10.5 3D Video over 4G Networks

As discussed in Chapter 6, fourth generation mobile networks provide a number of important improvements from the preceding 3G networks [33]. Among other improvements, 4G is capable of delivering connections at very high data rates (with target speeds of 100 Mbps or better). Circuit switching will not be supported in 4G networks, as they will be entirely based on an all-IP architecture. Consequently, it is expected that a major part of the total traffic will be from different IP-based multimedia services, such as voice over IP and video streaming. To support the large variety of services over the same IP architecture, 4G systems provide different mechanisms such as differentiated classes of service for varied quality of service (QoS) support and improved security to the end users. Recall from Chapter 6 that, depending how strict is the classification of a mobile system into the “4G” category, the two main 4G standards can be considered to be 3GPP LTE (long term evolution) and IEEE802.16e or their respective evolutions LTE-A (long term evolution – advanced) and IEEE 802.16m WiMAX (sometimes called WiMAX 2). Also, recall from Chapter 6 that, in broad terms, both families of standards share many similarities. Both standards are based on OFDM technology with low latency (e.g., by defining a 10 ms frame in LTE). The use of OFDM presents advantages in terms of high bit rate data transmission (due to the good performance in multipath environments) and results in a flexible implementation that allows the standards to be easily adapted to operate over different system bandwidths. Indeed, the flexibility provided by OFDM also allows for developers to more easily design products for one of the family using as starting point intellectual property, such as software libraries, already developed for products based on the other family of standards. The low latency helps enhance channel estimation in high Doppler scenarios, which enables the use of a number of techniques that improve transmission speed. In addition, both families of standards support time-division multiplexing (TDM) and Frequency-division multiplexing (FDM) uplink/downlink duplexing, incorporate multiple transmit and receive antenna technology (MIMO), adaptive modulation and coding (AMC), and hybrid automatic repeat-reQuest (ARQ) for flexible and efficient error correction.

A study of 3D video transmission over WiMAX was reported in [34]. The study is limited to the physical layer of IEEE 802.16e Mobile WiMAX and compared 3D video encoded into two separate left-right views at 3 Mbps each and using a color-plus-depth scheme with 4.8 Mbps allocated to color and 1.2 Mbps allocated to depth information. The work studies different setting for the AMC feature in WiMAX so as to deliver the total 6 Mbps required by the test video sequences. Mobility was considered in the study as the channel model was the ITU Vehicular A for a user traveling at 60 km/h. As a result of the study, it was observed that 3D video streams encoded using the two-view (left and right) method yielded better PSNR results than the color-plus-depth method. The measured difference in PSNR was between 1 and 5 dB.

The higher bit rate and lower latency that can be achieved with 4G technology are going to be key enablers to transmit 3DTV services and also new features such as view switching/selection or multi-view applications. At the same time, this will bring new challenges and considerations to the design of schedulers and resource allocation mechanisms. The 4G standards do not specify algorithms for resource allocation. Therefore, an important area for research and development is to design techniques to allocate resources for video transmission that exploit the flexibility offered by OFDM in terms of wireless spectrum resource assignments. For example, in [35], subcarriers are allocated and the encoding rates of multiple scalable video programs are calculated so as to meet quality, fairness, and efficiency goals by using the frequency, time, and multiuser diversity offered by the OFDM system. Another example of resource allocation for video transmission over 4G networks is the work in [36]. This work studies, as an application of a novel video frame synthetic generator, three scheduling algorithms for the allocation of WiMAX slots. A WiMAX slot consists on one subcarrier assignment lasting for a number of OFDM symbols. The three discussed scheduling algorithms are:

• earliest deadline first (EDF): the scheduler examines the packet at the head of each queue from flows requesting service and schedules first the one with the earliest deadline,
• deficit round robin (DRR): the scheduler allocates in a WiMAX frame those packets that meet a fair share criterion; the fair share criterion is derived from the queue length and the modulation and coding level as an indicator of the channel conditions,
• earliest deadline first with deficit round robin (EDF-DRR): the scheduler sorts the packets in order of earliest deadline and then applies the DDR criteria to make the final scheduling decision.

In [36] it is shown through simulations that EDF is the most unfair of the three scheduling algorithms and that DDR is approximately as fair as EDF-DDR but also has better performance for mobile video traffic.

As explained in Chapter 6, the use in all 4G standards of different variants of OFDM, leads to the need to allocate wireless medium access resources within a time–frequency grid, where the time dimension is given by the successive transmitted symbols, and the frequency dimension is given by different subcarriers allocation. A scheduler for time–frequency resource allocation in LTE is studied in [37]. This scheduler departs from the popular Proportional Fairness choice because it is argued that although it is possible to achieve long-term fairness in the resource allocation, there are no guarantees for delay-constrained real-time video services. In contrast, the scheduler in [37] takes a two-pronged approach where on one side a weighted round-robin (WRR) algorithm allocates time–frequency resources, and on the other side a cross-layer approach is used to perform AMC and choose source coding parameters based on channel conditions. Furthermore, the weights for the WRR algorithm, that determine the share of resources for a given call, is given as a linear combination of three weights, one incorporating the channel quality, a second considering the QoS constraint for an application and the third introducing an element of fairness that measures the historical average data rate.

As mentioned earlier, the resource allocation mechanism has not been specified in the 4G standard. Therefore, 4G system designers need to devise techniques that are able to couple the design of schedulers and resource allocation mechanisms with the QoS-related mechanisms defined within the standard. Both family of standards, LTE and WiMAX, support multimedia services by providing several QoS mechanisms [38]. Specifically, 4G standards include mechanisms to establish connections with different scheduling types and priorities, which support, for example, traffic with guaranteed maximum and minimum traffic rate. Also, the standards include different mechanisms that allow the uplink scheduler at the base station to learn the status of the buffers at the mobiles.