My Paper Review of Throughput Enhancement Schemes for IEEE 802.11ah based on Multi-layer

Cooperation YAN TING LIN

Authors

Throughput Enhancement Schemes for IEEE 802.11ah based on Multi-layer Cooperation

Outline• Abstract• Introduction• Contextualization• The proposed TIM compressed scheme• Simulation and Analysis• Conclusion

• The IEEE 802.11ah complements the other standards to provide support for machine-type communications (MTC) in local area network using a different spectrum from the highly congested WLAN operating at 2.4GHz/5GHz ISM bands. In order to satisfy massive station communications, the paper presents traffic indication map (TIM) compressed scheme for the Internet of Things and Smart Grid based on IEEE 802.11ah. The simulation results are discussed by evaluating the impact of the TIM compression on the system performance and cost. Results indicate that the proposed scheme has an acceptable cost and can support massive communications with higher performance enhancement. The proposed compressed TIM indication length can efficiently save the TIM bit map space and simultaneously be superior to the traditional structure. • Index Terms—IEEE 802.11ah, TIM compression, MAC layer

Abstract

Introduction• The exponential growth of data volume for wireless communication services

has attracted extensive and upsurge research interests into spectrum efficiency improvement using advanced spectrum reusability techniques, which potentially contain media access control (MAC) cooperation, machine to machine (M2M) network, device-to-device network and et.al [1]. • On the basis of IEEE 802.11ac [3] [4], IEEE 802.11ah works on a license-

exempt band with carrier frequency below 1GHz. By enhancing several functions on PHY and MAC, IEEE 802.11ah can associate with more than 2007 stations with coverage of up to 1Km, and its main application scenarios are sensors, measuring instruments and extending the range of Wi-Fi.

Introduction - 2Especially, the IEEE 802.11ah standard originating from IEEE 802.11ac, which also has been investigated in several aspects, such as interference avoidance in dense basic service set [6], network allocation vector enhanced mechanism [7].

The IEEE 802.11ah standard also aims to support the concept of the Internet of Things (IoT). Furthermore, IEEE 802.11ah adopts some new power saving mechanisms to increase low power stations’ life.

Introduction - 3• However, long-distance makes communication environment be more complex,

reflection and scatter will be more serious, and Doppler will cause a great harm on system performance. Meanwhile, in order to support lots of stations, AP has to use a longer TIM frames to indicate stations to receive their buffer data, which will cause unnecessary channel using and waste more power. • Combing with these problems and based on participating in the

standardization project, some protocols and key technologies used in PHY and MAC of IEEE 802.11ah are researched in this paper. • The main focus of this paper is on the use case relevant to MTC, as such use

cases pose several interesting challenges that require significant enhancement to the current IEEE 802.11 standard.

Contextualization• A. Channelization • B. Physical Layer and MAC Layer

Contextualization• A. Channelization • The IEEE 802.11 Task Group ah (TGah) is formed to work on

specification for sub 1 GHz WLAN. • Furthermore, IEEE 802.11ah can provide support for transmission

range up to 1km, as well as data greater than 100kbps for the low operating frequency [10]. • Due to the large coverage, the potential use cases for IEEE 802.11ah

standard fall into three general categories including sensor/meter network, backhaul for sensor and meter data, and extended range Wi-Fi for cellular data offloading [11] [12].

• In view of different countries have different available sub 1GHz ISM bands, and hence, IEEE802.11ah standard has described the respective available wireless spectra in various countries espe- cially including the United States, South Korea, China, Eruope, Japan, and Sigapore as Fig.3 [13].

Contextualization• B. Physical Layer and MAC Layer • In contrast, an OFDM symbol duration in 802.11ah is exactly ten times

than that of 802.11ac. Meanwhile, the number of data tones in 2MHz, 4MHz, 8MHz and 16MHz channels in 802.11ah are the same as those in 20MHz, 40MHz, 80MHz and 160MHz channels in 802.11ac. The set of supported MCSs is also the same as that of 802.11ac [13]. • The interaction between the AP and the users can be divided into three

stages, firstly the user may scan the channel list to decide to access which BSS, secondly, the users should transmit association frame to the desired AP and achieve the Association Identifier (AID). Thirdly, the users may need pass the authentication process using the available password.

• In a given network, AID is a unique ID in one BSS, through which the AP can indicate the associated stations. As a matter of fact, the legacy IEEE 802.11 can support up to 2007 station due to the limited length of the partial virtual bitmap of Traffic Indication Map (TIM) Information Element (IE), where each bit can indicate the corresponding stations’ AID located in the Signaling field. • With the increasing number of supportable stations, a hierarchical AID

structure is newly defined in 802.11ah as illustrated in Fig.4. It consists of 13 bits, and accordingly, the number of stations that it can express up to 213 − 1 = 8191. It is composed of four hierarchical levels, namely, page, block, sub-block, and station’s index in sub-block. The hierarchical AID structure is used to support stations’ power and control management.

• Meanwhile, the Fig.5 illustrates the usage of the TIM and page segmentation. We assume that a page is composed of 32 stations and the page is divided into 4 page segments since one Delivery Traffic Indication Message (DTIM) beacon interval consists of 4 beacon intervals in the Fig.5. Each beacon frame sequentially carries each page segment, and each beacon interval is used to accommodate traffic delivery of the corresponding page segement.

• In 802.11 system, AP may periodically transmits beacon frame, which contains TIM IE. The partial virtual bitmap field in the TIM IE conveys the information of the existence of buffered traffic destined to power saving stations. • In consequence, the power saving stations need to wake up to receive

a beacon frame periodically, based on which it can make sure and check the existence of the buffered packets destined to itself.

• However, the traditional protocols may be run normally in most cases, there still emerge several undesirable phenomena especially when a network exists a large number of stations. One is that the length of the beacon of the beacon frame could become extremely long due to the excessive length of the partial virtual bitmap in TIM IE. In addition, if the amount of the buffered traffic is too heavy to be accommodated within a beacon interval, several power saving stations inevitably keep in awake state to complete the receptions of their buffered packets.

• In order to solve the aforementioned problems, 802.11ah introduces a mechanism called TIM and page segmentation, which works as follows. Firstly, an AP splits the whole partial virtual bitmap corresponding to one page into multiple page segments, and each beacon is responsible for carrying the buffering status of only a certain page segment. Then, each power saving station wakes up at the transmission time of the beacon which carries the buffering information of the segments it belongs to. A new IE called segment count IE is defined to deliver segmentation information, such as the resulting number of segments after page segmentation and the boundary of each page segment.

• As shown in Virtual Bitmap, it can be known that the length of virtual bitmap can at most support 2007 one-to-one mapping. However, each BSS for IEEE 802.11ah may usually have 6000 STAs, the AID mapping position of which have randomness, therefore, the traditional TIM can not support to indicate massive STAs, furthermore, even if increasing the virtual bitmap length to 750 bytes to satisfy this requirement, then sometimes only several STAs’ indication still need fairly long TIM, which not only increase the TIM transmission time but also cause the unnecessary bandwidth occupation and increase the energy expense for receivers, then it is unfavorable to perform power saving. Therefore, we propose to design the TIM bitmap mechanism and simultaneously transmit through dividing bitmap field into multi- segment in order to reduce the length of each transmission.

The proposed TIM compressed scheme• Virtual Bitmap plays a significant role as TIM elements, which is owing

to the structure is consist of 2008 bits. Moreover, each bit is related to an associated identification (AID). The bit will be set to 1 when the buffer of AP has data to corresponding AID station, otherwise set to 0. The Fig.6 shows the frame structure of traditional TIM information element and each field can be presented as follows.

The proposed TIM compressed scheme - 2• In order to cut down the TIM length and satisfy to support massive

station communications, we propose to design the TIM hierarchical structure-TIM three level hierarchical structure, then implement to shorten the TIM length through encoding each layer. TIM three level hierarchical structure can be seen as Fig.7, three level of which are page, block and sub-block respectively.

The proposed TIM compressed scheme - 3• The paper presents three block methods, which are Normal, Single

AID and Inverse Bitmap, these three methods also can be used through combination and the usual group can be listed as Normal, Inverse plus Normal, Single AID and Inverse plus Single AID. The specific method of three block control methods are as follows.

• 1. Normal, namely the common map method, has mapping relationship with STA as AID = [Page Index(2 bits)Block Offset(5 bits)n(3 bits)m(3 bits)]. The nth bit in block bit map shows 1, which represents the nth sub-block bit map located in sub-block field, otherwise not located. The mth bit value in sub-block map shows 1, which represents AID has buffer data in AP with its corresponding STA.

• 2. Single AID, namely the block only indicates one STA, then the available 6 bits identification may not appear in sub-block field, the mapping relationship can be expressed as AID = [Page Index(2 bits)Block Offset(5 bits)Block Bitmap[5:0]].

• 3. Inverse Bitmap, namely when the number of 1 emerges so much in original bit map, then the bit map may firstly be inversed and encoded subsequently.

• In order to improve the performance of TIM three level hier- archical structure, a novel control method, namely OLB mode, which is the abbreviation of ’Offset, Length and Bitmap’ mode. In general, OLB mode should be chosen when sub-block number requires coding is more than eight. The corresponding coding compression scheme of OLB mode can be presented as follows.

• Based on OLB mode, 1 octet block bit map field may not denote the 8 subsequent sub-block map, instead the octet length of successive sub-block bit map. Then the AID has the mapping relationship with bit map as AID = [Page Index (2 bits)Block Offset (5 bits)zeros(6 bits)] + p, where p denotes the pth bit in sub-block map field, the bit of which states that AP has buffer data of STA if it filled with 1.

Simulation and Analysis

Conclusion• In this paper, we presented an analysis that characterized the

performance of IEEE 802.11ah networks. In view of the traditional TIM indication method is not favor to improve the system performance and utilize the bandwidth efficiently, then the three levels hierarchical TIM compression coding structure is proposed to support massive STAs communications in practical networks. The proposed scheme can achieve favorable performance improvement via numerical simulations, the results of which show that the compressed TIM indication length can efficiently save the TIM bit map space and simultaneously be superior to the traditional structure.