In the downlink of a wireless LAN, power-save mode is a typical method to reduce power consumption. However, it usually causes large delay. Recently, remote wake-up control via a low-power wake-up radio (WuR) has been introduced to activate a node to instantly receive packets from an access point (AP). But link quality is not taken into account and protocol overhead of wake-up per node is relatively large. To solve these problems, in this paper, a broadcast-based wake-up control framework is proposed, and a low-power WuR is used to receive traffic indication map from an AP, monitor link quality, and perform carrier sense. Among the nodes which have packets buffered at the AP, only those whose SNR is above a threshold will be activated, contending via a proper contention window to receive packets from the AP. Optimal SNR threshold, deduced by theoretical analysis, helps to reduce transmission collisions and false wake-ups (caused by wake-up latency) and improve transmission rate. Extensive simulations confirm that the proposed method (i) effectively reduces power consumption of nodes compared with other methods, (ii) has less delay than power-save mode in times of light traffic, and (iii) achieves higher throughput than other methods in the saturation state.
Wireless local area network (WLAN) [
As for (i), there has been much research on remote wake-up control [
Recently, the idea of wake-up control is also applied to WLAN. In the simple downlink wake-up control [
As for (ii), many efforts have been devoted to applying multiuser scheduling to improve transmission rate, and there is still active research in this field [
Compared with previous works that separately solve the above issues, in this work, we try to jointly solve these issues, applying multiuser scheduling to improve transmission rate, meanwhile reducing both the protocol overhead of wake-up control and the energy overhead of monitoring the channel, all by using the low-power WuR. To this end, we propose a new framework, broadcast-based wake-up control. In this framework, an AP periodically broadcasts by its WLAN module a WuM carrying a traffic indication map (TIM) [
The broadcast-based wake-up control brings new issues. When more than two nodes contend, there will be transmission collisions and false wake-ups. A WLAN module cannot transmit immediately after it receives wake-up instruction from its WuR in the presence of the wake-up latency. During the wake-up period of a node, other WuRs sense the channel as idle and falsely activate their WLAN modules, which lead to false wake-up [
Main contributions of this paper are threefold as follows: A new framework of broadcast-based wake-up control enables distributed wake-up scheduling. Broadcast-based wake-up control reduces protocol overhead and improves channel efficiency compared with wake-up per node. The three problems, multipath fading, transmission collisions, and false wake-ups, are solved simultaneously, by setting a proper SNR threshold.
Optimal SNR threshold is deduced by theoretical analysis. Extensive simulations verify that the proposed method (i) effectively reduces power consumption of nodes compared with the other methods, (ii) has less delay than power-save mode in times of light traffic, and (iii) achieves higher throughput than the other methods in the saturation state.
The rest of this paper is organized as follows: Section
The framework of the proposed method is shown in Figure
Framework for remote wake-up control of a mobile node. An AP transmits a wake-up message with a traffic indication map by applying the OOK modulation on the envelope of an OFDM signal. A node is equipped with a low-power WuR, which conducts envelope detection to receive a wake-up message, detects SNR, and performs carrier sense.
Different methods can be used to convey a WuM from a WLAN module (of an AP) to a non-WLAN WuR as follows. (i) Frame length of WLAN signals can be modulated to deliver wake-up message from a WLAN module (transmitter) to a WuR [
A WuM carried in the envelope of a WLAN signal is detected by a WuR using envelope detection. It should be noted that emulating the OOK modulation has its own cost. For example, at the rate of 100 kbps, transmitting a WuM with 40 bits will require 0.4 ms, which causes much overhead for wake-up per node. To reduce protocol overhead, a broadcast wake-up policy is adopted in this work, instead of letting an AP separately activate each node to receive packets. To this end, each WuM carries a TIM, which simultaneously notifies packet availability to multiple nodes. Then, it is nodes, instead of the AP, that contend to initiate the data receiving [
Each WuM initiates the transmission of all packets indicated by a WuM. The frame control field in each data frame carries a More Data flag [
A WuM has the same preamble as a WLAN signal. Therefore, a WuR can estimate SNR/RSSI of a WuM and on this basis performs carrier sense, contending to activate its colocated WLAN module. If multiple nodes wake up simultaneously, their contention may cause collisions. To reduce collisions, only WuRs with high SNR above the threshold and packets ready will participate in this contention. Due to the wake-up latency, the channel is continuously idle in the wake-up period of a node that has just won the channel contention. As a result, other WuRs sense the channel as idle and may falsely activate their colocated WLAN modules [
Here, normalized SNR [
The number of contending nodes with packets ready to be received changes with time, so does the SNR threshold. The AP continues transmitting all packets to the same node in a burst mode, where frames are spaced by SIFS (Short Interframe Space). The switch between nodes is indicated by a space of DIFS (DCF Interframe Space, longer than SIFS). By checking this DIFS, the number of contending nodes can be updated. Later, in Section
As link quality changes dynamically, the backoff counter is not resumed after each contention, but initialized per contention round using a fixed contention window (CW).
The main procedure of the proposed method is described as follows: An AP periodically broadcasts a WuM by applying OOK modulation on the envelope of WLAN signals, carrying a TIM indicating the availability of packets. A WuR measures RSSI/SNR on receiving a WuM, gets TIM from this WuM, and finds the number of contending nodes. Then, based on the number of contending nodes, each WuR determines its SNR threshold (based on a table). If there is a packet ready and its normalized SNR is greater than the threshold, a WuR initiates its backoff counter by a random value drawn from a CW and decreases its backoff counter per idle slot. A WuR immediately wakes up its WLAN module when its backoff counter reaches 0. An activated WLAN module performs carrier sense again. If the channel remains idle, a WLAN module transmits a CTS to initiate its receiving from the AP. Otherwise, the channel is already busy, and this is a false wake-up, and the WLAN module goes to sleep again. On receiving a CTS frame from a node, the AP transmits all packets to the node in a burst, with a SIFS between adjacent frames, and notifies the end of this burst by clearing the More Data flag. When the transmission to a node finishes, there will be a space of DIFS, after which other nodes will contend to initiate their receiving from the AP. A space of DIFS or longer indicates the decrease by 1 in the number of contending nodes, and the SNR threshold is updated. Then, the procedures from step
Occasionally, SNR of all remaining nodes (that have not received their packets yet) happen to be below the specified threshold. After DIFS, no nodes transmit and the channel remains idle, until the AP rebroadcasts a new WuM, which recovers the contention among the nodes.
A collision occurs when multiple nodes simultaneously transmit CTS. In this case, the AP, failing to receive an expected CTS, transmits a new WuM to resynchronize all the nodes.
A simple example is shown in Figure
Wake-up control for the downlink transmission scheduling in a WLAN with an AP and 3 nodes. The AP broadcasts a WuM, which carries a TIM. The WuR of each node, on receiving a TIM and detecting its normalized SNR above the threshold, performs carrier sense, contending to activate its WLAN module. An activated WLAN module initiates the receiving from the AP. Packets to the same nodes are spaced by SIFS, and packets to different nodes are spaced by DIFS and random backoff (
Here, we analyze the performance of the proposed method to deduce the optimal parameter. In the analysis, we use the following notations:
The length of wake-up period,
Main notations for the analysis.
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Time length of a DIFS |
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Time length of an SIFS |
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Time length of an EIFS |
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Time length of a wake-up message |
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Time length of a CTS frame |
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Time length of a DATA frame |
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Time length of an ACK frame |
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Power consumption at transmission state (1 watt) |
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Power consumption at receiving state (1 watt) |
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Power consumption at idle state (1 watt) |
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Power consumption of WuR (1 mill-watt) |
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Time length of a contention slot |
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Amount of slots in a contention window |
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Amount of slots corresponding to wake-up period |
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Amount of slots corresponding to sleep period |
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Number of nodes participating in the contention |
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Number of nodes with packets ready |
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Number of combinations choosing |
Among all nodes, there are
Consider the
There are three possible cases after each channel contention.
(i) All nodes have a normalized SNR below
Different cases of a transmission. (a) A contention window with
(ii) One node transmits without any collision (Figure
Of the other
(iii) Two or more nodes transmit simultaneously, which leads to a collision (Figure
Of the other
The average time for each round is the weighted sum of three parts (idle waiting, successful transmission, and collision) as follows:
First, energy efficiency, defined as the ratio of the number of overall bits to the product of average transmission time and average energy, is considered as follows:
Different definitions of energy efficiency (average absolute SNR is equal to 20 dB).
Energy efficiency
Energy efficiency
A problem is that average transmission rate
Therefore, we choose to remove
In the simulation evaluation, we compare the following 4 methods.
We built a packet-level simulator in the MATLAB environment. We consider a WLAN with an AP and a variable number of nodes (average absolute SNR is equal to 20 dB) and evaluate its performance with different traffic settings. Packet arrival follows the Poisson process. In the evaluation, we will consider four metrics: delay, channel utilization (the percentage of time that the channel is busy, including SIFS, DIFS, and backoff slots), power consumption per node, and throughput. The results are the average of 50 runs with different seeds. The transmission rate is decided by SNR [
Main parameters for simulation.
PHY | IEEE 802.11a, propagation: free-space & two-ray |
MAC | CSMA, |
Period | Beacon: 100 ms, TIM: 200 ms, WuM: 10 ms |
WuM | 0.4 ms in WuRMUS, 0.1 ms in WuR. |
Packet |
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Traffic | Packet arrival time follows Poisson process |
Bit rate | Decided by SNR |
CW | 50 slots |
Latency | Wake-up latency: |
Power | WLAN: receiving/idle 1 W, transmission 1 W; WuR: 1 mW |
In the proposed method, the WuM period plays an important role. Here we evaluate the performance of the proposed method under different WuM periods, and the results are shown in Figure
Performance of WuRMUS under different WuM periods (traffic rate: 50 packets/s per node).
Delay per packet (ms)
Channel utilization
Power consumption (mW)
Here we evaluate the performance of different methods with respect to the number of nodes in a WLAN. In this evaluation, the overall traffic is fixed to 500 packet/s and the amount of traffic per node decreases as the number of nodes increases. Because packets to different nodes arrive at different timing, the number of nodes that have packets buffered at the AP changes with time.
Figure
Performance of different methods under different numbers of nodes (overall traffic rate: 500 packet/s).
Delay per packet (ms)
Channel utilization
Power consumption (mW)
Figure
Figure
Next we fix the number of nodes to 20 and adjust the overall traffic to investigate the changes in delay, power consumption, and system throughput.
Figure
Performance of different methods under different volumes of traffic (number of nodes = 20).
Delay per packet (ms)
Power consumption (mW)
System throughput (Mbps)
Figure
Figure
In summary, WuRMUS reduces power consumption compared with other methods. In times of light traffic, WuRMUS also reduces delay compared with PSM and PSMAdapt and reduces channel utilization compared with WuR. In times of medium traffic or many nodes, the number of nodes that have buffered packets increases, which increases the SNR threshold. But the channel does not change completely randomly due to the limit of channel coherence time. Therefore, in WuRMUS, the probability that no nodes have SNR greater than specified SNR increases, which leads to large delay. How to solve this problem is left as a future work. In times of heavy traffic, the overhead of wake-up control in WuRMUS is not obvious after packet aggregation. Instead, applying multiuser scheduling helps to improve system throughput.
This paper studies the transmission scheduling in the downlink of a WLAN by using a wake-up radio to remotely activate a WLAN module to receive packets on demand. Instead of wake-up per node, we proposed a broadcast-based wake-up control framework to reduce the overhead and improve the transmission rate. This brings new issues like transmission collisions and false wake-up. These problems are solved by setting a SNR threshold. In this way, the proposed method reduces transmission collisions and false wake-ups and improves transmission rate. Simulation evaluations verify that the proposed method (i) effectively reduces power consumption of nodes compared with state-of-the-art methods, (ii) reduces delay compared with power-save mode in times of light traffic, and (iii) improves system throughput compared with other methods in the saturation state. In the future, we will also study how to further reduce the delay in the proposed method.
The authors declare that there are no conflicts of interest regarding the publication of this paper.