) is a low power radio frequency (LPRF) packet communications technology. Bluetooth enabled devices can create ad-hoc wireless networks (piconets) via short-range radio frequency hopping spread spectrum (FHSS) communication links in the 2.4 GHz frequency spectrum. These links may be of the order of 10 to 200 m.  
      A piconet is controlled by a Master and can contain up to seven Slaves. The piconet has a star-topology with the Master as the central node and the Slaves as dependent nodes. The timing of the piconet is controlled by the Master and the Slaves synchronize their Bluetooth clocks to the Bluetooth clock of the Master.  
      All communications within the piconet include the Master. A Slave cannot communicate directly with another Slave in the piconet, but instead communicates with the Master which then communicates with the other Slave.  
      The communications within the piconet are time divided into slots of 625 microsecond duration.  
      The frequency at which a communication is made is dependent upon the time slot at which it begins. The Master defines a frequency hopping sequence (FHS) that all the devices in a piconet share. The sequence is derived from the Bluetooth address of the Master. The normal frequency-hopping interval in a Bluetooth connection is a slot (625 microseconds). However, if the piconet uses the same channel mechanism, which is introduced in Bluetooth 1.2 Specification, the slave responds to the master on the same frequency as it received the master transmission.  
      Bluetooth devices with power control capability optimize the output power in a physical link. The receiving device measures RSSI and reports back to the transmitting device whether the transmission power should be increased or decreased if possible. This type of power control loop is also known in cellular telecommunication systems.  
      Normally communication within the Bluetooth network is via a radio communications channel having a frequency that hops between 79 potential frequencies.  
      The unlicensed ISM frequency band (2400-2483.5 MHz) is used by the variety of systems e.g. (IEEE 802.11 b/g and Bluetooth). In addition, microwave ovens, and e.g. harmonics of the GSM 850 and IS-95 cause interference in the band. If the interference or traffic of other system occurs at the same time on the same frequency channel as the Bluetooth network then the data throughput of the network is degraded. It would therefore be useful to use the frequency channels that have less interference.  
      Adaptive frequency hopping is a procedure that does this. N ‘good’ candidate frequencies (79&gt;N&gt;20) are selected from the 79 potential frequencies. The candidate frequencies are defined by a channel map, determined at the Master and communicated to the Slaves using AFH_channel_map PDU. The channel map identifies which of the potential 79 frequencies are in use and which are not in use.  
      The Master determines the channel map from channel classifications made by itself and/or from channel classification reports made by the Slaves and communicated to it in the AFH_channel_classification PDU. This PDU indicates whether the frequency channels are good or bad.  
      If a frequency channel has only good classifications then it is selected as a candidate frequency for the network and if it has a bad classification from any device then it is unused. This is recorded in the channel map. Each channel classifications therefore records the candidate frequencies selected by one of the devices performing the classification and the channel map records the candidate frequencies selected by the Master for the network  
      The manner in which the devices assess whether a channel is good or bad is not defined by the BLUETOOTH Specification at present.  
      Typically during the connection a device can measure whole band. These measurements can be based e.g. on Received Signal Level measurement or Packet Error Rate (PER) measurement. When the device compares those measured values to certain reference value it can estimate whether there is interference on that particular channel or not.  
      One approach is to use a fixed RSSI threshold. The noise signal in a frequency channel that is not being used to transmit a packet to or from the device is detected. If the RSSI for the noise signal is above the fixed threshold it is classified as bad, whereas if it is below the fixed threshold it is classified as good.  
      The quality of a communication link between a Master and a Slave depends in part upon the environment of the Master and Slave and the distance between the Master and Slave. If the Master and Slave are physically close to each other and in line of sight then the radio link between them is likely to be robust against interference. It is therefore possible that frequency channels will be classified as bad using the fixed threshold which are capable of being used for communication between the Master and Slave. If a Master and Slave are physically distant from one another and/or not in line of sight then the radio link between them may be susceptible to interference. It is therefore possible that channels will be classified as good using the fixed threshold which are incapable of being used for communication between the Master and Slave.  
     BRIEF DESCRIPTION OF THE INVENTION  
      According to one embodiment of the invention there is provided a device for communicating via a radio communications channel, having a frequency that can hop between a plurality of frequencies, in a network comprising a plurality of devices, the device comprising: a radio transceiver for communicating with a first one of the plurality of devices via a first radio link using the radio communications channel; and  
      means for determining a threshold for use in selecting candidate hop frequencies for the communications channel from the plurality of frequencies.  
      According to another embodiment of the invention there is provided a network comprising a first device and at least a second device and operable to use a radio communications channel having a frequency that can hop between a plurality of frequencies, comprising: a radio transceiver for communicating between the first and second devices via a radio link that uses the radio communications channel; means for determining a threshold for the network using at least an indication for the first radio link; means for selecting, using the threshold, candidate hop frequencies from the plurality of frequencies; and means for controlling the radio communications channel to hop between only candidate hop frequencies.  
      According to another embodiment of the invention there is provided a chipset for controlling a radio communications channel, in a network comprising a plurality of devices, having a frequency that can hop between a plurality of frequencies, the chipset comprising: means for controlling a radio link with a device that uses the radio communications channel; and means for determining a threshold, using an indication for the radio link, for use in selecting candidate hop frequencies for the communications channel.  
      According to another embodiment of the invention there is provided a computer program which when loaded into a processing unit provides: means for determining a threshold, using an indication for an established radio link, for use in selecting candidate frequencies for the communications channel.  
      According to another embodiment of the invention there is provided a method for use in a device communicating, via a radio communications channel, in a network comprising a plurality of devices, the method comprising: a) communicating with a first device via a first radio link, wherein the first link uses a radio communications channel having a frequency that can hop between a plurality of frequencies; b) determining a threshold using an indication for at least the first radio link for use in selecting candidate hop frequencies for the communications channel; and c) communicating with the first device via the first radio link using a radio communications channel having a frequency that hops between only selected candidate hop frequencies.  
      Using a quality indication for a radio link when determining the threshold used in selecting candidate hop frequencies improves the selection process as it is not assumed that every Slave device is in the same environment or position.  
      It is possible to determine a threshold for each link of the network and classify the frequency channels for a link using that link&#39;s threshold. The classifications of the links can then be used to select the candidate hop frequencies. It may therefore be possible to ensure that the candidate frequencies provide an adequate Signal to Noise ratio for each link of the network.  
      It is also possible to determine a threshold for the network and classify the frequency channels for a link using that threshold. The classifications of the links can then be used to select the candidate hop frequencies. It may therefore be possible to ensure that the candidate frequencies provide an adequate Signal to Noise ratio on all links of the network. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a better understanding of the present invention reference will now be made by way of example only to the accompanying drawings in which:  
       FIG. 1  illustrates an ad-hoc wireless network comprising a Master device and a plurality of Slave devices;  
       FIG. 2  illustrates a novel method for determining the candidate hop frequencies;  
       FIG. 3  illustrates a process for calculating the threshold;  
       FIG. 4  illustrates a device  10  that is operable as a Slave/Master in the network; and  
       FIG. 5  illustrates a record medium embodying computer program instructions. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION  
       FIG. 1  illustrates an ad-hoc wireless network  4  comprising a Master device  10   M  and a plurality of Slave devices  10   S1 ,  10   S2 ,  10   S3 . The network  4  is a Bluetooth network and has a star topology with the Master  10   M  at the centre. The Master  10   M  can communicate with any one of the Slaves, but each Slave can communicates with only the Master and not the other Slaves.  
      The network  4  uses a frequency hopping channel that hops between up to 79 different frequencies. The hop sequence that is followed by the network is determined by the Bluetooth Address of the Master and the timing of the hops is determined by the Bluetooth Clock of the Master.  
      In this example, the Master  10   M  is in active communication with the Slaves  10   S1 ,  10   S2 ,  10   S3  via the respective radio links  2   1 ,  2   2 ,  2   3 . A radio link  2  is formed according to the link establishment procedure and involves authentication of the devices.  
      When the network  4  operates without Adaptive Frequency Hopping, the network uses a frequency hopping channel that hops between up to 79 different frequencies.  
      When the network  4  operates with Adaptive Frequency Hopping, the network uses a frequency hopping channel that hops between up to N different candidate hop frequencies where 20&lt;N&lt;79.  
       FIG. 2  illustrates a novel method for determining the candidate hop frequencies for the Bluetooth network  4 .  
      At step  20 , the Master device  10   M  and Slave devices  10   S1 ,  10   S2 ,  10   S3  are communicating via respective radio links  2   1 ,  2   2  and  2   3  using a first communications channel. This channel is a frequency hopping channel that hops between up to 79 different frequencies.  
      At steps  22 ,  24  the Master device  10   M  determines a threshold using a quality indication for one or more of the radio links  2 . This threshold is for use in steps  30 , in the Master and Slave devices, where candidate hop frequencies are selected for the communications channel.  
      At step  22 , the Master device  10   M  measures the quality of the one or more radio links  2 . The quality measurement of a radio link is made at the same time as a radio packet is correctly received via that radio link. The timing may be synchronized by detecting the Access code of the correctly received packet. Typically the quality measurement is a Received Signal Strength Indication (RSSI). The RSSI measurement is a quality indication because it is a measurement relating to a desired signal. Other alternative types of quality indication may be used, such as for example Packet Error Rate (PER).  
      At step  24 , the Master device  10   M  determines the threshold.  
      In a first embodiment, at step  22 , the Master device  10   M  measures the quality of one radio link  2 . The radio link is selected based on a suitable criterion. For example, the most used radio link may be selected, or the radio link with the highest quality of service requirement may be selected or the radio link with the lowest latency requirement may be selected. The Master device  10   M  may measure multiple quality indications for that link over a set period of time. A quality indication is measured for one or more radio packets that are correctly received on the selected radio link during that period of time. Then at step  24 , an average of the measured quality indications is calculated and a predetermined value is subtracted from the average to give the threshold. This guarantees an adequate Signal to Noise ratio on the link. The predetermined value may, for example, be 20 dB. The threshold is, however, constrained to lie within upper and lower boundary limits.  
      In a second embodiment, at step  22 , the Master device  10   M  measures the quality of multiple radio links  2 . The Master device  10   M  may measure multiple quality indications for the radio links over a set period of time. A quality indication is measured for each radio packet that is correctly received on any of the radio links during that period of time. At step  24 , an average of the all measured quality indications from all the links is calculated and a predetermined value is subtracted from the average to give the threshold. The predetermined value may, for example, be 20 dB. The threshold is, however, constrained to lie within upper and lower boundary limits.  
      In a third embodiment, at step  22 , the Master device  10   M  measures the quality of multiple radio links  2 . The Master device  10   M  may measure multiple quality indications for the radio links over a set period of time. A quality indication is measured for each radio packet that is correctly received on any of the radio links during that period of time. At step  24 , a link threshold is separately calculated for each radio link and then one of the thresholds is selected. For each link, an average of the measured quality indications for that link is calculated and a predetermined value is subtracted from the average to give a threshold. The predetermined value may, for example, be 20 dB. The threshold is, however, constrained to lie within upper and lower boundary limits. One of the thresholds is then selected as the network threshold. The selection is based on a suitable criterion. The threshold that is selected may for example be the lowest threshold or may be associated with the most used radio link, the radio link with the highest quality of service requirement or the radio link with the lowest latency requirement. In this example, the subtraction of the predetermined value occurs before selection but it is also possible for selection to precede the subtraction of the predetermined value if it is constant for all the links.  
      In an alternative implementation of the third embodiment, instead of calculating a single network threshold from a plurality of link thresholds the Master sends each of the link thresholds along its respective link for use by the Slave terminating that link in the classification step  30 .  
      An example of a process for calculating the threshold is illustrated in detail in  FIG. 3 . At step  50  a new packet is correctly received and its RSSI is measured. At step  52 , a cumulative total of the measured RSSI is calculated. After a predetermined period of time, or after enough RSSI measurements have been made, the process moves to step  54 . At step  54 , the average RSSI is calculated. Then at step  56 , e.g. 20 dB is subtracted from the average value. If the result of the subtraction is greater than an upper threshold boundary limit (maximum_RSSI_threshold) then the threshold is set to the upper boundary limit at step  57 ,  58 . If the result of the subtraction is less than or equal to an upper threshold boundary limit (maximum_RSSI_threshold) then the threshold is set to the result at step  58 .  
      Returning to  FIG. 2 , at step  26 , the Master device  10   M  transmits the threshold to the Slave devices.  
      At step  28 , the Master device  10   M  and the Slave devices  10   S  measure a noise indication for the signals received at each of the 79 frequencies. The devices know the frequency hop sequence used by the communication channel of the network  4 .  
      The devices make their measurements at a time and at a radio frequency that avoids collision with the hop sequence of the network  4 . Consequently, the measured signal represents noise i.e. signals that do not originate from the network  4 . The noise indication measured may be the Received Signal Strength Indication (RSSI).  
      At step  30 , the frequencies are classified using the threshold distributed at step  26 . This involves selecting, using the determined threshold, local candidate hop frequencies for use in the communications channel from the plurality of frequencies. If the noise indication for a frequency exceeds the threshold then the frequency is classified as ‘bad’, that is unusable because of interference. If the noise indication for a frequency does not exceed the threshold then the frequency is classified as ‘good’, that is usable. The local candidate hop frequencies are those classified as good. These are ‘local’ candidates as they may be unsuitable for use in the network because of local interference elsewhere in the network. The candidate hop frequencies are identified in a classification report. Each Slave sends its classification report to the Master at step  32 .  
      At step  34 , the Master device  10   M  creates a channel map from the received classification reports and its own classification report. If a frequency channel has only good classifications in the classification reports then it is selected as a candidate frequency for the network as it is free of interference throughout the network. If a frequency channel has a bad classification in any one of the classification reports then it is not selected as a network candidate frequency.  
      At step  36 , the Master device  10   M  sends the channel map to the Slaves.  
      At step  38 , the Master and Slaves use a new hop sequence based on the channel map. They communicate using a new radio communications channel having a frequency that hops between only some or all of the network candidate hop frequencies.  
      Although in the above described embodiments, the Master device  10   M  measures the quality of the one or more radio links  2  at step  20 . In other embodiments it is possible for one or more of the Slave devices to measure the quality of the one or more radio links  2 . The Slave or Slaves would then transmit the measurements to the Master device. Measurement at the Slave(s) may be as an alternative or as an addition to measurement at the Master.  
      Although in the above described embodiments, the Master device  10   M  determines the threshold at step  22 . In other embodiments it is possible for one of the Slave devices to determine the threshold. The Slave would then transmit the threshold to the Master.  
      Although in the above described embodiments a single network threshold is calculated and then distributed, in other embodiments each Slave device that performs the step  30  of classifying the frequency channels can calculate its own link threshold for use in that step. In these embodiments, there is no step corresponding to step  26  and the steps  22 ,  24 ,  28  and  30  are performed in the same device. The step  22  when carried out by a Slave device measures the quality of the link with the Master and calculates a link threshold, for example, using the method illustrated in  FIG. 3 . This link threshold is then used to classify the frequency channels at the Slave device.  
      The method of steps  22  to  38  is then repeated at a later time, for example periodically. The method of steps  22  to  26  may occur independently to the method according to steps  28  to  38 . The method of steps  22  to  26  may occur with a first periodicity and the method of claims  28  to  38  may occur with a second, shorter periodicity (greater frequency).  
      The use of upper and lower boundary limits for the threshold is advantageous. The upper limit obviates the selection of a frequency channel that collides with and degrades other transmissions such as WLAN transmissions. The minimum level ensures that there are at least some candidate hop frequencies in conditions when the quality of the radio links is poor. The upper limit will be used if the Slave devices are very close to the Master and the lower limit will be used when the Slave devices are very far from the Master. The threshold limits may be adapted  
       FIG. 4  illustrates a device  10  that is operable as a Slave and as a Master.  
      The device comprises: a radio transceiver  70  for communicating using the radio communications channel; a processor  72 ; a memory  74 , circuitry  76  for measuring a quality indication of a frequency channel and functional circuitry  71 .  
      The processor  72  is connected to read from and write to the memory  74  and is connected to receive data from and provide data to the radio transceiver  70 .  
      The circuitry  76  for measuring a quality indication of a frequency channel is connected to the radio transceiver  70  and the processor  72 . In other implementations it may be integrated with the radio transceiver. The circuitry  76  is operable to measure the quality indications and the noise indications described above.  
      The memory  74  stores computer program instructions  78 , which when loaded into the processor  72  controls the operation of the device  10 . When the device is operating as a Master the computer program instructions control the device to perform the method steps to the left of  FIG. 2  that are performed by the Master. When the device is operating as a Slave the computer program instructions control the device to perform the method steps to the right of  FIG. 2  that are performed by the Slaves.  
      The computer program instructions  78  may be received at the device via an electromagnetic carrier signal received via a radio transceiver or may be transferred from a physical entity  90  such as a record medium e.g. CD-ROM, solid state memory etc as illustrated in  FIG. 5   
      The functional circuitry  71  is connected to the processor  72  and provides functionality not relating to the Bluetooth network  4 . It may, for example, provide a display, an input device and applications if the device is operable as a personal digital assistant, for example, or provide a display, an input device, a cellular radio transceiver and applications if the device is operable as a cellular mobile telephone.  
      In this example, the combination of processor  72 , memory  74 , computer program instructions  78  and circuitry  76  provide the means referred to below. In other implementations, the means may be provided by dedicated circuits such as ASICs.  
      The device  10  comprises:  
      a) means for obtaining a quality indication for a radio link by measuring the quality of the radio link as a radio packet is correctly received via that link.  
      b) means for determining a threshold using one or more quality indications for one or more radio links.  
      c) means for averaging a plurality of quality indications and determining the threshold using the average and calculating the threshold from the average. The quality indications may be for the same link or for multiple different links.  
      In the first embodiment, the means for determining the threshold is operable to calculate a average of the quality indications of a single link.  
      In the second embodiment, the means for determining the threshold is operable to calculate a single average of the quality indications of the multiple different links.  
      In the third embodiment, the means for determining the threshold is operable to calculate an average of the quality indications for each of the multiple different links and selects an average. The selection may involve one of: selecting the lowest quality average; selecting the average associated with the most used link; or selecting an average based on at least one Quality of Service requirement.  
      The combination of processor  72 , memory  74 , computer program instructions  78 , circuitry  76  and radio transceiver  70  may be sold as a chipset  73 . A chipset is a set of integrated circuits that have a specific purpose in a computer system. The purpose of the chipset is to enable a device using the chipset to participate in a Bluetooth network and to perform one or more embodiments of the invention. The radio transceiver circuitry and possibly the circuitry  76  may be part of a separate RF chipset. In this case the processor  72 , memory  74 , computer program instructions  78  for a base band chipset.  
      The chipset  73  consequently comprises means for controlling a radio link with a device that uses the radio communications channel; and means for determining a threshold, using a quality indication for the radio link, for use in selecting candidate hop frequencies for the communications channel.  
      The computer program  78  when loaded into the processor provides the means for determining a threshold, using a quality indication for an established radio link, for use in selecting candidate frequencies for the communications channel. The computer program instructions may be provided separately to the chipset, for example, on a record carrier such as a CD-ROM, etc  
      Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the spirit and scope of the invention.