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.

MAIN ASSUMPTIONS | PERFORMANCE ANALYSIS

It is worthy to stress on the assumptions made, since the list will lead us to the series of following papers that simply try to relax on these assumptions and therefore achieve higher accuracy.

1 Arrival Rate

 This assumption is crucial for the mathematical analysis since it simplifies the problem. Nevertheless in any case, this arrival process describes the way the packets are arriving at a distant network host, and therefore it is not straightforward to model without realizing a possible undergoing application. However, the type-1 class of sleep mode is also working when there is no application going on.

Some authors assume different arrival models without, but nevertheless, succeeding in reasoning these models. In an Erlang distribution is used as a highly customizable distribution.

A different model is proposed using multiple interrupted Poisson processes (IPPs). It would be a challenge to model the arrival distribution, because this would give insight into the hierarchical case of packet level sleep mode and flow level sleep mode. It is foreseen that the operation of the sleep mode algorithm can take place in two different states, the one between toggling on and off an application in the terminal (flow level) and the other under an ongoing application where packets have long interarrival durations (packet level). It is expected that this kind of modeling will unveil a balance between the gain of the algorithm in these two different layers.

2 Outgoing Traffic

Another reasonable assumption is that only the incoming traffic is considered. However, prove that taking into account the outgoing traffic improves significantly the access delay. This improvement actually stems from suppression of unavailability periods which in turn raises the power consumption and lowers the delay. The same result is found by Xiao in a parallel work.

3 Sleep Mode Setup Time

As explained in the previous section, the terminal must exchange MOB_SLP-REQ messages before initiating the sleep mode algorithm. This message transaction lasts for a period durating a small number of frames. It is proposed that this period must be subtracted from the interarrival time of the incoming packets to correctly compute the access delay. In this paper, the resulting interevent times are called packet residual interarrival times.

This time offset is usually small enough to neglect in case of interarrival times between two application streams. However, the effect could be dominating in case of using the sleep mode algorithm during an ongoing flow. In this case, the interarrival times would be generally smaller and comparable to this time difference. In this perspective, the sleep mode setup time remains an important issue.

4 Energy Cost of State Switching

Switching on and off the transceiver is known to consume a fixed amount of extra energy. The authors model this switching cost to compute the final energy consumption. However, modern specification sheets for WiMAX transceiver provide low consumption idle states where the switching to active state is immediate and consumes negligible amounts of energy. Therefore, it seems that modeling the cost of state switching is not worthwhile.

5 Multiclass Scenario

Most of the papers in the literature so far focused on PSC type-1 class sleep mode. As explained above, this class refers to terminals that have only low QoS ongoing flows or no flows at all. However, considering a realistic scenario, one would have to take into account multiple connections of several QoS levels and include possible network management operations and therefore, combine many different sleep mode algorithms (one of type-1 and possibly many of type-2 and type-3) for only one terminal. These several algorithms, running in the same time, define the common availability periods of the terminal which in turn define the final unavailability periods. The terminal is allowed to go to sleep only on that commonly accepted unavailability periods (periods that the attention of the terminal is not required by the BS).

Take into account two different type of sleep mode classes. However, their work is focussed on selecting one of the two possible classes depending on delay and energy consumption optimality. Therefore, the task to analyze the performance of multiclass scenario remains open.

What is more important is the type-3 class. This class is used by the BS itself to organize multicast communications, locate the MS, and perform periodic ranging. Locationing is especially needed in mobile IEEE 802.16e networks to keep track of the mobile distance from the BS and perform handover operations. Including mobility into sleep mode modeling seems to require the addition of at least one extra sleep algorithm of type-3. Mobile scenarios are always multiclass. This assumption seems to be very important for the final results since high speed mobility tends to impose very frequent location updates. Then, periodic ranging is also very important and sometimes requires an extra PSC instance. It is interesting to investigate the effect of management operations in power consumptions of WiMAX terminals through a hybrid multiclass scenario.

6 Full Model Analysis

Most of the published work focuses on estimating the expectation of access delay and energy consumption. The mean value is indeed an intuitive characteristic and clearly valuable for comparisons and testing. The possibility of computing the variance of these measures. On the other hand, derivation of high-order moments or even of the distribution of these measures can be possible for simple interarrival models and could prove to be insightful.

7 Queueing

The analysis described above as well as most of the literature assumes that once the terminal shifts to awake mode it immediately serves the packet. However, by assuming nonzero service time, and for high-rate arrivals it is expected that queueing phenomena will occur. Specifically that the sleep mode algorithm can be modeled by a server with vacations and a M/G/1/K queue. 

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.

SLEEP MODE OPERATION IN WIMAX RELAYING NETWORKS

In the forthcoming IEEE 80216j standard, sleep mode operation has also been incorporated. In such relay networks, a BS coordinates all communications to and from subscriber stations. Although the initial standard provides a large coverage distance of up to 50 km under line-of-sight (LOS) conditions and typical cell radii of up to 8 km under non-line-of-sight (NLOS) conditions, there was still a huge demand for enhancing the network coverage without compromising system throughput. One of the possible options to achieve this was to introduce relay stations (RS) which bridge the communications between an SS out of coverage and the BS [9]. The great advantage of this multihop communication concept is that coverage is extended with cheap devices at a small cost of throughput degradation. RSs are divided into three categories, namely fixed RS (FRS) which are installed at fixed locations, nomadic RS (NRS) installed for a specific time duration, and mobile RS (MRS) which are mobile units operating with battery cells (Figure 1).

 

Figure 1: Sleep mode in IEEE 802.16j multihop networks.

Useful deployment of MRS with a battery power source or low-power fixed/NRS powered by solar power or battery require high-power efficiency. Moreover, this feature reduces the interference generated by RSs. Here, we briefly describe how sleep mode can function in relay scenarios.

In a centralized scheduling system, where the network is coordinated by the BS (named in Relaying Networks as MR-BS), the MR-BS informs all the stations of the network including both the RSs and the MSs, about listening and sleeping windows. In a second phase, the RS coordinates with the MSs the finding of common availability and unavailability intervals. If all the MSs connected to a single RS are sleeping then the RS can enter the sleep mode too. Similar to the initial IEEE 802.16e standard, the RS can request activation of RS sleep mode by sending RS_SLP-REQ message to the MR-BS. Note that now respective message types begin not with MOB as before but with RS.

In order for the RS to generate the listening and sleep windows, via a RS_SLP-REQ, the RS shall keep record of the information sleeping patterns of associated MSs. When an RS enters sleep mode it can be awakened by the serving MR-BS or by itself. The MR-BS can use the existing MOB_TRF-IND to awaken a sleeping RS.

Sleep Mode can be divided into

• Full RS Sleep Mode: This mode is entered if there is traffic at any relay and access link while the RS stays in sleep mode. All associated RSs and MSs connections are suspended RS sleeping.
  
  
• Partial RS Sleep Mode: This mode is similar to the previous case. The difference is that certain management messages are sent at predefined intervals to support MS network entry, re-entry, and handover.

A question remains whether these mobile relay stations can withstand the heavy usage. To what extent can such a terminal survive only on battery, and how energy efficient can IEEE 802.16e be in case of distributing information to many nodes?

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.
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  Deduction 3. The access delay is improved when load is high, but not in a straightforward manner.

MESSAGES AND TRANSACTIONS | POWER MANAGEMENT

The 802.16 standards can support 255 different management type of messages. According to IEEE 802.16e, 66 messages are defined from which 50–66 are used explicitly for handover and sleep mode operation. 

In the following, we describe the most important messages for sleep mode operation. Each message has three parts. The first part is casually MOB (implying Mobility) while the second part carries the scope of the message (SLP, TRF, HO, BSHO, MSHO, PAG, etc.). For the case of sleep mode operation, we are mainly concerned with SLP (implying Sleep) and TRF (implying Traffic). The final part of each message is usually REQ (implying Request), RSP (implying Response), or IND (implying Indication).

MOB_SLP-REQ: Sleep Request Message, type: 50, Connection: Basic.

A MS sends this message to request definition/activation of several PSCs. The actual definition occurs when a MS suggests a PSC for an incoming connection. The message contains a Power Saving Class ID, which is a unique identifier for a group of PSCs associated with a MS. This ID is also used for the exchange of many similar message types and response messages. Other parameters are also defined like the Initial Sleep Window, Final Sleep Window Base, and Exponent quantities measured in IEEE 802.16e frames. Moreover if the MS handles more than one class, the Number_of_CIDs field carries this number. 

MOB_SLP-RSP: Sleep Response Message, type: 51, Connection: Basic.

This message is sent in response to a request for the definition/activation of a PSC from the BS to a MS using a broadcast CID (Connection ID) or the MS’s basic CID. If a new definition was requested, then a new PSC is defined and the assigned ID is returned. When the MS receives this message, it activates the defined PSC. The MOB_SLP-RSP message contains fields such as the Length of Data, for the number of bytes per PSC, the start frame number for the first sleep window, and the relative intervals. Additionally, CIDs and windows (base and exponent) are described in MOB_SLP-REQ.

MOB_TRF-IND: Traffic Indication Message, type 52, Connection: Broadcast.

By the time the MS wakes up, it starts listening for a possible MOB_TRF-IND message, sent from the BS on broadcast CID or sleep mode multicast CID. This message is sent from a BS to a MS in the listening interval to indicate if there has been traffic addressed to the MS while it remained in sleep mode and functions only when there are one or more PSC IDs defined for PSC type I (as described below). Any other MS ignores this message. An explicit occasion in which the BS may arbitrarily include a positive indication for a MS is if the MS’s periodic ranging operation is scheduled to start in the next sleep window.