How Quality of Service (Qos) Is Achieved in Wimax (IEEE 802.16)

How Quality of Service (QoS) is achieved in WiMAX (IEEE 802.16)

Johan ElgeredA, 1, A.Safaei MoghaddamA, 1, and Benjamin VedderB, 1

ADepartment of Computer Science, Chalmers University of Technology BDepartment of Signals and Systems, Chalmers University of Technology 1{elgered, safaeia, vedder}@student.chalmers.se

Abstract—This report deals with Worldwide Interoper- additional technologies were supported to guarantee a ability for Microwave Access (WiMAX) technology with minimum Quality of Service (QoS) while the mobile is focus on Quality of Service (QoS). The basics of the on move. This latest WiMAX version has pushed the technologies for the physical layer and the Media Access boundaries of wireless communication further. Control (MAC) layer are introduced. Also, a simulation Wireless communication has become a widely spread was made to evaluate how the Best Effort (BE) scheduler performs in different scenarios. method to exchange information, leading to more in- teractive information and thus a larger requirement of Index Terms—WiMAX, QoS, IEEE 802.16, MIMO, bandwidth. An important factor in the competition be- OFDMA, ns-2, Best Effort scheduling tween different wireless technologies is the capability of meeting Quality of Service (QoS). QoS is defined I.INTRODUCTION as the performance guarantees a network system can We are always in the need for higher communica- make regarding packet loss, delay, throughput and jitter tion speed. This need for more speed is not just in [2]. Different frameworks have been developed within transmission speed of information, but also we like to the field of QoS; Integrated Services (IntServ) provides move faster and do things faster. In other words be more individualized QoS guarantees to particular flows (packet efficient, which mobile communication offers. The need stream with common source address, destination address of mobility alongside with a high data transfer speed and port number), while Differentiated Services (Diff- brings the essential need of a new technology in wireless Serv) divides flows into separate classes [3]. Controversy communication to the picture. In addition, availability exists whether QoS is needed or not. Those who are is another essential aspect in today’s communication. against it state that when traffic has reached a level All this means that wireless communication is becoming beyond the capacity of the network, QoS will not manage more and more important. Worldwide Interoperability for to satisfy user demands and if the network has enough Microwave Access (WiMAX) technology was founded resources for all traffic, QoS is unnecessary. Those who in 2006 by Korea Telecom to fulfil these needs [1]. argue for QoS are of the opinion that it makes the WiMAX is based on the 802.16 standard and overseen by information exchange more fair by dividing resources Institute of Electrical and Electronics Engineers (IEEE). among users and also that it allows the network to run In the development of WiMAX various changes had to with a more intensive usage. be done in different layers of the OSI (Open Systems The main topic of this paper covers mostly 802.16e, Interconnection) model [8] in 3rd Generation Mobile where the focus lies on the lower layers connected with Telecommunications (3G). A bigger area coverage with QoS. These layers include the physical layer and the an increase in transmission speed was introduced in Media Access Control (MAC) layer. A simulation was 802.16d also know as WiMAX base. WiMAX base also made to analyse how the Best Effort (BE) scheduler introduced a dynamic bandwidth assignment to different performs. mobile users, based on their needs at a specific moment. Though, it did not solve the problem of high rate data II.PHYSICAL LAYER transfer to a moving mobile node with a vehicular The physical layer in mobile WiMAX has two key speed. In 802.16e-2005 also known as Mobile WiMAX, technologies; Orthogonal Frequency-Division Multiple munication systems. The capacity of the SISO additive white Gaussian noise (AWGN) channel is [5]:

2 C(t) = log2(1 + ρ|H| )bits/s/Hz (1) Where H is the channel matrix and ρ is the average signal-to-noise ratio (SNR). For this case one extra bit for the capacity needs 3dB extra power. For MIMO, when assuming the transmitter has M antennas and the receiver has N antennas and Channel Side Information (CSI) is not present, the capacity is [5]:

ρ C = ε (log [det(I + HH∗)]) (2) H 2 M N ≈ αmin(M,N)bits/s/Hz Fig. 1: Illustration of differentiation between SISO, where α is a constant, H is the channel matrix and ρ SIMO, MISO and MIMO wireless communication sys- is the average SNR at each receiver branch. tems [4] As the equation shows, the capacity is proportional to the minimum number of transmitter and receiver antennas. Therefore, it can be seen that when comparing Access (OFDMA) and Multiple-input and Multiple- to SISO, MIMO has great potential in increasing the output (MIMO). In the physical layer the delay, through- capacity when the bandwidth and the power are limited, put and jitter regarding QoS can be affected. which is very valuable in telecommunication transmis- MIMO is the use of multiple antennas at both transmit- sion [6]. ter and receiver. For example, 4 x 2 MIMO means using four antennas at the base station and two antennas at the A. Transmission models mobile device. An illustration about different antenna The 802.16 standard supports three MIMO models setups is shown in Figure 1. Comparing with the tradi- which are Matrix A (Transmit Diversity rate = 1), Matrix tional Single-input Single-output (SISO) antenna model, B (Transmit Diversity rate = 2) and Matrix C (Highest MIMO increases the capacity and the spectral efﬁciency rate, times 4). Diversity gain can also be achieved by of the communication system greatly. Therefore, the using Space Time Coding (STC) without transmitter system offers higher data rate with limited bandwidth, CSI. With STC, a single data stream is transmitted over which increases the throughput regarding QoS. MIMO multiple antennas. Different data bits are transmitted can deal with the multipath fading easily, however, it has over different antennas during the ﬁrst symbol period and no solution for frequency selective fading. the conjugate or inverse of the same bits are transmitted OFDMA is a multi-user technology which is the evo- again. This way, the received signal is more robust while lution of Orthogonal Frequency-Division Multiplexing the rate is unchanged. STC can be used to enhance the (OFDM). Mobile WiMAX uses MIMO together with coverage area and as a better channel allows for higher OFDM to achieve good performance on frequency selec- order modulations even the capacity can be increased. tive fading channels. Although OFDM has a lot of advan- Another method is to transmit different independent tages compared to other traditional modulation schemes, data streams over different transmitter antennas. This is there is no obvious improvement on the channel capacity. the technology known as Spatial Multiplexing (SMX). If On the other hand, MIMO could increase the channel the system could separate different data streams well, it capacity by increasing the number of antennas. So the would behave just like parallel channels. Therefore the technology of MIMO combined with OFDM could offer data rate is increased. The spatial multiplexing method a stable and low error service. Given these facts, the is important as it can increase the capacity without MIMO-OFDM model should be one of the most promis- additional power or bandwidth consumption [7]. Given ing and widely used technology in the future. multiple transmitter and receiver antennas, STC tech- Capacity is a very important measurement of telecom- nology can be used combined with SMX technology. How to combine STC and SMX is a trade off whether higher data rate or lower bit error rate is preferred. So, the combination of STC and SMX affects the delay, throughput and jitter regarding QoS.

B. IEEE 802.16e-2005 Mobile WiMAX attacks the problem of mobility in wireless communication. This brings the possibility of high data transfer speed at vehicular speed mobility. Also, it supports dynamic assignment of multiple modulations for both robust and high speed data transfer rate such as Quadrature Phase Shift Keying (QPSK) and Quadrature amplitude modulation (QAM). As the mobile device moves further away from the station, the modulation changes, providing a more robust connec- Fig. 2: Protocol layers of WiMAX [10] tion with lower bandwidth. By contrast, as the mobile moves towards the station, more channels are assigned to that device in order to provide a higher bandwidth. mobile devices where some uses 802.16e while others 802.16e-2005 can use more sub carriers with OFDM and use 802.16m can be served by the same base station OFDMA with 128 sub carrier Fast Fourier Transform on the same carrier. The difference between them is (FFT) up to 2K sub carrier FFT. Therefore, a higher that 802.16e uses one single carrier while 802.16m can speed rate can be offered [11]. use several carriers to increase data transfer rates. The requirement of downlink data rates in 802.16m is 100 III.MACLAYER Mbps in mobile and 1 Gbps in stationary. WiMAX The MAC layer is a sub-layer to the Data Link 802.16m was recently approved as a standard by IEEE Layer which exists in the layer 2 of the OSI model and will be introduced to the market 2012 [12]. Its [8]. The MAC layer refers to the different protocols basic improvements compared to 802.16e are the support and mechanisms to allow simultaneous users to access a for new service classes, increased mobility and better specific network, in this case a Mobile WiMAX network. Quality of Service gurantees [13]. It consists of three sub-layers: the convergence sub-layer (CS), the common part sub-layer (CPS) and the security B. QoS in 802.16m sublayer [9] (see Figure 2). In the IEEE 802.16m QoS is achieved by the in- CS approves arriving Protocol Data Unit’s (PDU) terpretation of flows of packets as service flows. Just from the higher layer, called MAC Service Data Unit’s like in the previous version, 802.16e, the flows occur (MSDU). It is within this layer that the fundamental QoS in a single direction and each flow is mapped to one mechanism in 802.16 is found, in the shape of functions transport connection. To increase QoS, the new features of classifying and mapping MSDU’s into adequate Con- in this standard are the improvements of the polling and nection Identifier’s (CID). After the CS has performed granting mechanisms. Several sets of QoS parameters mapping and classification, it transports the MSDU’s to can be chosen for each service flow. This is achieved CPS where common MAC functions are executed to in the interplay between Mobile Station (MS) and Base achieve QoS, for example fragmentation, packing and Station (BS), which negotiate the QoS parameters when concatenation. the service flow is configured. The process is executed in a flexible way because both BS and MS can separately A. IEEE 802.16m adjust the QoS parameters dynamically, according to Because WiMAX is competing with LTE in the 4G a predefined set if different QoS requirements would area, a group has been developing 802.16e further. This appear or if the traffic characteristics would change. An additional system to the IEEE 802.16 standard is called example of this is in the use of Voice over IP (VoIP) 802.16m, also known as WiMAX 2. A positive element applications. Initially when the connection between the with WiMAX is that the system is backwards compatible BS and MS is established, two different sets of QoS why 802.16e and 802.16m works in parallel. Thus, parameters can be decided. One set could be used during periods when voice is transported over the connection module [14]. The extended WiMAX module was de- and another set could be used during silent periods. veloped to add support for further scheduling services Through these manners, QoS is achieved by managing more than just the existing scheduler included in the the conditions that arise during the changes of states an module, which works as a BE scheduler. A comparison application pass through [13]. was made between the original BE scheduler and the new implemented BE scheduler, in a setting with several rtPS C. QoS Scheduling Subscriber Station’s (SS) and two BE SS:s. The outcome There exists five different scheduling service types in of the simulation was that the original BE scheduler 802.16 standard MAC layer. Through these, WiMAX of- provided the same BE throughput independently of the fers the ability to give different applications varying size traffic load. In comparison, the new implemented BE of bandwidth associated with the priority of the applica- scheduler provided lower throughput as the traffic load tion, in comparison to WiFi where each application has was increased. The conclusion drawn of this was that the the same priority. It is the BS scheduler that distributes new implemented BE scheduler was more realistic since the amount of bandwidth necessary for an application, BE connections do not have the strict QoS requirements depending on which QoS class the application has. With as other scheduling services. the QoS parameters (see Table I) given, the BS can offer In another study, simulation was performed with the polls and or grants at adequate occasions. aim to compare the different scheduling services UGS, The five scheduling service types are presented below. rtPS and ertPS [15]. The performance metrics used in 1) UGS (Unsolicited Grant Service): When a UGS the simulation were system throughput, packet delay and scheduling service type is used, the BS grants data signaling overhead as a function of traffic load in the sys- packets, at periodic intervals, which are fixed-sized. tem. The simulation was done in Matlab and consisted of Thus, it is suitable for real-time data streams such as movement algorithms for the SS:s to never move beyond VoIP. the coverage area of the BS:s. Also, different traffic models were used to make the comparison between the 2) rtPS (Real-Time Polling Service): Has the same scheduling services fair (eg. ertPS performance strongly function as UGS, but where UGS grants fixed-sized data depends on the rate of traffic size changes). The results packets, ertPS uses dynamic distribution. An example of showed that UGS is not a good alternative because an application is transportation of video, for example overall system capacity tended to decrease alot in its use. MPEG (Moving Pictures Experts Group). When ertPS was in use the best results was obtained; 3) ertPS (Extended Real-Time Polling Service): This system throughput was considerable higher, whereas scheduling service type is based on both UGS and rtPS. signaling overhead and packet delay were smaller in It is adequate for applications as VoIP, in which the data comparison to the other test cases. rate is variable. 4) nrtPS (Non-Real-Time Polling Service): Requires V. WIMAX NS-2 MODULE minimum data rates for data packets with variable size. For the simulation the ns-2 simulator [16] was used It is made with the intention of managing delay and is along with the NIST WiMAX module [17]. The ns-2 therefore convenient in the use of File Transfer Protocol simulator is principally targeted for network research. (FTP). The BS regularly offers request polls, which It has been under development since 1989 and is still makes it possible for the application to make requests in progress. Ns-2 uses two languages, C++ and OTcl. even though network congestion is in progress. C++ is suitable for detailed implementation of protocols 5) BE: This scheduling mechanisms can cause long because it is fast to run. Though, changing settings using delays when there is network congestion since it handles C++ would be rather slow why OTcl is to prefer, where applications on best available basis. It has no support changes can be made interactively. The developers state for applications which have requirements of minimum that it is not a polished and finished product and bugs are service guarantees. An example of an application that being discovered and corrected. However, many studies uses this scheduling service is E-mail. [10] are based on the use of the ns-2 simulator.

IV. COMPARISON OF SCHEDULING SERVICES VI.SIMULATION MODEL In one study, simulations of UGS, rtPS and BE were Simulation was done regarding packet loss in wimax made with the ns-2 simulator and an extended WiMAX network in two scenarios. Packet loss is described as Scheduling Maximum Minimum Request/transmission Tolarated jit- Maximum la- Traffic policy service sustained reserved traffic policy ter tency traffic rate rate UGS • (Can be present) • • • rtPS • • • • nrtPS • • • • BE • • • TABLE I: QoS parameters of the 802.16 scheduling services [10] the difference between the total packets transferred by the base station and the total number of packets received by all nodes. In order to evaluate the BE scheduler regarding mobility, two scenarios were chosen, scenario A and B. In both scenarios an omni antenna is used. The bit rate chosen was 11 Mb/s. Also, a 0.02 second gap was used between packet transmission of each node to avoid packet loss due to BW request collision. The base station coverage area was 500m radius. In scenario A, all nodes are in the same distance of the antenna in a static position. The simulation was executed with an increasing number of nodes from the beginning towards the end. The code below are examples of how some of the settings were set. Fig. 3: Simulation with up to 200 static nodes. # disable random motion $wl_node_($i) random-motion 0 ; $ns at 0 "$wl_node_($i) setdest 550.0 550.0 40.0" # network interface type set opt(netif) Phy/WirelessPhy/OFDM ; # MAC type set opt(mac) Mac/802_16/BS ; # antenna model set opt(ant) Antenna/OmniAntenna ;

# Traffic scenario: if all the nodes start talking # at the same time, we may see packet loss due to # bandwidth request collision set diff 0.02 for {set i 0} {$i < $nb_mn} {incr i} { $ns at [expr $traffic_start+$i*$diff] "$cbr_($i) start" $ns at [expr $traffic_stop+$i*$diff] "$cbr_($i) stop" }

VII.SIMULATION RESULTS Fig. 4: Simulation with up to 200 nodes moving towards As Figure 3 shows, as the number of nodes increased, the base station. the packet loss would increase also. In scenario B all the parameters were the same in the beginning. Over time the nodes would start moving towards the base station with a speed of 40m/s and then stop at the base station. PL = Ptx − Prx − Pbwc (3) Figure 4 shows that the packet loss increased compared to figure 3. Where PL is the packet loss of interest, Ptx is the total Packet loss may also occur due to collision. To avoid packets transmitted, Prx is the total packets received and this a 0.02 second gap between packet transmission of Pbwc is the packets lost due to bandwidth collision. each node was added, however, it may still occur. Thus, Consider PL1 as the packet loss from scenario A and the packet loss can be described as the following: PL2 as the packet loss in scenario B. 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