Wake-Up Radio – A key component of IoT?
Academic researchers have worked on wake-up receivers for some 10 years, with the goal of taking a disruptive step towards very low power consumption for wireless devices. In May 2016 the IEEE 802.11 Working Group took the first step towards standardizing wake-up receivers. This led to the formation of the 802.11ba Task Group, which is one of the groups within the IEEE 802.11 Working Group that attracts the most interest. The target is to have a draft 1.0 of the specification ready in May 2018.
The basic idea behind Wake-Up Radio
As the cost for the chipsets for IoT devices decreases, a bottleneck for massive deployment of IoT may be the ability – or lack of it – to provide these chipsets with energy, and preferably energy that will last the entire lifetime of the chipset.
Many devices intended for IoT applications will only transmit and receive very small amounts of data very rarely, say, a few times a day. In many cases, there is sufficient energy in a coin-cell battery for all useful communication in such a device’s lifetime. The problem is that in this scenario an IoT device does not know when to expect to receive data, and thus must perform energy-draining scanning to detect the presence of a packet.
Imagine a receiver in a very low power mode that would, in a magical way, wake up just in time for the data to be received. The magic trick to make the main receiver, hereafter referred to as the primary connectivity radio (PCR), wake up just in time is to add a small companion receiver, with the sole purpose to wake up the PCR, see Figure 1. The small companion receiver is referred to as a wake-up receiver (WURx).
At first, it may seem that this is just another receiver so why would this reduce power consumption? The core idea is that whereas the PCR is designed to transmit and receive data, potentially at Mb/s or even Gb/s, the WURx only needs to receive a single bit of information – the presence of a Wake-Up Packet (WUP).
When the WURx receives a WUP with the right content for waking up the PCR, the PCR is woken up and the PCR then transmits an acknowledgement (ACK) to the transmitter of the WUS to indicate that, in fact, the WUS has been received. In this way the magic of waking up the PCR just in time is achieved.
Figure 1. Illustration of how a WURx is added to an 802.11 transceiver. On the left-hand side, the transceiver is in the low power mode, and on the right-hand side the transceiver is fully operational.
Standardization in IEEE 802.11
In May 2016 work on the Wake-Up Radio in IEEE 802.11 was a first step towards standardizing a WURx. To reflect that a standardized solution includes important MAC features and is not just a power efficient receiver, the term Wake-Up Radio (WUR) is used rather than Wake-Up Receiver. From July to November 2016, the scope and targets of the standard were defined, which are summarized in the Project Authorization Request (PAR). There are three key requirements in the PAR:
- The WUR should have an active power consumption of less than 1mW.
- The WUR should coexist with legacy IEEE 802.11 devices in the same band.
- The WUR should meet the same range requirement as the primary connectivity radio.
The PAR was approved and the work to develop a specification for the WUR began in January 2017 within Task Group (TG) 802.11ba. Since both the PHY and the MAC of 802.11ba are supposed to be significantly less complex than, for example, the PHY and MAC of traditional Wi-Fi (e.g. 802.11ac or 802.11ax), the goal is to develop the standard in considerably less time than is usually needed for an 802.11 amendment. If things progress according to plan, the standard specification will be published at the end of 2019.
How are these three requirements fulfilled?
The requirement on very low power consumption means that the power is reduced to about 1% of the PCR. To allow for a power efficient receiver implementation, the data is sent using ON-OFF keying (OOK). A simple receiver architecture suitable for decoding OOK may be based on an envelope detector. Since this demodulation is non-coherent (does not require any phase reference), the requirements on the frequency generation circuitry – which is one of the more power consuming blocks in a receiver – can be significantly relaxed. In addition, since the precise power level of the signal is not important, gain control in the receiver can be both coarse and simplified.
The OOK is generated in a rather elegant way, in which the transmitter architecture of the PCR transmitter is reused. The PCR is using Orthogonal Frequency Division Multiplexing (OFDM), which means that the signal is generated by means of an inverse Fast Fourier Transform (IFFT) block. The OOK is also generated by the IFFT block, where an ON signal is generated by populating the 13 sub-carriers in the center with a predetermined non-zero sequence and an OFF signal is represented by setting these sub-carriers to zero.
Coexistence with legacy 802.11 devices means that other 802.11 devices must be able to identify that there is an ongoing WUP transmission and properly defer from accessing the channel. Since legacy devices cannot recognize a WUP modulated using OOK, a legacy 802.11 preamble is sent prior to the WUP, see Figure 2.
Figure 2. WUP pre-appended with Legacy Preamble. L-STF and L-LTF are the short and long training fields used by legacy devices for packet detection and synchronization. The SIG contains signaling information.
The structure of a WUP is shown in Figure 2. The legacy signaling field (L-SIG) contains information about the duration of the WUP, so once a legacy device has identified that it is not the intended receiver for the packet, it can defer from accessing the channel accordingly.
Possibly the most challenging requirement is to achieve the same range for the WUR as for the PCR. The main component to obtain sufficient range is to use a low data rate. It has been agreed within the 802.11ba Task Group that the lowest data rate for the WUP is 62.5 kb/s. This may be compared to the lowest data rate of, for example, 802.11n, which is 6 Mb/s. With almost two orders of magnitude lower data rate, a similar range is obtained while allowing for a very simple and energy efficient receiver.
Challenges and possible future directions for Wake-Up Radio
The WUP is sent on the same channels as used by the PCR. Therefore, the support of WURs by necessity reduces the data throughput that can be supported in the system. To limit the negative impact on the data throughput, the 802.11ba Task Group has decided that the duration of a WUP must not exceed 1ms. This corresponds to about 8 bytes of information at the lowest data rate when the overhead is also considered.
If the concept of WUR turns out to be successful, a possible future step may be to not only use the WUP to wake up the PCR, but also transmit a few bytes of data, for example, to activate actuator, sensors, etc.
With a possible much wider range of usage cases for the WUR, the number of WUPs (where now the notion of wake-up would be a bit misleading) could be significant. How to support an increase in medium occupancy without ruining the capacity for the PCR using the same channel is a challenge that needs to be addressed in future standardization activities.