Data-Plane and Control-Plane Functions in Base Stations and Mobile Stations

Figure below shows the user data processing path at the BS and MS. As shown in the figure, the user data traverses the path from network layer to physical layer and vice versa. In the transmitter side, a network layer packet is processed by the convergence sub-layer, the ARQ function (if enabled), the fragmentation/packing function, and the MAC PDU formation function, to form the MAC PDU to be sent to the physical layer for processing. In the receiver side, a physical layer SDU is processed by MAC PDU formation function, the fragmentation/packing function, the ARQ function (if enabled), and the convergence sub-layer function, to form the network layer packets. The control primitives between the MAC CPS functions and between the MAC CPS and PHY that are related to the processing of user traffic data are also shown below

Signal flow graph in data- and control-planes

The control-plane signaling and processing flow graph at the BS and the MS. In the transmitter side, the flow of control primitives from control-plane functions to data-plane functions and processing of control-plane signals by data-plane functions in order to construct MAC management messages and MAC header/sub-headers, to be transmitted over the air interface, are illustrated. In the receiver side, the arrows show the processing of the MAC control messages through data-plane functions and the reception of the corresponding control-plane signals by control-plane functions. The dotted arrows show the control primitives between MAC CPS functions and between MAC CPS and physical layer functions that are related to the processing of control-plane signaling. The control primitives to/from M-SAP/C-SAP define the network related functionalities, such as inter-BS interference management, inter/intra RAT mobility management, etc., as well as management-related functionalities, such as location management, system configuration, etc. 

Relay-Assisted Mobile WiMAX

There are two major technological and social trends significantly changing people’s lives: wireless communications and the Internet. Leveraging these two trends, worldwide interoperability for microwave access (WiMAX) creates a new utility enabling the development of new services and new Internet business models. In particular, the full potential of WiMAX will be realized when it is used for innovative nomadic and mobile broadband applications. With the finalization of the IEEE 802.16e standard and upcoming test and certification of WiMAX products, mobile broadband services are becoming a reality. The 802.16e standard provides broadband wireless Internet Protocol (IP) access to support a variety of services (such as voice, data, and multimedia) on virtually any device. The operation of WiMAX is currently limited to a number of licensed frequency bands below 6 GHz for reliably supporting non-line-of-sight (NLoS) operations. The 802.16e standard has also become a part of the IMT-2000 family.

WiMAX is often quoted as combining long transmission ranges (e.g., in a macrocell) with high data capacities (multi megabit per second throughput to end users). Power and spectral efficiency is key to a successful WiMAX deployment. The mobile WiMAX physical (PHY) layer is based on scalable orthogonal frequency division multiple access (SOFDMA) technology, which enables flexible channelization. The new technologies employed by mobile WiMAX result in higher data transfer rates, simpler mobility management, and lower infrastructure costs compared to current 3G systems. The underlying scenario for mobile WiMAX is an outdoor environment with multiple users within a cell. Hence scheduling (allowing a fair and efficient distribution of resources) and interference (from intracell and intercell) become important issues.

Radio relaying can address many of the challenges faced in the deployment of mobile WiMAX and its potential benefits. The relay system targets one of the biggest challenges in next generation mobile wireless access (MWA), namely the provision of high data rate coverage in a cost-effective and ubiquitous manner. Within a multihop relay network, a multihop link can be formed between the base station (BS) and a distant mobile station (MS) using a number of intermediate relay stations (RS). To avoid interference between the relay links, the simplest approach is to assign unique radio resources to each link. Using this approach, the multihop users will rapidly drain the system of valuable radio resource. The wireless medium is a precious infrastructure commodity and the situation is especially acute at lower frequencies (where the radio signal propagation characteristics are more favorable) when a significant amount of radio spectrum is needed to provide ubiquitous wireless broadband connectivity. Spectrum predictions for future cellular networks indicate large shortfalls by 2010, if not before. Hence the above approach can only be used for a very small number of very high-value MSs, or in applications where spectral efficiency is not vital, such as military or disaster relief communication networks. Given its commercial applications, relaying in the context of WiMAX must conserve radio spectrum and emphasize the need for high spectral efficiency. Enhancing radio resource efficiency is a key challenge in a competitive business development.

We focuses on the efficiency of relay transmission. We present leading edge techniques, and merge both theoretical analysis and practical application for a mobile WiMAX system with multihop relay. Directional distributed relaying is then proposed to achieve high data throughput with reduced demands on radio resource.

QUANTITATIVE PERFORMANCE STUDY | Multimedia over Mobile WiMAX

The delay-distortion performance of the position-value based resource allocation is compared in this section with traditional layer-based resource allocation in mobile WiMAX. The parameters of the simulation study are listed as follows. Default time frame duration T = 0.004 second, default channel BER is 0.0001, MAC header is 6 bytes, and fragmentation subheader is 2 bytes. Frequency bandwidth is 20 MHz, and 64-QAM with 3/4 coding rate and 1/4 cyclic prefix are used.

Figures 1 and 2 indicate the average loss ratio and expected delay trade-off for delivering a typical SDU with 1400 bytes using different fragmentation thresholds and SR-ARQ retry limit strategies. From these figures it is clear to see, with larger fragmentation number (lower fragmentation threshold and shorter PDU length accordingly) and higher SR-ARQ retry limit, the SDU packet loss ratio is decreased considerably. However, the penalty of such packet loss ratio decreasing is the prolonged delay of successful SDU delivery, mainly because of the retransmission latency. This is because mobile WiMAX is basically TDMA based scheduling, retransmission has to be reissued in the next frame duration. Thus, quality and latency form the trade-off that can be fine-tuned in mobile WiMAX transmission strategies optimization.

 

Figure 1: SDU loss ratio for an application layer packet with 1400-byte length, at channel BER 1e-4.

Figure 2: SDU delay expectation for an application layer packet with 1400-byte length, at channel BER 1e-4.

Figure 3 depict the delay-distortion performance comparison of the position-value based approach and layer-based approaches. For both of these approaches, image qualities with loose delay constraints are better than those with strict delay constraints. This is because with loose delay constraints, more network resource especially the SR-ARQ retransmissions can be allocated to the code stream, which improves the packet delivery ratio and thus the picture quality considerably. The position-value based approach achieves better delay-distortion performance than layer-based approach with the same latency budget constraint. Layer-based UEP approaches allocate resource according to the importance of different layers in code stream, and important layers containing coarse image information are more effectively protected while unimportant layers containing imagefine details are less protected. The position oriented approach allocates resource more efficiently by considering not only different layers’ unequal importance, but also the unequal importance of position and value information in each layer. With the position-value based resource allocation, the p-segments especially those in the coarse image quality layers are more effectively protected to improve image quality; and the v-segments especially those in fine details enhancement layers are less protected to reduce delay penalty. From this figure we can see, with 1e-4 channel BER the position-value based approach shows quality improvement up to 7–8 dB in terms of PSNR over traditional layer based approach with the same delay constraint. In the worst cases, i.e., with ultra tight or ultra loose delay constraints, the position-based approach has similar performance as layer based approach. This is because it has either not enough network resource or over excessive network resource for allocation. In those situations, the performance is not confined by efficiency of resource allocation, but the amount of network resource itself.

Figure 3: Image quality and delay-bound for different resource allocation schemes at BER 1e-4.

STATE-OF-THE-ART RESEARCH | Multimedia over Mobile WiMAX

Generally, real-time multimedia streaming poses significant challenges in wireless networks due to the time-varying nature of wireless channels, limited bandwidth, channel state fluctuation, inevitable bit error and packet loss, ambient noise, and interferences. Many solutions have been proposed to deal with the challenges for real-time media streaming over generic wireless networks. Research focusing on network resource allocation such as proposed effective solutions for improving performance of delay sensitive multimedia streaming over wireless local area networks (WLANs). These solutions used different error resilient protection techniques such as packetization and retransmissions to different media quality layers to achieve best effort multimedia quality with rate or delay constraints. Efficient network resource allocation problem as joint optimal selection of transmission strategies across PHY, MAC, and APP layers, which maximized multimedia quality or perceived peak signal noise ratio (PSNR) subject to rate and delay constraint. This approach to determine optimal cross-layer strategies based on classification and machine learning. Optimal MAC layer retry limits were predicted for various video packets transmitted over 802.11a WLANs, according to the perception importance of each video packet and current channel conditions. The unicast and multicast video streaming optimization problems over WLANs were addressed, where hybrid automatic repeat request (ARQ) combining PHY layer forward error correction (FEC) and link layer retransmission were described for unicast flows, and the multicast optimization problem was solved via combining progressive source coding and low layer FEC. Another hybrid ARQ scheme combing Reed–Solomon (RS) coding and rate compatible punctured convolution (RCPC) coding for H.263 coded wireless video streaming was proposed.

Unfortunately, cross-layer optimization with specific WiMAX consideration for multimedia streaming was not extensively discussed in most of the researches. A queuing-theoretic and optimization-based model for radio resource management in IEEE 802.16-based multiservice broadband wireless access (BWA) networks was proposed. Joint bandwidth allocation (BA) and connection admission control (CAC) were performed with packet level and connection level QoS consideration. They further presented the architecture for integrating hot spot 802.11 WLANs with 802.16 based multihop wireless mesh infrastructure to relay WLAN traffic to Internet. In that approach, the bandwidth allocation was presented with a bargaining game formulation for fair resource allocation, and an admission control policy was proposed to maximize the utilities for different types of connections. Via simulation, the effectiveness of rtPS, nrtPS, and BE in managing traffic sources, and the results highlighted that the rtPS scheduling service was a very robust scheduling service for meeting the delay requirements of multimedia applications.Addressed resource allocation problems regarding dynamic subcarrier allocation, adaptive power allocation, CAC, and capacity planning in OFDMA wireless metropolitan areas networks (WMAN). Research in Ref. [18] proposed an adaptive bandwidth allocation and admission control scheme for polling service (PS) in an IEEE 802.16-based WMAN. A noncooperative game was proposed, admission control policy was described, and the solution was determined by the Nash equilibrium for the amount of bandwidth allocated to a new connection, ensuring QoS for all connections in the system. Focused on scheduling and resource allocation in a cross-layer fashion. The principles of joint scheduling and resource allocation for IEEE 802.16 operating in adaptive modulation coding (AMC) mode were described, and the critical roles played by physical layer considerations, especially inter-cell interference estimation and channel state awareness were discussed. However, those mentioned researches focused on binary data transmission in mobile WiMAX, and transmission strategy optimization for multimedia content was not fully considered.Studied the performance of voice packet transmissions and BS resource utilization using the three types of scheduling services in IEEE 802.16-based backhaul networks. They demonstrated that while the UGS achieves the best latency performance, the rtPS service could utilize the BS resource more efficiently and flexibly, trading-off between packet transmission performance and BS resource allocation efficiency. According to their studies, the appropriate choice of the frame size was important in both the rtPS and ertPS services to reduce delay and packet loss. Similar research regarding VoIP over WiMAX was found. However, they specifically considered the characteristics of VoIP. Content-based unequal error protection (UEP) for video/image streaming was not considered. A good scheduling control was the key field to support coexisting real-time and nonreal-time traffic flows in mobile WiMAX. They especially suggested that for H.264/AVC-based scalable video coding, it was crucial to separate different video layer packets into different connections with different treatment of protection and retransmission. However, this was based on the assumption that no adaptive resource allocation exists in each connection, which may cause significant overhead on multiple connections’ management.

Since IEEE 802.16e-based mobile WiMAX is a relatively new development, very few protocol compliant resource allocation strategies with respect to multimedia streaming have been proposed in literature. The challenges for multimedia streaming over WiMAX networks entail the definition of a MAC effectively supporting multimedia streaming while efficiently exploring limited radio resources . The IEEE 802.16e standard already has build-in QoS features to support different classes of services, therefore, the radio resource allocation strategies and the scheduling algorithms for multimedia streaming between BS and SS are left open to specific vendor implementations. On the other hand, most of the previous works regarding wireless multimedia focus on traditional layer based UEP, and inequality between Position and Value (P–V) information has largely been ignored. Detailed description of position and value diversity in multimedia streaming. To sum up, the joint consideration of multimedia content and resource allocation with protocol compliance will provide significant potentials for improving delay sensitive media quality performance over standardized mobile WiMAX.

Multimedia over Mobile WiMAX

Mobile worldwide interoperability for microwave access (WiMAX) technology based on recent IEEE 802.16e specifications and Orthogonal Frequency Division Multiple Access (OFDMA) physical layer air interface has become one of the most promising broadband wireless protocols to support high throughput and mobility over large coverage areas. This technology can provide fast and inexpensive broadband access to markets that lack infrastructure such as rural areas and unwired countries. 

WiMAX can also serve as backhaul networks for client accesses to hot spots using different technologies including 802.11a/b/g as well as 802.16d/e. The most prominent characteristics of mobile WiMAX is that it can provide large distance data services for up to 31 mi with high data rate transmissions. With rapid growth of online multimedia services, supporting sensitive multimedia streaming with low latency over wireless networks especially mobile WiMAX becomes a focus of many research and development activities. 

Figure 1 illustrates an example of multimedia delivery applications over mobile WiMAX networks for railroads. The high-speed 802.16d protocol is applied to backhaul network, and mobile WiMAX (802.16e with SOFDMA) is applied to client access networks.

 

Figure 1: Multimedia applications over mobile WiMAX networks.

Multimedia streaming is bandwidth intensive, delay sensitive but loss tolerant, and bit errors or packet losses are inevitable in WiMAX networks due to the error-prone characteristic of wireless channel. Another important characteristic of multimedia streaming is unequal importance, where different packets in the stream have different perceptional values in terms of reducing distortion (i.e., some packets in the stream may be much more important than other packets). Fortunately, the connection oriented medium access control (MAC) layer of mobile WiMAX is designed to provide flexible quality of service (QoS) to different applications, which lays foundations to support multimedia streaming over WiMAX networks in two aspects: traffic differentiation among connections and resource allocation adaptation inside each connection. Each application traffic flow (e.g., video, voice, data) can be mapped to one or multiple service flows, and each service flow is further mapped into a logical connection with a unique 16-bit connection identifier (CID). 

The Service Data Unit (SDU, e.g., an H.264 video frame or a JPEG2000 image packet) from upper layer is dispatched with proper CID by SDU classifier, and MAC common part sublayer performs fragmentation and retransmission as well as QoS control. There are five QoS classes defined in mobile WiMAX: Unsolicited Grant Service (UGS), extended real-time polling service (ertPS), real-time polling service (rtPS), non real-time polling service (nrtPS), and best effort (BE). Such WiMAX flow scheduling architecture is illustrated in Figure 2, and the attributes of each service class in terms of traffic differentiation are described as follows. Example applications are also shown for each type of service class.

 

Figure 2: Packet delivery process in mobile WiMAX.

UGS: The UGS service class is especially designed to support real-time service flows which generate fixed-size data packets on a periodical basis, and the UGS service class offers real-time periodic bandwidth grants, which eliminates the bandwidth request overhead and latency . Typical applications for UGS are T1/E1 data flows, G.711 based voice-over-IP (VoIP) traffic without silence repression, etc. 

ertPS: IEEE 802.16e standard introduces extended real-time polling service, which allows 802.16e to manage traffic rates and transmission policies, as well as improves latency and jitter performance, where the ertPS service class is built on the efficiency of both UGS and rtPS. The base station (BS) provides unicast bandwidth grants in an unsolicited manner similar to UGS. The difference between UGS and ertPS is that UGS bandwidth allocations are fixed while ertPS allocations are dynamic. The advantages afforded by ertPS are especially important in support of VoIP applications without silence repression, which generate variable size data packets on a periodic basis.

rtPS: The rtPS service class is designed to support real-time service flows with variable data size packets on a periodical basis, and it offers unicast and periodical request opportunities real-timely, which allows the subscriber station (SS) to specify the size of desirable bandwidth grant. rtPS incurs more bandwidth request-grant overhead than UGS, but it improves data transmission efficiency and bandwidth resource utilization. Typical applications for rtPS include Moving Pictures Expert Group (MPEG) video streaming, video conferences, and IPTV.

nrtPS: The nrtPS service class is designed to assure service flows receiving bandwidth request opportunities even during network congestions, where BS offers unicast polling service to SS on a regular basis. It is especially suitable for delay tolerant data streams such as HTTP-based Internet Web browsing, FTP-based file transferring, etc.

BE: The intent of BE service class is to support data streams without minimum bandwidth allocation requirement. The BE service (for instance, e-mail service) is on a resource available basis, where no throughput or delay guarantees are provided.

Overall, the versatile QoS framework in WiMAX has significant flexibility to differentiate application streams (e.g., voice, video/image, data) and to provide different services to these streams. More important, the flexible scheduling architecture provides considerable advantages to the resource allocation adaptation and optimization in each media stream.

PERIODIC RANGING IN SLEEP MODE | Power Management in Mobile WiMAX

The standard defines periodic ranging in sleep mode functions in great detail. Ranging is a very important mechanism in IEEE 802.16 networks since the MS is using it to adjust the power and sychronize with the OFDM symbol. When the MS is in sleep mode, the ranging uplink transmission can be allocated in three different ways.

1. During the listening window, a BS may allocate an uplink transmission opportunity for periodic ranging.
   
   
2. A BS can activate a PSC type III to keep the MS in active state until the assignment of an uplink transmission opportunity for periodic ranging.
   
   
3. As mentioned above, the RNG-RSP (or MOB_SLP-RSP) can include the next periodic ranging TLV. From this the MS may know when the next periodic ranging opportunity shall occur. From the next periodic ranging TLV a MS may decode all consequent UL-MAP (uplink MAP) messages waiting for an uplink unicast transmission opportunity. When its own opportunity takes place the ranging procedure can be executed as normal.

It is important to mention that successful ranging does not deactivate the PSCs. After successful ranging, the BS announces the next ranging time in an RNG-RSP message. Additionally, if there is downlink traffic in the BS’s queue then a DL traffic indication is addressed to the MS. This way, the MS can exit the sleep mode algorithm earlier and reduce the imminent response delay.

HYBRID CASE AND IMPORTANT ISSUES | Power Management in Mobile WiMAX

As described in the standard, Figure 1 shows two PSCs: One for type I, in which the sleeping window is doubled as long as no traffic is addressed to it, and one for type II where the sleeping intervals are of constant length. Finally, availability and unavailability intervals are shown too. During unavailability interval the MS is said to be “sleeping.”

 

Figure 1: Hybrid case for IEEE 802.16e.

Usually most of NRT-VR and BE connections are handled by PSC type I, whereas each UGS connection requires a single type II PSC. This is because bandwidth allocation for the first two scheduling classes is fully under control by the BS and does not necessarily strictly follow the traffic arrival pattern. On the other hand, for the case of UGS, each traffic source might have an individual, periodic interarrival time (ON/OFF pattern). Assume two VoIP calls, for example, one with 20 ms and another one with 30 ms interarrival time. If the larger interarrival time is used as a sleep window then the first one would suffer a multiple of 10 ms additional delays for each transmission (without adding any other network delay). But if a sleeping window less than 30 ms would be used, then the ON interval for the second call could not fit into, which also means extra delay. If, however, both applications had the same interarrival time, then the sleep window is specified as the OFF periods between these calls. Nevertheless, management reasons of these two flows dictates that a seperate intance of PSC type II is used.

Finally, it has to be mentioned that as long as the number of UGS connections increases, the MS’s total ON time does increase as well. This relationship would be directly proportional if the MS would only hold PSCs of type II. In WiMAX networks, however, ranging and other management operations require the use of PSC type III, and follow a decoupled operation. Deriving the final unavailability periods becomes more complex in this real hybrid case. The above analysis yields some deductions for the PSC type (e.g., VoIP calls) and the sleeping time:

• Deduction 1. Each UGS and RTPS-CR/VR connection needs to be handled by a separate power saving class type II with individual parameters.
  
  
• Deduction 2. The energy efficiency is inversely proportional to incoming load.
• 
  
  Deduction 3. The access delay is improved when load is high, but not in a straightforward manner.

Power Management in Mobile WiMAX

Mobile devices tend to incorporate more and more processing units and functionalities which has a negative effect on battery lifetime. For this reason IEEE 802.16e workgroup has standardized those mechanisms that would augment battery lifetime in a worldwide interoperability for microwave access (WiMAX) network without affecting the quality-of-service (QoS) performance. This chapter gives an overview of those mechanisms, includes the methods to analyze them, and identifies those elements that could decrease the waiting time when in sleep mode.

1 INTRODUCTION

By definition, designed for mobile devices, whose utility is inherently constrained by limited energy supply, efficient power saving mechanisms are decisive criteria for IEEE 802.16e to become a compelling alternative for mobile broadband wireless access (BWA). This applies to any mobile worldwide interoperability for microwave access (WiMAX) device, regardless of its size and application and therefore ranging from a tiny personal digital assistant (PDA) to mobile phones to table PCs and notebooks.

Power saving becomes important as all these devices continue to incorporate more and more functionalities per integration unit, a development that has been predicted already years ago and which is known as Moore’s law. By doing so, manufacturers serve customer’s demand for consolidation; the ideal device is small, light, but feature rich. Hence, while early mobile phones were solely designed for voice services, today’s generation integrate MP3 player, camera, positioning service and many other energy-hungry features. The combined energy demand of such peripheral features, for instance, make up for almost 30 percent of the total for a Nokia 6630. Clearly, this also entails a whole set of new thermal requirements on mobile device design.

This development poses a considerable challenge to battery performance just as with computing power increasing energy accumulation is expected. Yet advances in battery capacity lacks considerably behind, like which claims that battery capacity has only improved by 80 percent in the last ten years while computing power doubles every 18 months according to Moore’s law. Nevertheless and on top of that, a recent Taylor Nelson Sofres (TNS) research project reveals that “Two-thirds of mobile phone and PDA users rate ‘two-days of battery life during active use’ as the most important feature of the ideal converged device of the future”. Similar to this finding, it is generally agreed that the operational time for a notebook shall be in the range of 2–5 hours. As of today, the only efficient means to cater to these user expectations is to counter this development by ever more efficient power saving mechanisms.

Motivating research on power consumption to overcome usability and performance constraints can be considered as the standard argumentation. But in fact, there is a much more important, frequently neglected reason: Saving energy means safeguarding our environment. In 2008, 70 million notebooks will be sold, roughly double the number than back in 2001. That means 70 million new power consumers and equal number of new batteries, yet one of the most expensive energy source in terms of production, which usually involves many valuable resources and hazardous chemicals, but also in terms of deployment. Disposing unusable batteries is a complicated business and the implicative environmental consequences do not require any further elucidation. Hence, saving power—and amid the total lifetime of a battery—can only be of utmost importance.

In conclusion, effective power saving mechanisms for IEEE 802.16e, which is supposed to seize the lead in BWA technology, are important to improve the usability of mobile devices and to accomplish this in the most environmentally friendly manner. The chapter is divided into two main parts. In the first part, we analyze how the power management mechanism functions in both point-to-point and relay architecture, and set some important deductions for the sleeping time. In the second part, we identify the tools for the performance analysis of power saving mechanisms and set the initial pace for some major, open research issues.

Mobile-Initiated Handoff | WiMAX HANDOFF CONTROL

An MS may decide to change its serving BS after losing signal quality or after detecting that a higher QoS can be disserved by another BS. In such situations, the MS will initiate the handoff. Nevertheless, IEEE 802.16e specifications did not specify the methodology of deciding whether or not to perform the handoff; they only focused on the mechanisms that should be implemented to collect information about the neighboring BSs for taking the handoff decision. In fact, each BS should transmit on the broadcast connection a mobile neighbor advertisement message MOB_NBR-ADV informing the listening MSs of the characteristics of any neighboring BS. Such a message includes the identifier of each neighboring BS, its frequency, the supported services, and its available radio resources such as its available channels. Upon getting such information, each MS should be able to take the handoff decision in the light of scanning of possible target BSs. The scanning procedure begins when the MS sends its serving BS a MOB_SCN-REQ message to inform the serving BS that it wishes to scan the neighboring BSs. The message indicates a length of time in frames for this interval and the type of association that will be used for scanning. The aim of association is to enable the MS acquiring and recording ranging parameters and service availability information to select the proper BS target. 

The specifications define three levels of possible associations, which are the association without coordination, the association with coordination, and the network-assisted association reporting. With the association without coordination, the target BS has no knowledge of the MS. Association with coordination means that the serving BS will coordinate association with the requested target BS and then respond to the requesting MS. With the network-assisted association reporting, the serving BS coordinates association with the requested target BS, a RNG-RSP message is sent back over the backbone to the serving BS. The latter collects all received RNG-RSP messages from all scanned BSs and then sends them to the MS in the form of a MOB_ASC_REPORT message. 

Next, the serving BS responds with a MOB_SCN-RSP message specifying the length of the approved scan and the association type that will be used. Upon receiving that message, the MS may scan its neighbors by synchronizing with a given BSs DL transmissions and estimating the quality of the physical channel. After performing scanning, the MS sends a MOB_MSHO-REQ message to its serving BS on the basic connection. That message includes a list of BSs recommended by the MS as targets. Upon receiving such message, the serving BS sends HO-prenotification messages to all BSs specified in the MOB_MSHO-REQ message and waits for the corresponding HO-prenotification-response messages to analyze them. Next, the serving BS generates the MOB_BSHO-RSP message indicating a list of target stations and sends it back to the MS on the basic connection. The MS may now perform or cancel the handoff; it informs its serving BS via a MOB_HO-IND message sent on the basic connection. If it performs handoff, the MS will inform its serving BS that it is leaving it while providing the parameters of the target BS and then it registers with the target BS. The target BS may, upon receiving the HO-prenotification message, include a fast ranging information element (IE) that provides the MS with a noncontention-based initial ranging opportunity to minimize the handoff process latency.

MOBILE WIMAX END-TO-END ARCHITECTURE

IEEE 802.16 standards family designed the physical and the MAC layers to provide broadband services access at the metropolitan scale for fixed, nomadic, portable, and mobile subscribers. The WiAMX forum, whose mission is to promote the interoperability of the broadband wireless access equipments implementing the IEEE 802.16 and ETSI HIPERMAN standards, has created a Network Working Group and a Service Provider Working Group to address higher-layer specifications such as intervendor inter-network interoperability for roaming, multivendor access networks, and intercompany billing. The result of these standardization efforts is the mobile WiMAX end-to-end architecture. The architecture is based on an all-IP platform implementing packet switching; it defines an access network and a common core network while decoupling the access from connectivity IP service. Moreover, the end-to-end architecture is designed to support loosely coupled interworking with existing wireless networks such as UMTS and existing wired networks such as DSL. Besides, a global roaming across WiAMX operator networks is achieved through the support for credential reuse, common billing and settlement, and consistent use of authentication, authorization, and accounting (AAA) services. The WiMAX forum designed the WiMAX network reference model (NRM), which aims at achieving interoperability through identifying the functional entities and reference points and the corresponding communication protocols and data plane treatment within a logical representation of the network architecture. As depicted in Figure 1, the MS, the access service network (ASN), and the connectivity service network (CSN) represent a set of functional entities that may be implemented by a single physical device or by different physical devices. The ASN is formed by at least one BS and one ASN gateway (ASN GW); it implements the access services and represents a boundary for functional interoperability with WiMAX clients and WiAMX connectivity service functions. For instance, the BS manages the MSs in its coverage while the ASN GW relays data to the CSN. On the other hand, the CSN may be defined as a network of Internet gateways, user databases, routers, servers, and proxies providing IP connectivity services to WiMAX subscribers.

Figure 1: The WiMAX NRM.

Mobile WiMAX end-to-end architecture is based on a security framework which provides basic security services and particularly authentication and confidentiality. In fact, each MS authenticates itself to the WiMAX network while the WiMAX network authenticates itself to the MS by implementing consistent and extensible authentication mechanisms. Besides, data confidentiality and integrity, replay protection and nonrepudiation services are guaranteed using applicable key length. Last but not least, MSs have the possibility to initiate and terminate specific security mechanisms such as virtual private networks (VPNs). The end-to-end architecture supports advanced IPv4-or IPv6-based mobility management mechanisms. For instance, vertical handovers can occur with wireless LANs or third generation wireless networks while roaming between different network service providers (NSPs) is supported. Seamless handover is also supported at up to vehicular speed while optimizing the overall network resources through the implementation of dynamic and static home address configurations, dynamic assignment of the home agent in the service provider network, and in the home IP network based on policies, etc. WiMAX mobility management will be further detailed.

MAC LAYER OVERVIEW | MOBILE WiMAX

The WiMAX forum has designed the physical and MAC layers of the mobile WiMAX based on the amendments of the IEEE 802.16e to offer broadband services including voice, data, and video at the metropolitan scale for mobile users. The mobile WiMAX network comprises BSs and subscriber stations (SSs). Each SS is assigned a 48-bit MAC universal address that it used to uniquely identify it toward a BS. Mobile WiMAX uses UL and DL maps to prevent collisions. More specifically, SSs implement the time division multiple access (TDMA) to share the UL while BSs use the time division multiplexing. 

 

UL and DL schedules are exchanged between the BS and the managed SSs in every frame using the UL-MAP and the DL-MAP messages. The MAC layer is connection-oriented; besides, all data communication is associated with a connection. Each connection with its QoS parameters forms a service flow and is identified by a 16-bit connection identifier (CID). MAC layer connections can be compared to TCP connections. 

 

In fact, thanks to TCP, a computer may have simultaneously different active connections for different applications using different ports. With MAC connections, an SS may have many connections to a BS for different services such as network management or user data transport; every connection is characterized by its own bandwidth, security, and priority parameters. When a new SS joins the network, the managing BS assigns to it three CIDs with different QoS requirements used by different management levels which are the basic, the primary, and the secondary management connections. The basic connection enables the transfer of short, time-critical MAC and radio link control messages; the primary management connection is used to transfer larger but more delay-tolerant messages while the secondary management connection is used to transfer standards-based management messages such as DHCP ones. Note that a CID may carry traffic for many different higher-layer sessions. The mobile WiMAX version adapts dynamic modulation and forward error codes to correctly serve the SSs located far from the BS in the rural areas and resists the weather conditions. 

 

Both the BS and the SS can adapt the transmission of the burst profiles by lowering the bandwidth for higher robustness. For instance, the BS always begins by adopting the most robust modulation and forwards error code scheme so that all SSs in the coverage area can correctly receive the DL-MAP and the UL-MAP messages. Meanwhile, an SS may ask its BS to have a longer UL window when needed. The BS and the managed SSs exchange MAC protocol data units (PDUs) carrying MAC management messages or convergence sublayer MAC service data units (MAC SDUs). The MAC PDU has a fixed MAC header, a variable length payload and a cyclic redundancy check (CRC) field. The MAC header may be a generic MAC header (GMH) or a bandwidth request header. The GMH is used to transfer the standard MAC management messages while the bandwidth request header is a header sent without payload to request additional bandwidth. 

 

Both the payload and the CRC fields are optional. It is worth noticing that the MAC header and the MAC management messages are never encrypted to facilitate registration, ranging, and normal operation of the MAC sublayer; however, this decision has opened the door to eavesdropping and other serious attacks. An SS which enters the network may be programmed to register with a certain BS; but generally speaking, it begins by scanning its frequency to detect an operating channel. Scanning consists in listening to each possible frequency until the frame preamble is heard. After detecting that channel, the SS tries to synchronize to the DL transmission by waiting for the DL-MAP stating the map of the timeslot locations in use for the frame. After that, the MS waits for the downlink channel descriptor (DCD) and the uplink channel descriptor (UCD) messages that are periodically broadcasted to specify the modulation and the FEC schemes used on the carrier. After gathering the required information describing the parameters needed for initial ranging transmission, the SS scans the UL-MAP to find an opportunity to perform the ranging. Initial ranging is used to determine the transmit power requirements of the MS to reach the BS. It is worth noticing that each SS should be informed about when to send the ranging request as many SSs may try to join simultaneously the network and highly affect the networks efficiency. Therefore, each new SS will send a ranging request (RNG-REQ) message and wait for the corresponding ranging response (RNG-RSP) message, indicating the timing advance, the power adjustment, and the basic and the primary management CIDs. After correctly determining the timing advance of SS transmissions, the SS and the BS will continue exchanging RNG-REQ and RNG-RSP messages until an acceptable radio link is established. Then, the SS should perform the authentication and the registration processes to enter the network. First, the BS asks the SS for strong authentication. Upon successful authentication, the SS will be able to register to the network. The SS will then establish a secondary management CID to receive secondary messages for different services. For instance, the SS will get an IP address through DHCP and a Trivial File Transfer Protocol (TFTP) address to request configuration files when needed.

QoS is guaranteed thanks to five QoS classes implementing different scheduling mechanisms. Those classes are the unsolicited grant service (UGS), the real-time polling service (rtPS), the extended rtPS (ErtPS), the nonreal-time polling service (nrtPS), and the best effort (BE). UGS fulfils the requirements of real-time communications occurring at periodic intervals such as voice over IP (VoIP); it is given a grant by BS that accommodates the maximum sustained traffic rate of such applications. rtPS uses unicast polling to support periodic real-time variable-size transmissions such as MPEG video streams, but generates more overhead than UGS. ErtPS is a combination of UGS and rtPS; in fact, the bandwidth is allocated without solicitation, but the allocation is done in a dynamic fashion. ErtPS fits well in the case of voice with activity detection like VoIP with silence suppression applications. nrtPS uses unicast polling regularly and allows contention requests to guarantee a minimum data rate for delay-tolerant applications, which generate variable-sized traffic such as FTP. Finally, the BE class guarantees a minimum QoS level for the associated traffic. Therefore, the SSs are never polled individually but are rather permitted to use contention requests and unicast requests.

PHYSICAL LAYER OVERVIEW | MOBILE WiMAX

802.16e WiMAX system profiles will cover 5, 7, 8.75, and 10MHz channel bandwidths for licensed spectrum allocations in the 2.3, 2.5, 3.3, and 3.5 GHz frequency bands. Targeting a worldwide coverage, the WiMAX forum intends to promote the allocation of frequency bands inferior to 6 GHz for civilian applications while considering the available spectrum all over the world. For instance, the 3.5 GHz frequency band is already assigned to fixed services in many countries, the 2.3 GHz band was reserved for the deployment of the WiBro solution in South Korea, while the 2.5 and 2.7 GHz bands have been assigned by the United States for fixed and mobile WiMAX deployment. 

 

Considering the modulation technique, mobile WiMAX is based on the orthogonal frequency division multiple access (OFDMA) technique and particularly on the SOFDMA variant. The frequency division multiplexing (FDM) principle consists in using multiple frequencies to transmit different signals in parallel. This is achieved by assigning a frequency range or subcarrier to each signal then modulating it by data. The multiple subcarriers used for transmitting different signals should be separated by guard bands to prevent interferences. 

 

Orthogonal FDM (OFDM) eliminates the guard bands by using overlapping subcarriers that are spaced apart at precise frequencies; thus achieving orthogonality. With OFDM, the center of the modulated carrier coincides with the edge of the adjacent carrier so that the independent demodulators performing a discrete Fourier transform see only their own frequencies. Redundancy may be guaranteed by scattering some bits over some sets of distant subcarriers. The OFDMA is a multiple access and multiplexing scheme that multiplexes data streams generated by multiple users onto the downlink (DL) subchannels and fulfils uplink (UL) multiple access by means of UL subchannels. The OFDMA symbol structure is made up of data subcarriers for data transmission, pilot subcarriers for estimation and synchronisation purposes, and null subcarriers used for guard bands and DC carriers. Data and pilot subcarriers are organized in a subset of subcarriers called subchannels. The minimum frequency–time resource unit of subchannelization is one slot equal to 48 data tones. 

 

It is worth noticing that there exist two types of subcarrier permutations for subchannelization which are the diversity permutation and the contiguous permutation. The diversity permutation organizes the subcarriers in a pseudorandom fashion to form a subchannel while the contiguous permutation organizes a block of contiguous subcarriers forming a bin. The bin is formed by nine contiguous subcarriers in a symbol with eight assigned for data, and one assigned for a pilot. Generally speaking, diversity subcarriers permutation achieves a high performance with the mobile applications while the contiguous subcarrier permutation fits well to fixed, portable, or low mobility environments. 

 

The SOFDMA technology optimizes the mobile access by assigning a set of subcarriers to particular users. For instance, subcarriers 1, 3, and 7 may be assigned to user 1 while subchannels 2, 5, and 9 to user 2 and so on. Users close to the base station (BS) will benefit from a larger throughput by getting an important number of subchannels with a high modulation scheme. SOFDMA offers multiple bandwidths to allow multiple spectrum allocation and fulfil different usage-model requirements. In fact, the scalability is guaranteed by adjusting the fast Fourier transform (FFT) size with respect to the available bandwidth while fixing the subcarrier spacing at 10.94 kHz. As the resource unit-subcarrier bandwidth and the symbol duration are fixed, scaling bandwidth will not affect higher layers. 

 

The first release of mobile WiMAX will adopt a system channel bandwidth of, respectively, 5 and 10 MHz, with a sampling frequency of, respectively, 5.6 and 11.2 MHz. The FFT size will be either 512 or 1024 while the number of subchannels will be either 8 or 16 and the useful symbol time will be fixed to 91.4 ms. The OFDMA symbol duration is 102.9 ms while the number of OFDMA symbol within a 5 ms frame is 48. The modulation scheme will gradually vary from 16 quadrature amplitude modulation (QAM) to quaternary phase shift keying (QPSK) (four channels) and even binary phase shift keying (BPSK) (two channels) at longer ranges while the power allotted to each channel will be increased. The adaptation of multiple-input multiple-output (MIMO) antenna technique along with advanced coding and modulation achieve peak DL data rates up to 63 Mbps per sector and peak UL data rates up to 28 Mbps per sector in a 10 MHz channel.

 

IEEE 802.16e supports both time division duplex (TDD) and frequency division duplex (FDD) duplexing modes. TDD is adopted when the license-exempt spectrum is used because a unique channel is shared between the UL and the DL traffics which occupy different time slots. Contrarily to TDD, FDD operates with two channels, one is dedicated to the UL traffic and the second is reserved to the DL traffic. Mobile WiMAX in its first release supports only the TDD duplexing mode although the WiMAX forum intends to address particular market opportunities by supporting FDD in future releases. With TDD, it becomes feasible to adjust the DL/UL ratio with respect to the nature of the ongoing traffic so that the DL/UL asymmetric traffic is efficiently supported. Besides, TDD guarantees channel reciprocity for better support of link adaptation, MIMO, and other advanced antenna technologies. Meanwhile, TDD offers a greater flexibility for adaptation to varied spectrum allocations as it uses the same channel for both UL and DL traffics. Last but not least, transceivers designed for TDD implementations are less complex and less expensive. An OFDM frame structure for a TDD implementation is divided into DL and UL subframes separated by transmit/receive and receive/transmit transition gaps (TTG and RTG) to eliminate DL and UL transmission collisions. The frame control information is carried by the preamble, the frame control header (FCH), the DL-MAP, and UL-MAP, the UL ranging, and the UL fast channel feedback (UL CQICH) fields. More specifically, the preamble field appears as the first OFDM symbol in the frame and it is used for synchronization. The FCH field identifies the frame configuration information including the MAP message length and coding scheme and usable subchannels. DL-MAP and UL-MAP provide subchannel allocation for the DL and the UL subframes while the UL ranging subchannel, which is allocated for the mobile stations (MSs), is used for closed-loop time, frequency, and power adjustments as well as bandwidth requests. Finally, the UL CQICH enables the MSs feedbacking the channel-state information to the managing entities.

The WiMAX forum included advanced physical layer features to enhance the mobile WiMAX network coverage and capacity. For instance, mobile WiMAX supports the mandatory QPSK, 16 quadrature amplitude modulation (QAM), 64 QAM schemes in the DL, and the optional 64 QAM in the DL. Meanwhile, both convolutional code (CC) and convolutional turbo code (CTC) coding schemes are supported along with two other optional coding schemes, which are the block turbo code (BTC) and low density parity check code (LDPC). The combination of various modulation and code rates results in a fine resolution of data rates so that the BS scheduler may determine the best suited data rate for each burst allocation, based on the buffer size and the channel propagation conditions at the receiver. Moreover, a channel quality indicator (CQI) channel is used for providing channel-state information from the MSs to the base station scheduler while other channel-state information can be provided to the BS by the CQICH, which includes the physical carrier to interference plus noise ratio (CINR), the effective CINR, the MIMO mode selection, and the frequency selective subchannel selection. Hybrid auto repeat request (HARQ) is supported to provide fast response to packet errors and improve the cell edge coverage through using N channel Stop and Wait protocol. The previously stated adaptive modulation and coding, CQICH and HARQ, guarantee robust link adaptation in mobile environments at vehicular speeds exceeding 120 km/h.

Frequency Allocation in Mobile WiMAX

In orthogonal frequency division multiple access (OFDMA), the frequency band is divided into subcarriers. These subcarriers are grouped into sets, called subchannels in WiMAX, which are allocated to users. We will consider in our numerical applications the case of an Fast Fourier Transform (FFT) size of 1024 subcarriers, with an intercarrier spacing of 7.8 kHz.

Two allocation modes are possible: distributed and adjacent.

Distributed Allocation

In the distributed subcarriers allocation, full channel diversity is obtained by distributing the allocated subcarriers to subchannels using a permutation mechanism. This mechanism is designed to introduce frequency diversity, thus minimizing the performance degradation due to fast fading which is characteristic of mobile environments. In addition to that, WiMAX standards specify two different distributed allocation modes: the fully used subchannelization (FUSC) mode where all subcarriers are used to form subchannels in each cell, and the partially used subchannelization (PUSC) mode where the frequency band is divided into three segments.

For illustration, with an FFT size of 1024 and after reserving the pilot and guard subcarriers, a FUSC allocation will correspond to 16 subchannels of 48 data subcarriers each, whereas a PUSC allocation will correspond to 30 subchannels, each containing 24 data subcarriers. Note that assigning subcarriers to subchannels in PUSC is a bit complicated, as it employs two permutations:

§  An outer permutation divides the subcarriers into six major groups of clusters using a specific renumbering sequence.

§  An inner permutation operates separately on each major group, distributing subcarriers to subchannels within the group and is based on the FUSC permutation with distinct parameters for the odd and even major groups.

This is illustrated in Figure 1, where two groups are assigned to one segment corresponding to a sector of the cell. Note that a segment can also be allocated to a cell in an omni-directional setting.

Figure 1: Construction of groups and segments in the partially used subchannelization allocation mode.

Adjacent Allocation

This method uses adjacent subcarriers to form subchannels. It corresponds to the WiMAX AAS (Advanced Array Systems) mode, designed to support Multiple-Input Multiple-Outpt (MIMO) techniques and adaptive modulation. Note that, to achieve a frequency diversity, mobiles using adjacent allocation may hop rapidly between different subchannels during their communication times.

Overview of the Mobile WiMAX Specification

IEEE 802.16 Standard

The IEEE 802.16 standard, which includes Medium Access Control (MAC) and physical (PHY) layer specifications, aims at supporting Internet services over wireless metropolitan area networks (WMAN). It is also an alternative to traditional wired networks, such as Digital Subscriber Line (DSL) and cable-modem. There are two modes defined in WiMAX networks: point-to-multiple-points (PMP) mode and mesh mode.

In the PHY layer, the IEEE 802.16 standard adopts the orthogonal frequency division multiplexing (OFDM), which is a multicarrier modulation scheme. The IEEE 802.16 standard has two OFDM-based modes: OFDM and orthogonal frequency division multiplexing access (OFDMA). Both of these technologies allow subcarriers to be adaptively modulated (e.g., QPSK, 16-QAM, and 64-QAM), depending on transmission distance and noise. Moreover, OFDMA has scalability to provide efficient use of bandwidth.

The MAC layer of IEEE 802.16 standard was originally designed for the PMP mode. On the later amendments of the IEEE 802.16a and the IEEE 802.16d, the mesh mode was included. The IEEE 802.16a adopts OFDM to provide greater spectral efficiency and to mitigate interference. IEEE 802.16b covers most of the quality of service (QoS) aspects. The IEEE 802.16e introduces scalable OFDMA into the standard, and supports mobile communications. With handover mechanisms, WiMAX is thus able to support mobile communications at vehicular speeds. We summarize the history of the evolution of the IEEE 802.16 standard in Figure 1.

Figure 1: Evolution of the IEEE 802.16 standard.

The IEEE 802.16 working groups on broadband access standards developed the IEEE 802.16 WirelessMAN standard for WMANs. On the other hand, the WiMAX forum was formed in June of 2001 to ensure interoperability among 802.16 products from different vendors. These groups and their activities may help popularizing WiMAX networks and systems by bringing vendors together and improving the specifications.

According to the IEEE 802.16 specification, the non-line-of-sight (NLOS) transmission range is 4 miles. With the combination of soft-switch technologies, a WiMAX network can work as a wireless "last mile" and make a viable alternative to the Public switched telephone network (PSTN) for VoIP services. In addition, a WiMAX network can work as a point-to-point backhaul trunk with a transmission capability of 72 Mbps at a transmission distance over 30 miles. With its technological advantages of throughput, power, transmission range, and versatility, WiMAX might be a strong competitor of other technologies, such as WiFi and 3G. Therefore, from both economical and technical points of view, WiMAX could be an appealing choice for broadband wireless services.

WiMAX Network Architecture

WiMAX has an IP-based wireless access architecture, which contains three parts: user terminal devices, access service network (ASN), and core service network (CSN). A user terminal device can be a fixed or portable/mobile terminal device, which supports the fixed/nomadic/mobile usage scenarios. Each device can establish a connection link to a WiMAX Base Station (BS), and perform authentication and registration through an access gateway in the CSN. The system architecture is illustrated in Figure 2.

Figure 2: Internet Protocol (IP)-based wireless access architecture of WiMAX.

A mobile WiMAX network has a similar architecture as a cellular network, where PMP links are between each BS and multiple Subscriber Stations (SSs). Each BS provides frequency and timing reference to SSs for synchronization purpose. The detailed MAC layer protocols and message sequences will be described later.