原文Detecting A Wlan Signal Using A Bluetooth Receiver During Bluetooth Scan Activity

背景知识

• WLAN STA周期性唤醒以检测WLAN AP的信标

Two types of devices may engage in WLAN communication, an Access Point (AP) and a station (STA). In one common operational scenario, the STA is not attempting to transmit to a WLAN AP, but the STA is out of radio range of the WLAN AP and is in a low power mode referred to as Out-of-Service (OoS). A WLAN AP periodically transmits beacon packets. If an OoS STA were to come into radio range of a WLAN AP, then beacon packets from the WLAN AP would periodically be receivable at the OoS STA. In such a situation, the STA should detect the beacons and begin communicating with the WLAN AP to join the WLAN network. To accomplish this, an OoS STA periodically scans to detect beacons by waking its receiver scan the WLAN channels. There are thirteen such channels. Although the WLAN receiver is inactive for about ninety percent of the time, the WLAN receiver of the STA is active for about ten percent of the time even though the OoS STA is only scanning and is not actually in use. This amounts to a large amount of power consumption. In mobile devices it may be desirable to reduce this power consumption of an OoS STA.

• 现有方法使用BT接收器来搜索WLAN信号

Some types of mobile devices include a BT transceiver in addition to the WLAN transceiver. If a BT transceiver and a WLAN transceiver exist in the same device, the BT and WLAN transceivers are said to coexist. To avoid wasting power, proposals have been made to use the BT receiver to search for WLAN signals. Rather than using the more power hungry WLAN receiver for this purpose, the BT receiver is used. If energy in the 2.4 GHz unlicensed band is detected using the BT receiver, then the WLAN radio is activated to perform subsequent normal WLAN communications. Published U.S. Patent Application US20081081155, for example, describes using a BT receiver to detect WLAN energy. The BT receiver is tuned to several frequencies that are spaced over the WLAN channel frequency band. An energy sample is taken at each frequency and the samples are analyzed to detect WLAN energy. In another example, the BT receiver is tuned to the center frequency of the WLAN channel frequency band. After accumulating the received energy for a length of time, the energy of the sample is compared to a threshold and if the threshold is exceeded then the WLAN radio is activated. Alternative and better ways of using BT receivers to detect WLAN energy are sought.

• A PTA (Packet Traffic Arbiter) coexistence mechanism 194 couples the WLAN and BT functionalities together to facilitate scheduling of WLAN transmissions in quiet periods between BT transmissions.

• BT receive chain 175 includes a BT receiver RF front end 201, a pair of Low Pass Filters (LPF) 202 and 203, and a pair of adjustable gain amplifiers 204 and 205. Output signal 198 of the BT receiver RF front end 201 actually includes an In-phase (I) signal on conductor 206 and Quadrature-phase (Q) signal on conductor 207. BT receiver RF front end 201 includes a baseband filter 208, Low Noise Amplifier (LNA) 209, and a pair of mixers 210 and 211 interconnected in a quadrature configuration as shown. The control information received onto BT RF transceiver integrated circuit 148 via serial bus 186 adjusts the local oscillator 216 of the BT receiver RF front end by indirectly via other interface circuitry not shown here. BT receiver RF front end 201 has a bandwidth that is appropriate for receiving BT signals and may be too narrowband to be used to receive a WLAN signal such that the WLAN signal can then be successfully demodulated. The receive bandwidth generally is substantially less than 20 MHz and in the present example is about 1 MHz. The BT receiver FR front end 201, however, is usable to detect WLAN energy as described in further detail below.

• A BT scan window in the BT page scan mode is about 11.25 milliseconds in duration. The conventional BT page scan interval, however, is 1280 milliseconds in duration. The BT receiver RF front end is not powered, is not operational, and is not used to receive any type of signal for most of the BT scan interval. The WLAN energy detector circuit 195 that processes the output signal 198 of the BT receiver RF front end 201 in this particular example is not be capable of detecting WLAN energy during the majority of the BT page scan interval.

• The upper waveform illustrates the sequence of BT scan intervals. The value pBT is the 1280 millisecond BT scan interval. The value WBT is the duration of the BT scan window. The value m is an index identifying the interval. The value m is zero for the first interval, is one for the next interval, and is two for the next interval, and so forth. The second waveform down in the diagram of FIG. 7 illustrates the sequence of WLAN intervals. The value pWL is the 100 millisecond WLAN interval. The value wWLBK is the 1.25 millisecond duration of a WLAN beacon. The value x is an unknown time offset between the beginning of the m=0 BT scan interval and the n=1 WLAN interval. Because it is unknown, it can be modeled as a uniform random variable.

• When there is no active traffic except beacons, then there is only one WLAN beacon packet transmitted per WLAN interval. A WLAN AP periodically broadcasts beacons at the lowest supported data rate to announce the network parameters for a WLAN STA to set up a connection. The payload of a WLAN beacon normally is about 100 bytes in length. When transmitted at 1 Mbps, the lowest supported data rate in most WLAN 802.11b/g networks, a beacon has a duration of about 1.25 milliseconds. The WLAN interval is, however, normally about 100 milliseconds. It is possible, depending on the starting time offset between the sequence of WLAN intervals and the sequence of BT page scan intervals, that a BT scan window will never overlap a WLAN beacon transmission.

• FIG. 8 is a table that illustrates how a BT scan window may never overlap a WLAN beacon transmission depending on the interval values, BT scan window duration, WLAN beacon duration, and time offset x. In the table, the first column shows the m index for a sequence of BT page scan intervals. For the BT page scan interval of the row, the value in the second column indicates when the BT scan window starts and the value in third column indicates when the BT scan window ends. In the simplified example of the table of FIG. 8, the BT scan window is 10 milliseconds. The BT scan window is 11.25 milliseconds but the useful portion is 10 milliseconds because that is the longest amount of time where it can be guaranteed that the WLAN beacon is entirely contained in the BT scan window. For the BT scan interval of index m=0, the BT scan window starts at time 0 milliseconds and ends at time 10 milliseconds, as indicated by the values in the second and third columns of the first row of values. Similarly, for the BT scan interval of index m=1, the BT scan window starts at time 1280 milliseconds and ends at time 1290 milliseconds, as indicated by the values in the second and third columns of the second row of values. The third row is the index n of the WLAN beacon that occurs during the time of the BT scan window of the row. Accordingly, WLAN beacon of index n=0 would occur during the time of the BT scan window of index m=0 if the time offset between the two sequences of intervals is in a range from zero milliseconds to ten milliseconds. The second row indicates that the WLAN beacon of index n=12 will occur during the BT scan window of index m=1 if the time offset between the two sequences is in a range from 80 milliseconds to 90 milliseconds. Inspection of the table of FIG. 8 reveals a periodic repeating of the time offsets in the right column. Inspection of the values in the right column of FIG. 8 further reveals that there are values of the time offset x for which there is no row entry. For these values of time offset x there is no WLAN beacon that falls in any the BT scan windows of a sequence, regardless of how long the sequence is. For example, a time offset value of x=12 appears in no row of the table.

• FIG. 9 is a table that illustrates how a BT scan window is guaranteed to overlap a WLAN beacon transmission for BT scan window duration, the WLAN beacon duration, and the WLAN interval of the example of FIG. 8, where the BT scan intervals are varied in duration as indicated in the bottom waveform of FIG. 7. Note that the start of the BT scan window for the m=1 BT interval starts at time 0 and ends at time 10, and that the start of the BT scan window for the m=1 interval starts at time 1280 and ends at time 1290, and that the start of the BT scan window for the m=2 interval starts at time 2560 and ends at time 2570. Note that this dithering of the BT interval corresponds to the dithering shown in the bottom waveform of FIG. 7. For each row, the value in the rightmost column of the table shows the range of values of the time offset x where the indicated WLAN beacon of indicated in the row will overlap the BT scan window of the row. The time offset x can have a value from zero milliseconds to the WLAN interval duration of 100 milliseconds. As indicated in the rightmost column of FIG. 9, a WLAN beacon will overlap a BT scan interval for any time offset value x.

• Mathematically, the required relationships (between the BT interval duration, the WLAN interval duration, the BT scan window duration, the WLAN beacon duration, and the time offset) is shown by the relation of equation (4) that should be satisfied for a WLAN beacon to fall within a BT scan window:

          pBT·m≦pWL·n+x≦pST·m+wBT−wWLBK  Eq (4)

[0072] In equation (4), m and n are integers; pBT is the BT scan interval; pWL is the WLAN beacon interval; wBT is duration of the Bluetooth page scan window; wWLBK is the duration of a WLAN beacon; and x is the timing offset. The timing offset x is a value between 0 and pWL. If pBT=1280 milliseconds and if pWL=100 milliseconds, and if wWLBK=1.25 milliseconds, and if wBT−wWLBK<20 milliseconds, then a timing offset x may exist such that no combination of integers m and n satisfy equation (4). This means that no WLAN beacon will fall into a BT scan window. The resulting detection probability for such a time offset value of x is always zero no matter how good the WLAN energy detector circuit may be.