WO2018141354A1

WO2018141354A1 - Method for improving the scan time for low energy, sub-noise-floor signals by interleaving scans across multiple channels

Info

Publication number: WO2018141354A1
Application number: PCT/EP2017/051990
Authority: WO
Inventors: Christopher Lowe; Cedric VAN DEN BERGH; Justin Penfold
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2017-01-31
Filing date: 2017-01-31
Publication date: 2018-08-09
Anticipated expiration: 2019-07-31

Abstract

A receiving device for detecting a transmitted signal from a communications entity on at least one of a predetermined set of channels, the transmitted signal having a repeating feature, the receiving device being configured to: repeatedly perform the following step (i): receive signals on a first one of the channels for a first period; correlate the signals received in each iteration of step (i) to form a correlated signal; and based on the correlated signal, estimate whether the repeating feature of the transmitted signal is present on the first channel.

Description

METHOD FOR IMPROVING THE SCAN TIME FOR LOW ENERGY, SUB-NOISE-FLOOR

SIGNALS BY INTERLEAVING SCANS ACROSS MULTIPLE CHANNELS

BACKGROUND

This invention relates to scanning for signals on a plurality of channels, for example radio channels. Scanning for a low-power signal on one of a plurality of channels can be a time-consuming process.

Some signals can be detected by searching for a known synchronisation sequence on one of multiple channels. To detect this type of signal, a receiving device, for example a user equipment (UE) device, can tune to a channel and correlate the signal received on that channel with an expected synchronisation sequence. If a signal matching the expected sequence can be extracted from the noise then the sequence can be assumed to be present on that channel. Otherwise, the receiver can switch to another channel and try to receive the sequence there. For some protocols, the repeat period of the synchronisation sequence can be relatively long when compared to the tune time of the receiver. Therefore, it can take a considerable amount of time to acquire a correlatable signal on each channel, particularly when the signal has a low signal-to-noise ratio. The signals that are being received may be affected by slow fading, where there are periods of reduced signal strength and quality, and this may mean that the signals must be received over an extended period in order to detect or rule out the presence of the synchronisation sequence.

Normally, a receiving device will receive on a single channel for a sufficient period to confirm that there is no synchronisation sequence present on that channel, before moving to the next channel and repeating this process. This can be time consuming, especially when the signal requires significant processing or correlation to be identified.

Some implementations perform a quick scan of each channel in case the synchronisation sequence can be readily received, and then a slow scan of each channel if the quick scan was unsuccessful. This is faster than performing a slow scan when the reception conditions are good, but when reception conditions are poor it takes longer than simply performing a slow scan. US 8300613 describes a WLAN radio that scans for a synchronisation beacon for a listening duration on each of multiple channels. The device rotates through all channels until an entire beacon period is listened to, with the number of iterations chosen to minimise the overlap of successive listening durations, reducing the total scan time. However, using this method, it is difficult to detect a beacon with a low signal-to-noise ratio because the device scans the non- overlapping parts of the beacon signal on each channel only once.

It is desirable to reduce the time taken for a device to confirm that a low-power signal is present on one of a plurality of possible channels.

SUMMARY OF THE INVENTION

According to one aspect there is provided a receiving device for detecting a transmitted signal from a communications entity on at least one of a predetermined set of channels, the transmitted signal having a repeating feature, the receiving device being configured to: repeatedly perform the following step (i): (i) receive signals on a first one of the channels for a first period; correlate the signals received in each iteration of step (i) to form a correlated signal; and based on the correlated signal, estimate whether the repeating feature of the transmitted signal is present on the first channel.

The repeating feature may be periodic and the receiving device may be configured to perform each iteration of step (i) at the periodicity of the repeating feature. This may allow a correlatable signal to be acquired over multiple iterations of the same portion of the signal.

The receiving device may be configured to repeatedly perform the following step (ii): receive signals on a second one of the channels for a second period. This may allow the device to determine which of the first and second channels the signals is present on. The receiving device may be configured to performs steps (i) and (ii) alternately. This may allow for efficient detection of the signal.

The duration of the repeating feature may be greater than 50% of the period of the repeating feature. The first and second periods may be less than the duration of the repeating feature. This may allow a correlatable signal to be acquired for each channel by scanning only a portion of the repeating feature. The first and second periods may be equal to or greater than the duration of the repeating feature. This may provide for faster acquisition of the signal in a slow fading environment.

The first and second periods may be spaced apart in time. In each repetition of steps (i) and (ii) those steps may be performed during a single instance of the repeating feature. This may allow a correlatable signal to be acquired for each channel by scanning only a portion of the repeating feature.

The signal may be below the noise floor of its channel at the receiving device. The device may be able to detect and correlate such signals.

The first and second channels may have different carrier frequencies. Aspects of the system discussed above may mitigate against the tune time needed to acquire signals on a particular frequency.

The receiving device may be configured to: correlate the signals received in each iteration of step (ii) to form a second correlated signal; and based on the second correlated signal, estimate whether the transmitted signal is present on the second channel. This may allow the device to assess which of the first and second channels carries the transmitted signal.

The receiving device may be configured to, in response to estimating that the transmitted signal is present on one of the first and second channels, use that channel for subsequent data communications. This may allow the device to select the appropriate channel. The receiving device may comprise a receiver front end for receiving the signals, and a processor configured for correlating the signals and estimating whether the transmitted signal is present. The channels may be wireless channels and the receiving device may be configured to receive the signals as wireless signals. This may allow the above techniques to be used in wireless telecommunications networks.

The receiving device may be configured to correlate the received signals by, for each channel, accumulating the signals received on that channel to form an accumulated signal and subsequently correlating the accumulated signal with an expected synchronisation signal. Alternatively, the receiving device may be configured to correlate the received signals by, for each channel, accumulating a correlation of the signals received on that channel against an expected synchronisation signal. Alternatively, the receiving device may be configured to correlate the received signals by, for each channel, accumulating an autocorrelation of successive repetitions of the received signals. These approaches may provide an efficient means of correlating the signals.

According to another aspect there is provided a method for detecting a transmitted signal from a communications entity on at least one of a predetermined set of channels, the transmitted signal having a repeating feature, the method comprising: repeatedly performing the following steps (i) and (ii) in turn:(i) receiving signals on a first one of the channels for a first period; and (ii) receiving signals on a second one of the channels for a second period; correlating the signals received in each iteration of step (i) to form a correlated signal; and based on the correlated signal, estimating whether the transmitted signal is present on the first channel.

BRIEF DESCRIPTION OF THE FIGURES The present invention will now be described by way of example with reference to the accompanying drawings.

In the drawings: Figure 1 shows a communication network for sending signals from a network entity to a user equipment device.

Figures 2 illustrates the receipt of repetitious signals by a device on different channels for the encircled time portions of the signal.

Figure 3 illustrates the receipt of repetitious signals by a device on different channels for the encircled periods of the signal.

DETAILED DESCRIPTION OF THE INVENTION

The system to be described below may be used in a communications network, as illustrated in Figure 1. A server 1 is connected, via a network 2 such as the internet, to a core network 3. The core network is connected to a base station 4, which comprises a wireless transceiver 5, a processor 6 and a memory 7, with two parts for storing code and messages respectively. A wireless transceiver 8 in the base station communicates with a user equipment device 9 via wireless transceiver 10. The user equipment device also comprises a processor 1 1 , a memory 12 with two parts for storing code and signals respectively, and a user interface 13 for presenting information orfor sensing environmental data. The user interface 13 may comprise a mechanism for communicating or interacting with the device's environment or user, for example a display, touch screen, or one or more transducers.

The server 1 can send a signal to the core network 3 via the network 2. The signal includes a destination address. Based on that destination address, which is the address of the receiving device to which the signal is intended to be delivered, the core network routes the signal to a suitable base station 4. At the base station, the signal is sent on to the user equipment device 9. The signal is sent over a wireless link from wireless transceiver 8 in the base station to a wireless transceiver 10 in the device. The network 2 may be a wide or narrow band network. It may be an internet of things network.

The device 9 may be capable of receiving signals from a plurality of different base station of the type 4. Each base station may send signals on a single frequency channel. Alternatively, a single base station 4 may be capable of sending signals on a plurality of different frequency channels. When a user equipment device is to communicate with a base station, it may need to establish which channel it should communicate on and/or it may need to synchronise its clock with the operations of the base station. This situation may, for example, arise when a UE device has first been turned on, or when it moves to a new location. Similar considerations may arise when receiving devices that are not UEs need to connect to other devices: for example if a base station is to communicate wirelessly with another base station. The following description describes methods by which a receiver can detect a synchronisation signal.

Figure 2 illustrates one embodiment. A receiving device, such as device 9 in Figure 1 , is configured to receive signals on three channels: 1 , 2 and 3. The channels could, for example, differ in frequency, spreading code or some other feature. Channels 1 , 2 and 3 each carry a repetitious signal. The repetitious signal may be a synchronization sequence. In this example the synchronization sequence is illustrated as being "ABCDEF". The receiving device configures itself to receive on channel 1 and scans (i.e. receives on) that channel for the duration of the part "A" of the synchronization sequence, as indicated at 14. The receiving device then configures itself to receive on to channel 2 and scans that channel for the duration of the part "C" of the synchronization sequence, as indicated at 15. The receiving device will then configure itself to receive on channel 3 and scan that channel for the duration of the part "F" of the synchronization sequence, as indicated at 16. It is assumed that some time is taken for the device to configure itself to receive on to a respective channel. This is the device's tune time. As a result of this delay there is an interruption in the receiving device receiving signal as it switches from one channel to the next.

The receiving device then repeats these steps during the next period of the repetitious signal ABCDEF, returning to channel 1 and scanning for period A at 17, to channel 2 for period C at 18 and to channel 3 for period F at 19. The device then repeats this process for subsequent periods of the repetitious signal. Each reception event on a particular channel is of the same duration. The start of each reception event on a particular channel is spaced apart by the repeat period of the synchronization sequence, or an integer multiple of it. In this way, if the synchronization sequence is present on a channel then the same part of the synchronisation sequence will be received on each reception event.

After each reception event on a channel, the receiving device correlates the data received so far on that channel with a part of the synchronization sequence. If the receiving device is already time-synchronised to the source of the synchronization sequence then it may correlate with just the part of the synchronization sequence that is expected to be received at the time of the reception event (e.g. "A" for channel 1 , in the present example). Alternatively, the receiving device may correlate the received data with a sliding window on the whole synchronization sequence. Ways in which the signals can be correlated are described in more detail below. The receiving device repeats the process of cycling through the channels until a synchronization sequence or part of one is detected, or until the process times out.

Thus, in summary, the receiving device attempts to receive just a portion of the repetitious synchronization sequence on one channel, synchronised such that the same portion is received for each repeat period of the sequence. While the device is not receiving on that channel, it tunes to another channel and performs the same steps for its respective time period. In this manner, the device scans over several channels in an interleaved manner.

The synchronization sequence could be used for time synchronisation only or for channel detection only or for both purposes. The time periods sampled on each channel could be spaced apart in time over a repetition period. For example, in Figure 2 the time periods when A, C and E might be received are spaced apart by periods corresponding to portions B, D and F. Alternatively, the device could move to a sample on a subsequent channel immediately after sampling for a period on the previous channel and the periods could only be separated by the tune time of the subsequent channel. The correlation of the received signal with the expected synchronisation signal may be performed in a number of ways. In one approach, for each channel, the repetitions of the portions of the received signal may be accumulated, before the accumulated signal is correlated with the expected portions of the synchronisation signal. Alternatively, the device may accumulate the correlation of the received signal portions on each channel against the expected portions of the synchronization signal. Alternatively, it may accumulate the autocorrelation of successive repetitions of the received signal portions on each channel.

Thus, using this portion-based approach, when scanning for a synchronization sequence that must be accumulated over multiple repeats in order to detect it in poor signal-to-noise conditions, receptions from different frequency channels can be interleaved such that the overall detection process is performed in parallel on multiple channels, rather than searching sequentially on different channels.

This approach may lead to faster overall scanning than some prior art approaches if some frequency channels have a relatively good signal to noise ratio, so that the device can detect the signal quickly even using just the small proportion of the repetition period.

For this portion-based approach, exemplified by Figure 2, the technique may in some situations give a factor of N improvement in scan times over scanning each channel sequentially, where N is the level of interleaving achieved, providing the signals can be correlated with just a subsection of the period being sampled on each channel.

The device may use this portion-based approach if the synchronization sequence has repetitions that are relatively long, allowing the device to receive for a sufficient period to permit correlation during just a portion of the synchronization signal.

Alternatively, instead of scanning each channel for just a portion of the repetitious signal, the device could scan for entire repetitions on one channel, and then switch to the next channel to do the same. By receiving in this manner the receiving device my obtain some benefit of time diversity, to counter the effects of slow fading. Slow fading may be encountered when the transmission of the signal to the device is blocked by a large object, such as a building or a tree. By receiving multiple times on a channel spread over an extended period of time the receiving device may be able to negate the effects of slow fading, without having to receive constantly on the channel. This can reduce the time needed to acquire the synchronization sequence in slow fading conditions. Figure 3 illustrates this alternative implementation. The repetitious signal received on each of channels 1 , 2 and 3 has a repeat F. The device scans for a period corresponding to at least the complete duration of repeat unit F on channel 1 , at 20. The device then connects to channel 2 and scans for at least a period corresponding to at least the complete duration of repeat unit F on that channel at 21 . The device then connects to channel 3 and scans for at least a period corresponding to at least the complete duration of repeat unit F at 22. The time taken for the device to connect to a respective channel is the respective tune time.

The device then repeats this process for subsequent periods of the repetitious signal. Each reception event on a particular channel is of the same duration. The start of each reception event on a particular channel is spaced apart by an integer multiple of the repeat period of the synchronization sequence. In this way, if the synchronization sequence is present on a channel then the repeat unit of the synchronisation sequence will be received in a time- synchronised manner at each reception event. After each reception event on a channel, the receiving device correlates the data received so far on that channel with the repeat unit of the synchronization sequence. If the receiving device is already time-synchronised to the source of the synchronization sequence then it may correlate with the repeat unit in expected synchronization with the received data. Alternatively, the receiving device may correlate the received data with a sliding window on the synchronization sequence. The receiving device repeats the process of cycling through the channels until a synchronization sequence or part of one is detected, or until the process times out.

For each channel, the repetitions of the received signal may be accumulated before the accumulated signal is correlated with the expected synchronisation signal. Alternatively, the device may accumulate the correlation of the received signal on each channel against the expected synchronization signal. Alternatively, it may accumulate the autocorrelation of successive repetitions of the received signal on each channel. The device may use this period-based approach if the synchronization sequence has repetitions that are relatively short. The device may receive on an individual channel multiple times over an extended period, correlating the signals received on that channel to assess whether a synchronisation sequence is present on that channel. Between receive operations on the channel, the device may receive on one or more other channels, correlating the signals received on each of those channels to assess whether a synchronisation sequence is present there. The extended period may be greater than 1 second, greater than 5 seconds, greater than 10 seconds or greater than 30 seconds.

This period-based approach gives a degree of time diversity which can counteract the effects of slow fading. If the device is operating on a channel that is subject to multipath fading with a Doppler period that is long compared with the repetition period of the synchronization sequence, the device can receive one, two or more repetitions of the signal in succession on one frequency channel. The number of repeats to be received in succession can be selected such that it is unlikely of itself to be sufficient to detect the signal in poor signal-to-noise ratio conditions. While the device is not receiving on that channel, it will tune to another channel and do the same.

Using the period-based approach, on any given channel, the synchronization processing is spread over a longer total duration in time than if the channels were each scanned sequentially for multiple repeat periods. This means that the receiver can benefit from more time diversity of the channel. If the signal moves in and out of fades over time, according to the Doppler period, the signal is less likely to be stuck in a fade for the whole duration of the synchronization attempt, which can result in insufficient data to correlate the received signal with the expected synchronisation sequence. Therefore, the increased time diversity achieved on each channel can reduce the active time that the receiver must dwell on any given channel in order to be sure of detecting a wanted signal if it is present. Since multiple channels are being received in an interleaved manner, the average time needed to scan each channel is reduced. Thus, using the above method, the scans from different channels are interleaved to reduce the overall time to identify the channel carrying the required signal using the portion based approach, or to improve time diversity using the full period approach.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1 . A receiving device for detecting a transmitted signal from a communications entity on at least one of a predetermined set of channels, the transmitted signal having a repeating feature, the receiving device being configured to:

repeatedly perform the following step (i): receive signals on a first one of the channels for a first period;

correlate the signals received in each iteration of step (i) to form a correlated signal; and

based on the correlated signal, estimate whether the repeating feature of the transmitted signal is present on the first channel.

2. A receiving device as claimed in claim 1 , wherein the repeating feature is periodic and the receiving device is configured to perform each iteration of step (i) at the periodicity of the repeating feature.

3. A receiving device as claimed in claim 2, wherein the duration of the repeating feature is greater than 50% of the period of the repeating feature.

4. A receiving device as claimed in any preceding claim, wherein the receiving device is configured to repeatedly perform the following step (ii): receive signals on a second one of the channels for a second period.

5. A receiving device as claimed in any preceding claim, wherein the receiving device is configured to performs steps (i) and (ii) alternately.

6. A receiving device as claimed in claim 4 or claim 5, wherein the receiving device is configured to:

correlate the signals received in each iteration of step (ii) to form a second correlated signal; and

based on the second correlated signal, estimate whether the repeating feature of the transmitted signal is present on the second channel.

7. A receiving device as claimed in claim 6, wherein the receiving device is configured to, in response to estimating that the transmitted signal is present on one of the first and second channels, use that channel for subsequent data communications.

8. A receiving device as claimed in any preceding claim, wherein one or both of the first and second periods is/are less than the duration of the repeating feature.

9. A receiving device as claimed in any preceding claim, wherein one or both of the first and second periods is/are equal to or greater than the duration of the repeating feature.

10. A receiving device as claimed in any of claims 4 to 9, wherein the first and second periods are spaced apart in time.

1 1 . A receiving device as claimed in any of claims 4 to 10, wherein in each repetition of steps (i) and (ii) those steps are performed during a single instance of the repeating feature.

12. A receiving device as claimed in any preceding claim, wherein the signal is below the noise floor of its channel at the receiving device.

13. A receiving device as claimed in any of claims 4 to 12, wherein the first and second channels have different carrier frequencies.

14. A receiving device as claimed in any preceding claim, the receiving device comprising a receiver front end for receiving the signals, and a processor configured for correlating the signals and estimating whether the transmitted signal is present.

15. A receiving device as claimed in any preceding claim, wherein the channels are wireless channels and the receiving device is configured to receive the signals as wireless signals.

16. A receiving device as claimed in any preceding claim, wherein the receiving device is configured to correlate the received signals by, for each channel, accumulating the signals received on that channel to form an accumulated signal and subsequently correlating the accumulated signal with an expected synchronisation signal.

17. A receiving device as claimed in any of claims 1 to 15, wherein the receiving device is configured to correlate the received signals by, for each channel, accumulating a correlation of the signals received on that channel against an expected synchronisation signal.

18. A receiving device as claimed in any of claims 1 to 15, wherein the receiving device is configured to correlate the received signals by, for each channel, accumulating an autocorrelation of successive repetitions of the received signals.

19. A method for detecting a transmitted signal from a communications entity on at least one of a predetermined set of channels, the transmitted signal having a repeating feature, the method comprising:

repeatedly performing the following step (i): receiving signals on a first one of the channels for a first period;;

correlating the signals received in each iteration of step (i) to form a correlated signal; and

based on the correlated signal, estimating whether the repeating feature of the transmitted signal is present on the first channel.