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Nov 25 2015

LTE small cells greatly improve QoE for video

Mobile operators know higher performance means higher quality of experience (QoE) — and that translates into more satisfied customers who consume more services and remain more loyal. That’s why they’re creating high-performance zones in urban areas, using LTE small cells that complement the macro network.

The customer service opportunity is enormous. According to Ovum, “the quality of the mobile broadband experience is the leading driver for mobile operator churn.

Looking more closely at what end users consider to be “poor service,” 45% are dissatisfied with video quality, making this service the most problematic issue. Mobile video consumption — watching YouTube or sharing videos on Facebook — drives wireless traffic growth. In fact, Bell Labs estimates that by 2016 streaming video will be the dominant wireless traffic type. And video traffic is expected to increase by 34x in the next 5 years. Clearly, providing higher quality video is key to offering better service.

What video QoE really means

Good video quality of experience implies minimal delays and few or no stalling occurrences — in addition to higher resolution content consistent with user equipment. Poor video quality means more unexpected events, including delay, buffering, and degradation of video quality.

A common metric for quantifying video quality is the Mean Opinion Score (MOS). A subjective measure of user experience, MOS can be used to compare a reference video with lower quality versions of the same video. These could include a network-impaired or a low-resolution video.

Using a standard ITU-T definition, an MOS of 4 indicates good QoE, while a MOS of 3 is fair. The difference in viewing time between a rating of fair and good can be dramatic, with abandonment rates increasing rapidly for videos with QoE MOS of 3 or lower.

Bell Labs analysis has shown (see the small cells QoE white paper presented at Wireless Congress 2015) that an MOS of 4 or higher yields a retention rate of 80%, while retention rates at MOS 3 or lower drop below 30%.

A look at network constraints

The network performance required depends on 3 primary factors:

  • Video encoding rate (bits)
  • Device resolution and screen size
  • Viewing distance

Considering these factors, the Bell Labs research cited previously estimates that the minimum throughput needed for a MOS of 4 or 5 is 1 Mbps on an iPhone5. That’s the minimum throughput needed to have a good video viewing experience on this terminal. Not surprisingly, larger screens and higher resolution devices need more throughput to maintain QoE. Figure 1 shows how throughput increases in step with screen size and resolution to keep pace with expectations.

Figure 1. Pressure on the network increases with screen size.

Achieving 1 Mbps throughput depends on radio signal propagation and the number of active users in each macro cell. As we see in Figure 2, the Bell Labs data shows user throughput -- for 7 or 20 active users -- assuming a uniform user distribution across the cell coverage area. This represents the average number of active users per cell at busy hours in Europe today.

For 20 active users per macro cell, operators can only offer the minimum 1 Mbps to 50 percent of the users within the cell. As a result, half of the iPhone5 users suffer from fair or worse video quality. That’s a problem.

Figure 2. Typical macro network user throughput distribution for 20 MHz LTE cell throughput.

Enter HetNet

HetNets—heterogeneous networks with small and macro cells—can be used to meet this challenge. Throughput must be enhanced in some places, and the macro cell offloaded in others. By adding LTE small cells where the macro signal is too weak, user throughput is boosted and video QoE meets the MOS-4 threshold. What’s more, remaining macro cell users benefit from having fewer users served by the same macro cell.

Figure 3 shows how, in 2 scenarios, HetNets increase the probability of good QoE scores over the macro-cell-only baseline scenario:

  • By adding 2 small cells for 20% coverage of the macro cell
  • By adding 4 small cells for 40% coverage of the macro cell

In both cases, throughput and video quality of experience increase significantly.

Figure 3. Enhancing QoE

With improvements such as less stalling and buffering, as well as increased resolution, operators can expect additional video usage at peak hours on both small and macro cells. Why? The better the quality of experience, the higher the probability that users will watch the video until its end, resulting in higher consumption of video.

In fact, as indicated in the white paper, Bell Labs modeling shows that increased coverage and throughput provided by small cells can increase video consumption by 200 – 300% (with 2 or 4 small cells deployed). This also increases customer loyalty, and increases the potential for users to migrate up to the next data plan cap.

Other apps benefit, too

Using small cells to increase throughput makes for across-the-board quality improvements.

Web browsing

Our web browsing experience is changing. The growing use of video and image-rich content has seen web-page size double from 2012 to 2015. By adding small cells, the median duration of a web page download is accelerated by 3 to 4 times.

Downloading and file transfers

Downloading music and file transfers are better, too. For 5 minutes or 5 MB of music, it would take as little 6 seconds with 40% of the area covered by small cells as compared to 40 seconds for a macro-only network. For time-crunched users, 34 seconds saved opens up possibilities for more browsing and viewing resulting in higher MOS ratings.

All this means the time has come to raise the bar on quality of experience. HetNet deployments will let operators achieve higher MOS. With LTE small cells, operators can cost-effectively increase macro cell network throughput while keeping pace with the growing appetite for mobile video.

Related material

Small Cells Solutions web page

Small cells – a solution for meeting QoE demands today infographic

To contact the author or request additional information, please send an email to networks.nokia_news@nokia.com.

About Bill Krogfoss
Mr. Krogfoss led technical teams and is also an individual contributor in the areas of application performance modeling, video performance and QoE, Internet architectures and net neutrality, content delivery networks, IPTV, and metro transport. Mr. Krogfoss has published over 20 technical papers in BLTJ, IEEE, CNS, OFC and Wireless Congress. He has also filed 6 patents in the areas of content delivery and QoE. Bill has also been an invited speaker at BEREC (European Regulator Body ), Wireless Congress, IEEE conferences, and US Senate Antitrust Subcommittee on topics from net neutrality to video performance and application QoE. He holds a BSEE degree from St. Cloud State University, Minnesota.
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