Spectrum Sharing between IEEE 802.16 and IEEE 802.11 based Wireless Networks Mohammad M. Siddique, Bernd-Ludwig Wenning, Carmelita G¨org Department of Communication Networks University of Bremen, Germany Email: [mms, wenn, cg]@comnets.uni-bremen.de
Abstract—Due to the high scarcity and high costs of radio spectrum, more and more radio services are occupying unlicensed bands for their operation. Due to this, there is a high risk of destructive interference which degrades the performance and fails to support Quality of Service (QoS) for systems operating in these bands. IEEE 802.11 based wireless networks are already operating in unlicensed band. A new competitor for unlicensed bands is the IEEE 802.16 based wireless metropolitan area network. Therefore, spectrum sharing between coexisting competing wireless systems like 802.11 and 802.16 is an upcoming challenge. To understand the characteristics of interference in such a heterogeneous scenario, an analysis of possible interference is presented and the performance of the legacy systems is evaluated. Then a spectrum sharing concept is proposed which can generally be applied to both systems. In this paper, the proposed concept is adapted for coexisting 802.16 and 802.11e based systems, which is an extention of 802.11. In this case, the 802.11e Hybrid Coordination Function (HCF) Controlled Channel Access (HCCA) is extended to provide a protocol for airtime sharing. Simulation results are presented showing that the proposed algorithm provides excellent improvement of system performance in the context of capacity and channel utilization compared to the case without applying any spectrum sharing method. Index Terms—IEEE 802.11, IEEE 802.16, Coexistence.
I. I NTRODUCTION
AND
R ELATED W ORK
Licensed spectrum is becoming more and more expensive. So from the economical point of view, there is a high probability in the future that WiMAX (aka IEEE 802.16 [1]) will operate in the same unlicensed bands (e.g. U-NII bands) where WLAN (aka IEEE 802.11 [2]) is operating [3]. Aside from economical consideration, technical recommendations considering the evolution of technologies are also going to that direction. According to the recommendation from e.g. the 4G draft [4], International Telecommunication Union Radiocommunication (ITU-R), the next generation wireless networks will be an integration of different wireless standards like WLANs, WiMAX and cellular networks. This increases the probability that multiple different radio access technologies will be present in the future, all or some of which have to operate in the same (unlicensed) frequency band. In such a case several possible coexistence scenarios could occur. One example scenario which appears in apartments or office buildings in dense urban areas can be defined as follows. An IEEE 802.11 system starts using the same unlicensed channel c 978-1-4244-7265-9/10/$26.00 2010 IEEE
Maciej Muehleisen Chair of Communication Networks RWTH Aachen University Germany Email:
[email protected]
which is used by an IEEE 802.16 system or vice versa, because an alternative channel is not available. This is denoted for further reference as a heterogeneous networks coexistence scenario. One of the main drawbacks of an unlicensed band is unpredictable interference. If the systems are not managed to use the spectrum properly, this interference leads to poor spectral efficiency and performance. So there is an increased requirement to efficiently utilize the unlicensed spectrum bands by means of spectrum sharing or coexistence methods. The more systems operate within a mutual range, the more they require methods for coexistence or even cooperation. The objective of this paper is therefore twofold. First, an analysis of possible interference occurring in a heterogeneous coexistence scenario is shown. Secondly, a generic spectrum sharing algorithm, which is developed in the framework of the ”Policy-based Spectrum Sharing for unlicensed Mesh (PoSSuM)” project [5], is described and applied to the same heterogeneous scenario and the system performance is evaluated. The IEEE standard draft 802.16h [6] proposes methods for 802.16 system coexistence. In [7] Rapp evaluates the coexistence of HiperLAN/2 [8] systems. Scheduling policies creating Silent Periods as transmission opportunities for other systems are introduced. A similar coexistence scheme is presented in [9] and [10]. In [9] a scheme is presented and evaluated analytically on how this idle period can be exploited by letting a second system fill the subframes from the other time direction. A scheme that reschedules data and allows multiple systems to coexist by shifting their frame start is described and analyzed in [10]. In [11], a concept of using a busy tone signal to protect 802.16 transmissions in a heterogeneous coexistence scenario is presented, but not evaluated. The IEEE Standards Coordinating Committee 41 (SCC41) is currently working on enabling network coexistence through dynamic spectrum access and a cognitive approach. The rest of the paper is structured as follows. Section II provides background information about different channel access methods in the case of 802.11 and 802.16, Section III gives an analysis of possible interference in the heterogeneous scenario. Section IV is about the spectrum sharing method and its adaptation to coexisting 802.11 and 802.16 systems. The simulation setup and results are provided in Section V. The
can be initiated during a Contention Period (CP) or during a Contention Free Period (CFP). The CAP may span across multiple consecutive polled TXOPs. The HC can start a CAP by sending a CF-Poll or a data frame (in the case of uplink or downlink respectively) when the medium is idle for more than a PCF Interframe space (PIFS) period. Frame n ! 1 Preamble FCH
Frame n
Downlink Subframe
MAP PDU 1
PDU bD
Idle
Frame n + 1 T T G
Uplink Subframe PDU 1
R T G
PDU bU Idle
Random Access
TTG: Transmit / receive Transition Gap RTG R RTG: Receive i / ttransmit it Transition T iti Gap G PDU: Protocol Data Unit FCH: Frame Control Header
Fig. 2. Fig. 1.
IEEE 802.16 Time Division Duplex MAC Frame [9]
IEEE 802.11e superframe structure
C. IEEE 802.16 paper is concluded in Section VI. II. M EDIUM ACCESS C ONTROL M ETHOD A. IEEE 802.11 The IEEE 802.11 standard defines channel access mechanisms for WLAN namely the Distributed Coordination Function (DCF) and the Point Coordination Function (PCF). The DCF [2] is a contention based random channel access scheme based on the Carrier Sense Multiple access/Collision Avoidance (CSMA/CA) protocol. However, it has been widely found like in [12] that the DCF and the PCF have limitations supporting QoS. This motivates the development of 802.11e [13] to provide user level QoS. Wireless multimedia extension (WME) [14] is the commercial version of IEEE 802.11e based WLANs. B. IEEE 802.11e and Hybrid Coordination Function IEEE 802.11e [13] defines a coordination function called Hybrid Coordination Function (HCF). The HCF includes two channel access mechanisms: the Enhanced Distributed Channel Access (EDCA) and the HCF Controlled Channel Access (HCCA). Fig. 1 gives an overview of the IEEE 802.11e superframe structure in the time domain. In HCCA the Hybrid Coordinator (HC), which is located in the access point (AP), has control over the channel. One main feature introduced in HCF is the Transmission Opportunity (TXOP). A TXOP specifies the duration of time in which a station can occupy the medium uninterrupted and exchange multiple consecutive frames with only Short Interframe Space (SIFS) spacing between an acknowledgement (ACK) and the next data frame. A station is granted a TXOP (called polled TXOP) by the HC through a CF-Poll frame. Other stations in the network set their network allocation vector (NAV) according to the duration field of the CF-Poll frame to stop their transmissions. Another special improvement in the HCCA is the contention free burst, known as Contention Access Phase (CAP), which
The IEEE 802.16 standard defines a centrally controlled wireless communication protocol. Subscriber Stations (SSs) associate with the Base Station (BS) forming a cell. IEEE 802.16 supports Frequency Division Duplex (FDD) and Time Division Duplex (TDD) operation but TDD is mandatory for unlicensed operation [10]. The IEEE 802.16 system follows a periodic MAC frame as shown in Fig. 2. If TDD is used, each frame consists of a downlink (DL) and an uplink (UL) subframe. Each frame starts with a preamble followed by the Frame Control Header (FCH). Besides general information about the system, the FCH provides the first part of the so called Medium Access Pointer (MAP). The MAP is formed by the scheduler at the beginning of each frame deciding the exact structure of the current frame. A SS can register its traffic demands through bandwidth requests in the Random Access phase. The scheduling algorithm is not defined by the standard. It is common to fill the subframes by Protocol Data Units (PDUs) in ascending time order. Idle periods occur at the end of the downlink and uplink subframe if they are not fully utilized. The Transmit Transition Gap (TTG) and the Receive Transition Gap (RTG) provide the required guard time for the transceiver to change from receive to transmit mode or the other way round. In the following we focus on Orthogonal Frequency Division Multiplexing (OFDM) based systems. III. I NTERFERENCE IN H ETEROGENEOUS S CENARIOS Fig. 3 shows the interference and collision events (numbered for reference and explained in the following) which occur during the channel access of 802.16 and legacy 802.11 DCF systems. Systems are within mutual interference range and operate in the same channel. The Upper part shows the 802.16 channel access and the lower part shows the 802.11 DCF channel access. Events like (4) and (5) are described in [11]. 1) When the channel is busy due to transmissions of the 802.16 system, 802.11 channel access is deferred up to when the channel is idle.
Fig. 3. Timing diagram for channel access by collocated IEEE 802.11 and 802.16 systems; possible interference and collisions events
2) However, the channel access by the 802.16 system is not deferred though the channel is being used by the 802.11 system. This results in very high probability of losing interfered packets in both systems. 3) The same as (1) happens during the uplink subframe of 802.16. 4) The same as (2) can happen when the next 802.16 frame starts. It is even more critical because in this case the preamble, FCH and MAP of the 802.16 frame would be lost. 5) When Subscriber Stations with no queued PDUs are not accessing the channel though they are scheduled to do so, this generates an idle period. The 802.11 system can take over the control of the channel which would cause interference to following 802.16 PDUs. It is important to note that the 802.11 system can take over the control if required when the idle period duration is equal or more than DIFS, depending on its backoff state. This characteristic of 802.11 DCF channel access is one of the main problems in the context of collocated wireless networks, mainly when the other system is following tightly scheduled medium access control like Time Division Multiple Access (TDMA) and TDD as in IEEE 802.16. In such a case, coexistence performance can be improved by making the channel access of 802.11 more regular like 802.16 and adapting the periodicity, service starting point, and service period length of both systems, which is introduced in the following section. IV. C OEXISTENCE A LGORITHM Three following assumptions have been taken into account in this paper for the spectrum sharing mechanism: 1) the systems have a method to estimate their own traffic demand and the traffic demands of other systems, 2) the systems have a method to detect the beacons/FCHs from other systems and shift their own frame starting time referring to the beacons/FCHs and 3) systems follow a common periodic interval to serve their stations. We refer the methods mentioned in the first two assumptions as ’detection and identification methods’, the outcome of which is considered as input to the spectrum sharing algorithms for decision making and scheduling. Development and integration of those methods are considered as related but separate research topics in the framework of this paper; they are ongoing work. These
detection methods are required for the adaptation of resource allocation in (significant) varying traffic conditions and varying scenario topology in larger time scale. The adaptation is considered as transient phase of the spectrum sharing method. In this paper the steady state phase is considered where mean traffic load does not change significantly from the system’s perspective, which justifies the consideration of the first two assumptions. These methods can be developed by using radio resource measurement. For example in the case of 802.11, measurement techniques on the basis of IEEE 802.11k can be applied to identify the idle/busy periods and to develop the detection methods. Measurement of spectrum utilization helps to estimate the traffic demand of other systems [3]. A method of detecting the beacon and estimating the number of systems based on that in the vicinity is given in [15].
Fig. 4. Systems provide idle periods and shift their frame start to enable coexistence
The proposed spectrum sharing technique allows systems to coexist by multiplexing their channel access in the time domain, which can be called ’TDMA between systems’ as shown in Fig. 4. Each system leaves some capacity which can be used as ’spectrum opportunities’ or ’idle periods’ for other systems. Distributing idle periods randomly results in collisions. Hence one of the main features of this scheme is that it occupies the channel and keeps idle periods in a regular pattern. Therefore, we refer to the scheme as ’Regular Channel Access (RCA)’. It helps, on the one hand, systems to reliably predict the length of the idle periods and their offset in the superframe during the detection phase; on the other hand, it helps systems to utilize the idle periods for own transmissions causing less or no collisions with each other due to orthogonality in time. It is worth to mention that it is often experienced that the capacity or bandwidth requirements of the applications used in the systems are less than the channel capacity. In the case of high bandwidth requirements, the system reserves some capacity for the admission of other systems. The duration of occupying the channel by the own system and the duration of idle periods for other systems can be adapted by estimating traffic demands of the own system and other systems. The time period or the air time allocated to the own system is calculated to T allocown =
T allocothers =
T Down × RI T Down + T Dothers T Dothers × RI T Down + T Dothers
(1)
(2)
TABLE I S YSTEM PARAMETERS
Here, TD (traffic demand) is defined as a ratio which is within the range [0,1], where 1 means the demand is equal to the channel capacity and RI means RCA Interval described below.
Carrier Frequency MCS Bandwidth
5.470 GHz BPSK 1/2 20 MHz
802.11
Slot Duration SIFS, PIFS and DIFS CWMin and CWMax ACK Duration
9 µs 16, 25 and 36 µs 15 and 1023 44 µs
802.11e
RCA Interval CF-Poll and QoS-Null Duration
10 ms 56 µs and 56 µs
Frame length 1 Symbol duration Preamble+FCH MAP DL subframe TTG UL subframe Random Access RTG
10 ms (720 Symbols) 1/72 ms 3 Symbols 4 Symbols 355 Symbols 2 Symbols 328 Symbols 26 Symbols 2 Symbols
Common
A. RCA in Heterogeneous Networks Fig. 5 shows the RCA in the case of collocated 802.11 and 802.16 systems. Here, the 802.16 frame length is considered as RCA Interval. The 802.16 system schedules all downlink (DL) and uplink (UL) subframes at the beginning of the RCA Interval in such a way that there is only a TTG duration gap between downlink and uplink. In such a case transmissions during the downlink and uplink subframes can be viewed logically as one continuous busy period. TTG is shorter than DCF Interframe Space (DIFS). The rest of the airtime in the superframe is kept as idle period for other systems. The air time (resource) allocation for 802.16 (DL+UL) transmissions inside the 802.16 frame can be dynamically adjusted considering the traffic load of the own system and others, using equation (1) and (2). It is assumed that each individual system does not require full bandwidth. To enable RCA in 802.11, an
802.16
V. S IMULATION S ETUP AND R ESULTS
Fig. 5. Timing diagram for channel access by collocated IEEE 802.11 and 802.16 system with Regular Channel Access (RCA) method; mitigate the interference and collisions
802.11e based system, like Wireless multimedia extension is considered. By introducing a MAC scheduler in the HC of the 802.11 system, a regular channel occupation and provision of idle periods is possible as shown in the lower part of Fig. 5. The basic functions are as follows: The interval between the start of two successive channel occupations by 802.11 system is realized as RCA Interval which is configured to be equal to the 802.16 frame length. The required air time allocation for 802.11 transmissions inside RCA Interval can be calculated considering the equation (1) and (2). To fill up the idle period left by the 802.16 system, the 802.11e system schedules its PDUs with an offset from the beginning of 802.16’s FCH. The offset is calculated to T ime Shif town = T allocothers × RI
(3)
By this proposed approach, the operation of coexisting systems is coordinated and synchronized indirectly with the help of regular channel occupation on the one hand and with the use of measurement techniques on the other hand.
The evaluation has been done by a simulation environment called Open Source Wireless Network Simulator (openWNS) [16]. In the framework of this paper a combined simulation platform for simulating the heterogeneous scenario is modeled and published as open source under Lesser General Public License (LGPL) in [17]. To understand the characteristics of the effects of interference and collisions in the systems, a scenario with one Base station and one Subscriber Station for the 802.16 system and one Access Point and one station for the 802.11 system is considered. This also resembles the apartment scenario mentioned before, where other orthogonal frequency channels in the unlicensed band are occupied by other systems. The system parameters are listed in Table I. Both systems are using the most robust modulation scheme of binary phase shift keying (BPSK) with a coding rate of 1/2, which can provide 6 Mbit/s of Data rate at the physical layer. Each Simulation is run for 500 seconds. In each simulation run, the static offered traffic for both systems during the span of simulation duration is considered. The results are shown in three steps. 1) Scenario1: Legacy 802.11 and 802.16 systems. 2) Scenario2: Legacy 802.11 and RCA enabled 802.16 systems 3) Scenario3: RCA enabled 802.11 and 802.16 systems. For evaluation, throughput measured on top of the MAC layer is considered. A. Scenario1: Legacy 802.11 and 802.16 systems Here, 2 Mbps (1 Mbps DL + 1 Mbps UL) traffic load and Protocol Data Units (PDUs) of 375 bytes in the case of 802.16 system and 2 Mbps traffic load and PDUs of 1480 bytes in the case of 802.11 system are configured as offered traffic load. PDU Inter arrival times follow an exponential distribution. The
6
x 10
5
Throughput / bps
802.16 802.11
− legacy (DCF) in 802.11 and legacy 802.16 − 2 Mbps 802.16 Traffic
4
3
2
1
0 0
0.5
1
1.5
2
2.5
3
3.5
Offered IEEE 802.11 Traffic / bps
4 6
x 10
Fig. 6. Mean throughput of both systems in Scenario1 against 802.11 traffic
802.16 system uses a periodic frame of 10 ms length as shown in fig 2. Fig. 6 shows the mean throughput over offered 802.11 traffic. A stacked graph is used showing the throughput of each system and each traffic direction as well as the total throughput of both systems. The 802.16 uplink and downlink throughputs are decreasing significantly with increasing 802.11 traffic up to 2 Mbps; because it increases the probability of collision events like (2) and (4) shown in 3. However, the 802.11 throughput is not affected as lost packets are retransmitted in the idle time period inside the current frame. For offered IEEE 802.11 traffic of more than 2 Mbps, the 802.11 throughput reaches saturation. The effects of ’selfish’ characteristic of 802.11 during channel access is visible in the curves. Overall, it has been found that around 2.5 Mbps capacity can be achieved, in other words, 40% of the channel capacity can be utilized. B. Scenario2: Legacy 802.11 and RCA enabled 802.16 systems
severe. In this case, the channel is idle for about half of the time. At low loads of 1 Mbps and 2 Mbps, no data loss occurs in the IEEE 802.11 system. A trend of decreasing 802.16 throughput like in Scenario1 is observed, the level of throughput is higher than in the case of Scenario1 as the number of collisions between the systems is less. The overall throughput is improved which resembles better utilization of resources (20 percent more than in Scenario1). The downlink only results give a hint towards the achievable performance if the 802.16 downlink and uplink traffic flows are scheduled directly after each other in the beginning of the superframe or the uplink subframe is filled from the back as presented in [10]. To see the impact of offered 802.16 traffic in a coexistence scenario, Fig. 8 is depicted. Increasing the offered load of both systems results in more collisions and overheads. 6
6
x 10
− legacy (DCF) in 802.11 and RCA model in 802.16 5
Throughput / bps
6
0.5 Mbps 802.16 Traffic 1.0 Mbps 802.16 Traffic 1.5 Mbps 802.16 Traffic 2.0 Mbps 802.16 Traffic 2.5 Mbps 802.16 Traffic 3.0 Mbps 802.16 Traffic
4
3
2
1
0 0
Fig. 8.
0.5
1
1.5 2 2.5 3 Offered IEEE 802.11 Traffic / bps
3.5
4 6
x 10
Overall mean throughput of both systems in Scenario2
From the above results it is found that unrestricted and uncoordinated channel access of 802.11 severely degrades the performance. C. Scenario3: RCA enabled 802.11 and 802.16 systems
6
6
x 10
− legacy (DCF) in 802.11 and RCA model in 802.16 − 2 Mbps 802.16 Traffic
802.16 802.11
6
6
5
x 10
4
5
3
4
Throughput / bps
Throughput / bps
− RCA in both 802.11 and 802.16 − 2 Mbps 802.16 Traffic
2
1
0 0
0.5
1
1.5 2 2.5 3 Offered IEEE 802.11 Traffic / bps
3.5
4
802.16 802.11
3
2
1
6
x 10
0 0
Fig. 7. Mean throughput of both systems in Scenario2 against 802.11 traffic
0.5
1
1.5 2 2.5 3 Offered IEEE 802.11 Traffic / bps
3.5
4 6
x 10
Fig. 9. Mean throughput of both systems in Scenario3 against 802.11 traffic
In this scenario downlink traffic is configured in the 802.16 system equal to the sum of downlink and uplink (2Mbps) in Scenario1, to model the scheduling of downlink and uplink directly after each other. We can consider this as a model of regular channel access in an 802.16 system. Fig. 7 shows the individual and overall system throughput against offered 802.11 traffic. As expected, the impact of interference is less
The IEEE 802.11 system uses RCA with an RCA Interval of 10 ms (which is the superframe length of 802.16). The results in Fig. 9 show individual and overall throughput against offered 802.11 traffic when the 802.16 offered traffic is 2 Mbps. In this case the airtime is equally divided between the systems. In this RCA case, the 802.11 system utilizes the idle
periods by shifting its channel access starting time and limiting its own channel occupation to fit into those idle periods. This process decreases interference, resulting in lower probability of losing packets, which eventually increases the throughput performance of the 802.16 system. Due to fixed allocation of airtime, the 802.11 system does not interfere at all. The 802.11 throughput goes to saturation at less offered load due to fixed allocation, however better fairness between systems is achieved. Fig. 10 shows the overall throughput. Here the airtime in the superframe is equally divided between the systems. Due to the fixed capacity allocation of 50%, the 802.11 throughput is not varied much for different 802.16 offered load and the 802.16 throughput is almost equal to the 802.16 traffic up to 3 Mbps. The dark line in the figure shows the coexistence performance when both systems have equal traffic load and airtime is allocated accordingly. The maximum achievable throughput is almost 5.5 Mbps which shows that proper sharing can improve the spectrum utilization up to 90 percent. The same is true for differing traffic demands of the systems. 6
6
x 10
− RCA in both 802.11 and 802.16
Throughput / bps
5
Airtime allocated according to traffic demand estimation
4
3
2
0.5 Mbps 802.16 Traffic 1.0 Mbps 802.16 Traffic 1.5 Mbps 802.16 Traffic 2.0 Mbps 802.16 Traffic 2.5 Mbps 802.16 Traffic 3.0 Mbps 802.16 Traffic
1
0 0
Fig. 10.
0.5
1
1.5 2 2.5 3 Offered IEEE 802.11 Traffic / bps
3.5
4 6
x 10
Overall mean throughput of both systems in scenario3
VI. C ONCLUSION
AND
O UTLOOK
In this paper, an analysis of possible interference that can occur in the heterogeneous coexistence scenario with legacy 802.11 and 802.16 systems is identified and shown. Simulation results are evaluated considering such a scenario which resembles next generation coexistent wireless networks in an ’apartment scenario’. It shows only 40% capacity could be utilized. A generic spectrum sharing method is developed and described enabling the systems to operate in harmony and to mitigate interference, resulting in increased spectral efficiency. Applying the method in a first step only on 802.16 system shows the channel utilization is increased up to 20%. When both systems are following the method, the improvement is even higher and fairness between the systems is observed. From the implementation point of view the main advantages of the adapted scheme are: The method does not change or violate the standards, the implementation complexity of this algorithm is rather low. However, there are some open issues like, the consideration of QoS parameters of the systems in
equations (1) and (2), adaptation and performance evaluation of the method in large scale network setting, etc. For future work, the performance of coexistence of legacy 802.11 and 802.16h based systems where Listen-Before-Talk is applied will be evaluated. Application of the proposed regular channel access based coexistence method in 802.11 and 802.16h based systems and a performance comparison will be done. ACKNOWLEDGMENT The authors would like to thank the German Research Foundation (DFG) project ”Policy-based Spectrum Sharing for unlicensed Mesh” (PoSSuM) which has funded the work presented. R EFERENCES [1] ”IEEE 802.16, IEEE Standard for Local and metropolitan area networks, Part 16: Air Interface for Fixed Broadband Wireless Access Systems,” October 2004. [2] “IEEE 802.11, IEEE Standard for Local and metropolitan area networks – Specific requirements – Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications,” 1999. [3] L. Berlemann, ”Distributed Quality-of-Service Support in Cognitive Radio Networks,” Thesis (PhD), Chair of Communication Networks, RWTH Aachen University, Aachen Germany, Mainz, 2006. [4] D. Rouffet, S. Kerboeuf, L. Cai, and V. Capdevielle, ”4G Mobile,” Tech. Rep., 2005. [5] A. K¨onsgen, M. Siddique, C. G¨org, L. Berlemann, G. Hiertz, S. Mangold, S. Max, and M. M¨uhleisen, ”Coexistence and Radio Resource Optimization of Wireless Networking Technologies,” Aachen, Wissenschaftsverlag Mainz, 2008, no. 1000, pp. 43–55. [6] ”IEEE Std 802.16h/D4, IEEE Standard for Local and Metropolitan Area Networks. Part 16: Air Interface for Fixed Broadband Wireless Access Systems. Improved Coexistence Mechanisms for License-Exempt Operation,” February 2008. [7] J. Rapp, ”Hiperlan/2 System Throughput and QoS with Interference Improving Strategies,” in IEEE VTS 53rd Vehicular Technology Conference, vol. 4, 2001. [8] M. Johnsson, ”HiperLAN/2: The Broadband Radio Transmission Technology operating in the 5 GHz Frequency Band,” 1999. [9] M. M¨uhleisen, R. Jennen, M. Siddique, and C. G¨org, ”Analysis of a TDMA Coexistence Approach for IEEE 802.16 Systems,” in MMBnet 2009, Hamburg, Germany, 2009, pp. 65–69. [10] M. Muehleisen, R. Jennen, M. Siddique, and C. G¨org, ”IEEE 802.16 Coexis tence through Regular Channel Occupation,” in Proceedings of the European Wireless, European Wireless, 2009, pp. 211–215. [11] L. Berlemann, C. Hoymann, G. Hiertz, and B. Walke, ”Unlicensed Operation of IEEE 802.16: Coexistence with 802.11(a) in Shared Frequency Bands,” in Proceedings of the 17th Annual IEEE International Symposium onPersonal, Indoor and Mobile Radio Communications, Helsinki, Finland, 2006. [12] Q. Ni, ”Performance Aanalysis and Enhancements for IEEE 802.11e Wireless Networks,” Network, IEEE, vol. 19, no. 4, pp. 21–27, 2005. [13] “IEEE Standard for Local and metropolitan area networks – Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications Amendment 8: Medium Access Control (MAC) Quality of Service Enhancements,” November 2005. R [14] ”Wi-fi MultimediaTM (WMM )” [Online]. Available: http://www.wi-fi. org/knowledge center/wmm [15] Yue Fang, Daqing Gu, McDonald, A.B., Jinyun Zhang, ”A two-level carrier sensing mechanism for overlapping BSS problem in WLAN,” The 14th IEEE Workshop on Local and Metropolitan Area Networks, Sept 2005, pp.6-18 [16] “OpenWNS - Open Wireless Network Simulator: A Simulation Platform for Wireless and Multi-cellular Mobile Communication Systems,” 2009. [Online]. Available: http://www.openwns.org [17] ”OpenWNS SystemTest Wifi and Wimac Coexistence,” November 2009. [Online]. Available: http://launchpad.net/ openwns-systemtest-wifi-wimac-coexist
