報告人：陳柏偉 日期： 指導老師：林永松

 INTRODUCTION  PRELIMINARIES  PROPOSED CRRM SCHEME  PERFORMANCE EVALUATION  CONCLUSION 2

 INTRODUCTION  PRELIMINARIES  PROPOSED CRRM SCHEME  PERFORMANCE EVALUATION  CONCLUSION 3

 Recently, demands for high quality multimedia services through the wireless networks are increasing due to the signi fi cant growth in the use of portable digital devices, such as smart phone and tablet devices.  The IEEE wireless local area network (WLAN) is one of the popular network technologies due to the low-cost for installation of APs, the support for relatively high data rate, and the free use of industrial, scienti fi c and medical (ISM) band [2][3]. 4

 IEEE WLAN speci fi cation is originally designed for primarily single-network, and setting of access point (AP), e.g., operating channel, transmission power, maximum allowable number of devices, is considered as the trivial matters.  Since medium access of IEEE WLAN is contention based, signal interferences between different WLAN hotspots may intensify the contention among APs and mobile stations (STAs) and reduce performance of communications [6]. 5

 IEEE medium access control (MAC) protocol employs distributed coordination function (DCF) as a fundamental medium access function.  if more STAs are contending for frame transmission, WLAN users may experience lower throughput and longer delay because of the heavy congestions in the shared medium [2][5].  Approaches to solve the problem can be classi fi ed into power control and channel management. 6

 Overlapped areas of different WLANs can be reduced by appropriately adjusting transmission powers of neighboring APs.  In addition, by making neighboring APs to avoid the use of the same frequency channel, signal interference between them can be reduced. 7

 However, since WLAN is intended to be easily installed by non-professional users, and the con fi guration may vary according to the changes in network environments, the power control and the channel assignment cannot be performed by human agency.  Therefore, it is required to develop an effective self- con fi guration scheme for APs which can make APs to automatically perform the power control and channel assignment considering the varying network environments. 8

 Radio resource management (RRM) techniques are useful to mitigate the co-channel interference in WLANs by allowing APs to automatically adjust their transmission power and select operational channel based on the network environments [8][9].  Especially, cooperative RRM (CRRM) can improve the WLAN environment by cooperatively managing the use of radio resources among neighboring APs. 9

 In CRRM, in order to maintain the ease of installation for WLAN, AP should have ability to automatically recognize neighboring APs and setup session between RRM modules in the APs for interworking [7].  Base on the interworking of RRM modules in the APs, various RRM techniques for WLAN have been studied, e.g., dynamic channel assignment (DCA), transmit power control (TPC), load balancing (LB) and coverage hole detection and correction algorithm (CHDCA) 10

 In this paper, we focus on the problem of signal interference between STAs and APs in difference basic service sets (BSSs) using the same channel.  In order to solve the problem, we propose a self- con fi gurable CRRM scheme for WLAN APs.  By performing power control and channel assignment based on interworking between neighboring APs. 11

 INTRODUCTION  PRELIMINARIES  PROPOSED CRRM SCHEME  PERFORMANCE EVALUATION  CONCLUSION 12

 IEEE standard speci fi es MAC and physical (PHY) layer functionalities for wireless local area networking [2].  A BSS is de fi ned as a group of STAs that are under the direct control of a single coordination function, i.e., DCF and point coordination function (PCF).  IEEE MAC layer uses DCF as a default to manage medium access of multiple STAs [2]. 13

 IEEE b/g standard de fi nes 13 different frequency channels for the access of wireless medium in WLANs.  Each channel occupies 22 MHz bandwidth for direct sequence spread spectrum (DSSS), and the center frequency of two adjacent channels is separated by 5MHz.  even though co-located BSSs use different frequency channels, they can interfere each other if they use adjacent frequency channels. 14

 WLAN APs can be freely deployed for any types of use without any permission or registration.  However, if a AP is newly deployed and con fi gured without consideration of the neighboring environments, it may not only have poor performance but also degrade performance of existing WLANs. 15

 Tow adjacent APs using the same channel may cause high signal interference to each other.  In this case, APs and STAs may not properly receive frames from transmitters in the overlapping coverages of APs using the same channel.  In order to minimize the overlapping region, dynamic channel assignment (DCA) and transmit power control (TPC) are considered. 16

 DCA scheme minimizes the coverage overlapping coverages of adjacent APs using the same channel by making some of those APs to change their channels to non- overlapping channels.  The optimal set of channel assignments is determined to minimize probability of frame collision [10].  However, since the number of non-overlapping channels is limited, DCA cannot perfectly resolve the overlapping coverages of APs using the same channel, if APs are densely deployed. 17

 In order to overcome the limitation of DCA, TPC assists DCA by controlling transmission powers of neighboring APs using the same channel.  By utilizing both DCA and TPC, frame collisions and heavy contentions occurring in the overlapping coverages of co- channel APs can be effectively resolved. 18

 Generally, STA is connected to the AP with the strongest signal power.  Therefore, AP having wide coverage or high density of STAs in its coverage may suffer from heavy loads.  In order to reduce the performance degradation, load balancing (LB) functionality is considered as a part of RRM [9]. 19

 If TPC and LB reduce transmission power of a AP for their purpose, some STAs cannot be attached to any AP.  In order to solve the problem, CHDCA makes STAs to detect area of radio coverage where the power level from the nearest AP is less than a speci fi c threshold.  If STAs report occurrence of coverage hole to the AP, the AP increases its transmission power in order to eliminate the coverage hole considering both TPC and LB functionalities. 20

 The RRM functions proposed in [9] can improve the performance of WLAN by considering various metrics.  However, those functions are targeted to be implemented in enterprise WLAN systems which adopt extra RRM controllers to manage APs.  Thus, the RRM functions are dif fi cult to be implemented in the general personal WLANs. 21

 INTRODUCTION  PRELIMINARIES  PROPOSED CRRM SCHEME  PERFORMANCE EVALUATION  CONCLUSION 22

PROPOSED CRRM SCHEME  For simplicity, we consider the following assumptions  Each AP uses a channel within the group of nonoverlapping channels specified in IEEE g standard with 2.4GHz operational bands, e.g., channel 1, 6 and 11.  Peer-to-peer sessions for RRM between neighboring Aps for their cooperation are already established.  Each AP maintains a list of neighboring APs, and the list is periodically updated. 23

PROPOSED CRRM SCHEME  When a new AP is installed, the AP firstly selects any temporal channel, and it does not permit attachment of new STAs until it determines the optimal channel based on the cooperation with neighboring APs.  We denote an AP initiating the CRRM function and its target AP as AP H and AP N, respectively. 24

 The purpose of the channel assignment capability is to assign a channel minimizing co-channel interference to WLAN APs.  The channel assignment procedure consists of two phases, neighboring BSS discovery phase and channel assignment phase. 25

 In neighboring BSS discovery phase, an AP gathers existence and basic information, such as BSSID, ESSID, and proximity, of neighboring APs by utilizing scanning capability of attached STAs.  All APs periodically broadcasts the beacon frame as specified in IEEE standard.  Each STA receiving the beacon frame from APs organizes a list of scanned APs containing their ESSID, BSSID, and received signal strength (Step 1). 26

 According to a certain period of time or explicit command, AP H requests its attached STAs to report their lists of scanned APs by broadcasting the AP scanning request message (Step 2).  When a STA receives the AP scanning request message, the STA sends AP scanning response message including the list of scanned APs to AP H (Step 3). 27

 If there exist a AP (AP N ) which is discovered for the fi rst time, AP H considers that the AP N is newly installed, and it initiates channel assignment phase for AP N (Step 4).  First, AP H sends a channel con fi guration request message to AP N.  The message includes information on the channel number used by AP H and the number of STAs located in the overlapping coverage of AP H and AP N (Step 5). 28

 If AP N receives the request messages from all neighboring APs, AP N selects a channel to use considering the information in those messages.  AP N selects the channel used by the AP having the minimum number of STAs in the overlapping coverage with AP N (Step 6).  AP N broadcasts channel con fi guration response message including the decided channel number to the neighboring APs (Step 7), 29

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 Even the channel assignment procedures can reduce the use of same frequency channel for neighboring APs, it may not suf fi cient to minimize the signal interference between the APs in dense deployment of APs.  In order to minimize the remaining signal interference after the channel assignment phase, the proposed CRRM scheme includes power control procedure for APs. 31

 The procedure consists of neighboring BSS discovery and power control phases.  From the information on the list of neighboring APs and the results of channel assignments, AP H can perceive APs using the same channel with itself.  If there exists an AP using the same channel (AP N ), AP H decides to initiate the power control phase in order to adjust transmission power of both AP H and AP N. 32

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 INTRODUCTION  PRELIMINARIES  PROPOSED CRRM SCHEME  PERFORMANCE EVALUATION  CONCLUSION 39

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 The path loss for each link is modeled referring the sitespeci fi c validation of ITU indoor RF model at 2.4 GHz, where the power decay indexes are N = 28 and N = 38 for distance up to 16 m and for greater distances, respectively.  The path loss of ITU indoor RF models is calculated as 41

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 INTRODUCTION  PRELIMINARIES  PROPOSED CRRM SCHEME  PERFORMANCE EVALUATION  CONCLUSION 46

 In order to resolve the problem, effective self-con fi gurable CRRM scheme is proposed in the paper.  By considering both autonomous channel assignment and power control by neighboring APs, the proposed CRRM scheme can reduce signal interference and unnecessary contentions in the co-channel AP deployments.  From the performance evaluation, we veri fi ed that the proposed CRRM scheme can effectively reduce coverage overlapping areas for APs using the same channel, and it can improve performance of WLANs. 47