The present invention relates to mobile communications systems in which access points provide service to one or more mobile devices having a main receiver and a wake-up receiver, and more particularly to detecting when the wake-up receiver of a mobile device is not within range of a wake-up signal transmitted by the access point.
In mobile communications technology, the reduction of power consumption in various types of user equipment has long been a topic of interest and continues to be an important consideration in the design of next generation systems. The need to reduce power consumption is often very pronounced for devices targeting the Internet-of-Things (IoT), which can be considered an Internet-enabled connection of devices beyond those traditionally used for person-to-person communication. A large number of these IoT devices are expected to be powered by coin-cell batteries, which means that minimization of energy consumption is of utmost importance. In the future, such devices may be designed to harvest their own energy, and such a development would even further increase the importance of low energy consumption.
The supported data rates are typically low for these kind of applications, both with respect to peak data rates and aggregated data rates during, for example, an average day. This means that a major part of the power consumption is not related to transmitting or receiving data, but rather is expended when the devices are listening to the radiofrequency spectrum to determine whether there is a transmission for which it is the intended receiver.
As a result, a large part of the total energy consumption is due to the device's listening for a potential transmission that very often is not there, and this has motived the development of so-called wake-up receivers (WUR). A wake-up receiver is a device having extremely low power consumption and whose only purpose is to wake up another receiver, a so-called “main transceiver” (main receiver and transmitter), in the device. An IoT device with a wake-up receiver therefore conserves energy by not needing to turn on the main receiver to scan for a potential packet; it instead turns on the wake-up receiver. If in fact there is data for the IoT device, a wake-up signal (WUS) will be sent to the WUR. When the wake-up receiver has detected this WUS, and determined that there is data present it will then wake up the main receiver and transmitter, and a communication link can be established.
An Access Point (AP) may support a number of devices, or “stations” (STA), all having wake-up receiver capability, in which case it needs a way of directing the WUS to a particular wake-up receiver whenever there is data available for the STA containing that wake-up receiver. One way of doing this is by means of a Traffic Indication Map (TIM) element transmitted within a beacon transmission. In this arrangement, the AP enables one or more bits within the TIM element, where the particular bits enabled are those corresponding to the association identifier of the stations. If a bit is enabled, it indicates the availability of downlink data to the corresponding STA. This strategy helps a station to be in power save mode and to wake up for the beacon transmissions. If the AP indicates the availability of downlink data, the station may send the Power Save-Poll (PS-Poll) to get the downlink data.
For the stations that are not in power save mode, and have their transceivers always powered on, the AP may directly transmit the downlink data without requiring the polling process.
Initially, wake-up receiver technology was mainly the subject of academic research, but recently it has also been brought in for consideration in IEEE 802.11 standardization, with the wake-up radio study group being started. The intention is to have low power wake-up radios as a companion radio for the main 802.11 transceiver. The wake-up receiver wakes up the main 802.11 transceiver when a packet is available for reception, but the 802.11 main transceiver is switched off during the rest of the time. The use of a wake-up receiver enables energy efficient data reception without increasing latency. The wake-up signal transmitted by the access point to the specific station is intended as an indication that there is downlink data available for it to receive.
The wake-up receiver processes the WUSs carrying few bits of information. The wake-up receiver then wakes up the main receiver only if the identifier section of the wake-up signal matches with the expected bits. For example, Wen-Chan Shih et al., “A Long-Range Directional Wake-Up Radio for Wireless Mobile Networks”, JOURNAL OF SENSOR AND ACTUATOR NETWORKS, 2015, ISSN 2224-2708 discloses a wake-up signal carrying 40 bits, out of which 16 bits represent station identifiers (STA-IDs) and 24 bits represent additional information.
Minyoung Park et al., “Proposal for Wake-Up Receiver (WUR) Study Group)”, document: IEEE 802.11-16/0722r1, dated May 18, 2016 and available at https://mentor.ieee.org/802.11/dcn/16/11-16-0722-01-0000-proposal-for-wake-up-receiver-study-group.pptx discloses an 802.11 compatible wake-up packet design that, as illustrated in FIG. 1, includes:                Legacy 802.11 preamble (OFDM) consisting of a Legacy Short Training Field (L-STF), a Legacy Long Training Field (L-LTF), and a Legacy SIGNAL Field (L-SIG) thereby providing coexistence with the legacy STAs        A new Wakeup signal waveform based on On-Off Keying (OOK) containing a Wakeup preamble, a Medium Access Control (MAC) header (receiver address), a Frame body, and a frame check sequence (FCS)        
A station may inform an access point that it is wakeup receiver capable. Armed with this information, the access point may then use the wakeup signal to indicate the availability of downlink data to those one or more stations that indicated that they are wake-up capable. Upon receiving the wakeup signal, the wake-up receiver wakes up the 802.11 transceiver. The station may acknowledge the reception of WUS by transmitting a PS-Poll packet. The access point may transmit the corresponding downlink traffic on receiving the PS-Poll from the station.
There still remain problems with this technology that need to be addressed. For example, wake-up receivers are expected to be less power consuming compared to the 802.11 but this is achieved by making certain design tradeoffs, such as by designing receivers to have comparatively lower receiver sensitivity. This means that, although the wakeup signal may be transmitted at what are normally adequate signal levels for serving a main transceiver, the transmission range of the wakeup signal intended for wake-up receivers may be less than the legacy transmissions. This comparison is illustrated in FIG. 2, which shows five stations 201, 203, 205, 207, and 209, all served by an access point 211. As further illustrated, all of the stations 201, 203, 205, 207, and 209 are within range to engage in communication with the access point 211 using their main, 802.11 transceivers. But only three of the stations 201, 203, and 205, are within range to receive a wake-up signal from that same access point 211.
In this type of situation, the wake-up receiver can essentially be used for only a fraction of the coverage area of the actual data transmission. The usage of a wake-up receiver may still be beneficial because the wake-up receiver is often within range. However, since this is not always the situation, the STA may need to know whether the station is within the WUS transmission range of the access point or whether it is out of range.
In a related problem, the access point may also need to know which stations are within the transmission range of the WUS.
Hence there is a need for technology that addresses the above and/or related issues.