|Jinyang Guo, email@example.com (A paper written under the guidance of Prof. Raj Jain)||Download|
There are many measures for saving the energy in wireless networks protocols. Some are focusing on saving the energy in different modes such as active/sleep modes. Some are concerned about reducing interference and achieving higher signal-noise ratio with the same transmission radio power. Some are concerned about increasing the speed according to the application and environment to save the time working under active modes.
Different wireless networks protocols take one or several energy efficiency measures to reduce the power consumption according to their payload, cover area, and the demand for energy. Therefore, the protocols integrate the energy efficiency characteristics into applications to achieve balances of power consumption between the energy efficiency and functions.
From what is discussed above, the energy efficiency protocols are demanded by governments, manufacturers, service providers, societies and final users. It should not be surprised that most wireless networking protocols use various measures to make wireless devices run more efficiently.
Before 802.11e was adopted, there is a power saving polling mechanism available in wireless network protocols. However, it does not have Scheduled Automatic Power Save Delivery (S-APSU) mode, thus the Access Point (AP) needs power save polling signals between AP and stations typically after each beacon which is similar to Unscheduled Automatic Power Save Delivery (U-APSU) in 802.11e. The poll signals would cause more traffic and more collisions. With the applications of S- APSD in 802.11e, AP only needs to start service with stations on predetermined time, therefore AP could reduce power consumption.
In Bluetooth protocol, there are more sleep modes than active/sleep modes because Bluetooth is more concerned about the battery life of peripheral devices. In Bluetooth protocol, there is a connected mode which is the same as active mode in other protocols.[Jain14] It has a 3 bit active member address, and be able to transmit both in synchronous and asynchronous connections. However, Bluetooth has three kinds of sleep modes. In Hold mode, the device keeps 3 bit active member address and communicates only in synchronous connection oriented but not asynchronous connection. In Sniff mode, the device does not have any communication but only listens to the master for every a fixed sniff interval. In Park mode, the device gives up its 3 bits active member address and get 8 bits park address, and wakes up and listens to beacons. By using different sleep modes in Bluetooth protocols, the device could be able to save more energy according to the communication demand. This is very helpful for energy efficiency and can have a longer battery life.
Tab 1: Different modes in Bluetooth
|Modes||Address||Communication||Max sleep period|
|Connected||3 bits Active member address||Both synchronous and asynchronous||NA|
|Hold||3 bits Active member address||Only synchronous||500ms|
|Sniff||3 bits Active member address||Only listen after a fixed Sniff interval||50ms|
|Park||8 bits Park address||Only listen to beacon after a period||10s|
By using different sleep modes in Bluetooth protocol, the device is able to save more energy according to the communication demand. This is very helpful for energy efficiency and thus the device can have a longer battery life.
For protocols that support beacons such as 802.11ad, 802.15.e/ZigBee, when stations work under beacon-enabled mode, they can support active/sleep modes and Guaranteed Time Slot (GTS) mechanism in the frame structure. (See Fig.1) [Tennina13] Therefore, it is possible to achieve power saving goal by achieving low duty cycle and GTS allocation if the payload is not large, for device only needs to send or listen to the beacon when it is its turn.
Fig.1 Frame Structure for 802.15.4 [ISA01]
For this reason, beacon could reduce power consumption further in active mode.
In device with Directional Radio, it is very important to find the best antenna direction and parameter quickly and rightly. Otherwise, the benefit of directional radio would be compromised by inefficiency overhead payload of antenna searching and non-optimal antenna direction or parameters. In 802.11ad protocol, a station uses a two-stage antenna training to find optimal antenna configuration with its recipient. The first stage is Sector Level Sweep (SLS) which sends training signals in all sectors and finds the optimal sector. The second stage is Beam Refinement Procedure stage which finds the optimal parameter in the optimal sector [Sai12]. Through this two-stage antenna training, the AP will be able to find the optimal sector and parameters for each recipient with few signal test data. There is also antenna alignment procedure which makes sure that the antenna is aligned with beam during the transmission. ECMA-387 uses almost the same antenna training and tracking procedure to make sure two stations having the optimal antenna configuration for communications between them [ECMA-387].
Hence, by using Directional Radio, wireless networking devices can reduce the power consumption while has the same SNR for transmitter and receiver.
Cognitive radio is able to sense available wireless channels and decide which channels to use according to programs and configurations. The cognitive radio can find the spectrums that are less interfered, can also bond several channels together to achieve high transmission speed. Because the cognitive use less interfered channels, it could save power in transmission to attain the same Signal-Noise ratio. What is more, by increasing the Signal-Noise ratio, it could achieve a higher speed, therefore, the device needs less time in active mode which could also save energy.
Both 802.22 and 802.11af use Television white space (TVWS) as their channels for communication. 802.11af is designed for wireless LAN for range up to 5 km while 802.22 is designed for wireless regional area networks for range up to 100 km. In order to avoid interference between these two protocols or between different devices that use the same protocols, both standards use cognitive radio technology to avoid interferences. In cognitive layer of both standards, they all have interfaces with geolocation device and TVWS database to get the spectrum resource information. 802.22 also has interface with spectrum sensors and has quiet periods for spectrum sensing [Lekomtcev12]. Because FCC does not require spectrum sensing for use of TVWS and these two standards is still under development, what kind of cognitive plane of the final versions of these two standards will use are still uncertain now. But it is very important that the protocols will have cognitive radios that are able to configure the radio to different TV channels to avoid interference by other devices.
So, although we do not know the final version of 802.22 and 802.11af, because they use the same TVWS channels resource, they need Cognitive radio to avoid interference and therefore more energy efficiency
Because both 802.22 and 802.11af use TVWS spectrum, they need a coexistence method to avoid interferences among stations using TVWS spectrum according to the location. The 802.19.1 proposal protocol is designed to solute the coexistence problem in TVWS. [Shellhammer10] The protocol uses discovery to find white space objects (WSO) that could affect each other's performance and then uses decision algorithm to classify different WSOs and allocates channels accordingly.
802.19.1 only solutes the coexistence problem, while 1900.4 optimizes the usage of spectrum resources in the whole area. It includes three use cases, which are dynamic spectrum assignment, dynamic spectrum sharing and distributed radio usage optimization. Through 1900.4, we could achieve optimizing the spectrum usage while satisfying each station requirement. (See Fig. 2) The Network Reconfiguration manager (NRM) could able to use the database to achieve the optimization of the channel resource and send the reconfigure commands to devices [NICT09]. 1900.4a is the amendment of 1900.4 in TVWS.
Fig.2 1900.4 system archtecture [NICT09]
Therefore, the whole coexistent TVWS would achieve the most energy efficiency with 1900.4a. The local wireless networking could achieve maximum energy efficiency via interference management.
By using Orthogonal Frequency Division Multiplexing (OFDM) method, wireless protocols can divide a frequency band into a large number of small frequency bands; therefore each subcarrier has a smaller data rate which would increase symbol duration. Therefore, it can have less inter-symbol interference, thus can transfer in higher speed.
In modern wireless transmission, m-ary Quadrature Amplitude Modulation is widely use because it can transfer more bits in one symbol. It combines both the amplitude-shift key (ASK) and phase-shift key (PSK) technology to attain a high speed. [IEEE802.22-2011] Many wireless protocols apply QAM technology to achieve high speed, such as 802.11a, 802.11g, 802,11n and 802.22. In 802.22, the protocol uses different QAMs from 64-QAM, 16QAM and QPSK, to attain a balance between high speed and transmission error rates because of distance.(See Fig.3) [Pentz] In the proposal 802.22b there is 256-QAM which can increase symbol bit ratio from 6 bit/symbol of 64-QAM to 8 bit/symbol. [Zhao12]
Fig.3 802.22 QAM Modes and distance [Pentz]
Channel bonding uses the primary channel and adjacent channel together to achieve a higher speed. In 802.11n, the protocol is able to bond 2 20MHz channels into one 40MHz channel, thus can transfer more data. 802.11af can bond with contiguous and non contiguous channel, thus can use the channel resource more efficiently and enhance the transmission speed. [Lekomtcev12] The protocols can double or quadruple their speeds.
With the application of OFDM, QAM Channel Bonding technology in wireless networking protocol, we can attain much higher speed than before and can transmit the data in shorter time.
In wireless sensor networks, it is very important to keep the on-board battery's life because it is very hard to supply those sensors with direct power source. Energy harvesting is very important for implementing a wireless sensor networks because it can get energy from renewable energy thus extend the life of non-renewable battery. Radio Frequency (RF) energy is becoming attractive because it can convert and store easily, and it also can easily send from the central station. Generally, there are three components in RF energy harvesting, transducer, condition and store. There are many protocols that deal with how to harvest energy efficiently. In [Eu12] proposed Adaptive Opportunistic Routing Protocol which can attain a higher throughput of energy harvesting.
As what is discussed above same protocols might use several measures to achieve the best energy efficiency. For example: 802.22 implements almost all measures mentioned above according to its environment. These technologies work together to save more energy.
Different wireless protocols adopt these measures according to the application of the payload and the environment conditions of applications. New technology in hardware will soon be integrated into software protocol to take advantage its energy efficiency property.
[Tennina13] Stefano Tennina, "IEEE802.15.4 and ZigBee as Enabling Technologies for Low-Power Wireless Systems with Quality- of-Service Constraints, Springer, 2013, ISBN978-3-642-37368-8.
[Sai12] Sai Shankar N, et.al, "WiGig and IEEE 802.11ad For Multi-Gigabyte-Per-Second WPAN and WLAN", arXiv:1211.7356, ZTE Communications http://arxiv.org/pdf/1211.7356v1 2012
[ECMA-387]ECMA-387 standard http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-387.pdf
[Annapureddy11]Annapureddy, Venkata, "Interference management in wireless networks", https://www.ideals.illinois.edu/handle/2142/29769 2011
[lekomtcev12] Demian Lekomtcev, "Comparison of 802.11af and 802.22 standards - physical layer and cognitive functionality", elektrorevue, http://www.elektrorevue.cz/en/download/comparison-of-802-11af-and-802-22-standards---physical-layer-and-cognitive-functionality/ June 2012
[Shellhammer10] Steve Shellhammer, "IEEE 802.19.1: Coexistence for TVWS", In-Stat, https://mentor.ieee.org/802.19/dcn/10/19-10-0013-00-0001-coexistence-architecture-of-802-19-1.pptx Jan. 2010
[IEEE802.22-2011] IEEE Standard 802.22-2011, http://standards.ieee.org/getieee802/download/802.22-2011.pdf Jan. 2010
[Zhao12] Bingxuan Zhao, "Link Budget Analysis for Broadband Services in IEEE 802.22b", IEEE 802.22-12/0071r0, IEEE Standards Association, July 2012, , https://mentor.ieee.org/802.22/dcn/13/22-13-0081-00-000b-proposed-phy-text-for-the-ieee-802-22b.docx, Jan. 2010
[NICT09] National Institute of Information and Communication Technology of Japan, "Mobile Wireless Router Conforming to World's First Cognitive wireless Standard (IEEE 1900.4)", http://www.nict.go.jp/en/press/2009/03/03-1.html. March, 2009
[Pentz]Brian Pentz, "IEEE 802.22 WRAN (Cognitive Radio)", http://ecee.colorado.edu/~ecen4242/cognitive/index.html
[Eu12] Zhi Ang Eu and Hwee-Pink Tan, "Adaptive Opportunistic Routing Protocol for Energy Harvesting Wireless Sensor Networks" IEEE ICC 2012 - Ad-hoc and Sensor Networking Symposium, http://www1.i2r.a-star.edu.sg/~hptan/publications/icc2012_aor.pdf.2012
[Jain14], Raj Jain, "Wireless Bluetooth and Bluetooth Smart" , http://www.cse.wustl.edu/~jain/cse574-14/j_11ble.htm.2014
[Wikipedia01] Power_optimization , http://en.wikipedia.org/wiki/Power_optimization_(EDA)
[ISA01] ZigBee short on power by design, http://www.isa.org/Images/InTech/2004/May/20040579.html
[Wikipedia02], Energy Efficient Ethernet , http://en.wikipedia.org/wiki/Energy-Efficient_Ethernet
|APSD||Automatic Power Save Delivery|
|ECMA||European Computer Manufacturers Association|
|EIRP||Equivalent Isotropically Radiated Power|
|FCC||Federal Communications Commission|
|LAN||Local Area Network|
|MQAM||m-ary Quadrature Amplitude Modulation|
|NRM||Network Reconfiguration manager|
|OFDM||Orthogonal Frequency Division Multiplexing|
|QAM||Quadrature Amplitude Modulation|
|QPSK||Quadrature Phase-Shift Key|
|S-APSU||Scheduled Automatic Power Save Delivery|
|SLS||Sector Level Sweep|
|TVWS||Television White Space|
|U-APSU||Unscheduled Automatic Power Save Delivery|
|WirelessHD||Wireless High Definition|
|WLAN||Wireless Local Area Network|
|WSO||White Space Object|