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Addressing Two Challenges to High-Density Wi-Fi Deployment

When the movie Star Wars: The Force Awakens was released, the phrase “May the force be with you” swept the world again. Similarly, as Wi-Fi networks are popular around the world nowadays, it has become popular to say “May the Wi-Fi be with you.” People have gotten so used to using mobile phones or tablets to watch videos, read news, and update WeChat Moments or microblogs, that it could be argued that Wi-Fi networks play an even more elemental role in our universe than “the force” does in its fictive Star Wars counterpart.

With the joint efforts of governments, carriers, and companies, Wi-Fi networks have been deployed in every corner of cities. In shopping malls, hotels, train stations, exhibition halls, and conference centers, you can use your mobile phones to quickly search Wi-Fi networks. In practice, Wi-Fi networks in such high-density scenarios are not as good as those at homes or in offices. Users often suffer from Wi-Fi access failures or extremely slow network speeds. It may take several seconds to open a web page and videos may constantly buffer and cannot play. What causes these problems?

Poor User Experience: What Are the Major Causes?

Many people attribute this problem to poor AP performance or imprecise network planning. Actually, these are not the major causes. Over the last few years, basic performance of APs has matured. In an office environment, an AP allows over 30 users to access a Wi-Fi network simultaneously. Network planning solutions are also maturing and can address common interference problems. Then why is users’ network experience still so poor? 

This must be analyzed from the perspective of infrastructure solutions. The industry’s mainstream solution for indoor high-density access is cellular deployment of APs. APs in this solution have built-in omnidirectional antennas. The coverage of an omnidirectional antenna is like an apple in all directions and in a wide range. This solution is applicable to scenarios with high coverage requirements, such as semi-open offices.

Figure 1. Apple-like Coverage of an Omnidirectional Antenna in an AP

However, if the preceding deployment solution is used in open environments such as stations and conference centers, signal interference between APs may easily occur and exponentially degrade radio performance. In such wireless environments, technical specialists can find no way to optimize save for reducing the transmit power of APs or narrowing down coverage using obstacles. This is of little use in fixing the overall problem.

Figure 2. Wireless Coverage in an Open Public Environment Imagined as a Hypothetical Ball Pool

Wireless signals are invisible and it is difficult to locate related faults. Upon receiving reports of poor user experience, most carriers and Wi-Fi companies anchor their hopes in deploying more APs to solve the problem. Additionally, many enterprises deploy their own APs to increase network speeds so as to attract customers. As more and more APs are deployed, the space will become increasingly congested. Therefore, radio performance further deteriorates.

High-Density Small-Angle Antennas

Signal interference is a key cause of deteriorated radio performance. It is a common problem but somewhat hard to eliminate. In recent years, a simple and effective solution has been deployed in some large stadiums and stations.

A typical case is the WLAN solution deployed at the 2015 IAAF World Championships in Beijing. During the event, the free Wi-Fi network allowed spectators to interact with friends and share exciting moments on social media. According to statistics, the Wi-Fi network provided access for a rough total of 680,000 spectators in eight days. The total network traffic was 16.4 TB, equivalent to a transmission of 8.6 million digital photos or 1.6 billion social media interactions.

The solution deployed at Beijing National Stadium uses high-density small-angle antennas, with greatly improved coverage effect. 

Figure 3. Huawei Outdoor High-density Deployment Solution at Beijing National Stadium

This solution has wireless coverage advantages in outdoor high-density environments. However, it is not an ideal choice in indoor environments as it poses new problems such as construction and labor costs. As antennas and APs need to be installed separately, the installation cost and time almost double those of the indoor AP solution. Additionally, an antenna is far larger than an iPad Air and antenna installation may affect interior design.

Compact Smart Antennas: Addressing the Indoor High-Density Problem

This problem has challenged the industry for many years, and has finally been solved with release of Huawei AP4050DN-HD.

The AP4050DN-HD has built-in smart high-density antennas that provide precise signal coverage. It effectively reduces inter-AP interference in high-density scenarios, and improves radio performance and users’ access experience. The AP4050DN-HD applies to indoor high-density scenarios such as lecture halls and supermarkets to provide Wi-Fi coverage. The AP4050DN-HD is compact, equivalent to the size of an iPad, and weighs no more than 1.5 kg. An AP4050DN-HD can be installed by one person in only 30 minutes. With all of these characteristics, the purchasing, installation, and deployment costs of the AP are reduced. In addition, the AP4050DN-HD supports the PoE out function, reducing required ports on PoE switches by 50%.

High User Concurrency Rate: The Biggest Challenge in High-Density Scenarios

Signal interference is not the only challenge for Wi-Fi deployment in high-density scenarios. In some interference-free Wi-Fi scenarios, a high user concurrency rate presents another challenge. Although there are a large number of people in high-density scenarios such as stations, shopping malls, and stadiums, not each of them needs to connect to the Internet at any one time. In these scenarios, the user concurrency rate is generally about 30%. In some special scenarios such as multimedia classrooms and e-classrooms, almost all concerned need to access the Internet at the same time. The user concurrency rate reaches almost 100%. Users have different service experience even if AP performance is the same in these two types of scenarios.

Figure 4. Almost 100% of the User Concurrency Rate in e-Schoolbag Scenarios

Let’s say that an AP (three spatial streams) providing the highest performance of 900 Mbit/s and 60 tablets (single spatial stream) are tested in an e-schoolbag scenario. When the user concurrency rate is 30% (that is, 18 tablets connect to the AP at the same time), the average available bandwidth for each tablet is about 6 Mbit/s, and the overall throughput is 110 Mbit/s. When the user concurrency rate is 100% (that is, 60 tablets connect to the AP at the same time), the average available bandwidth for each tablet is about 1.2 Mbit/s, and the overall throughput is only 72 Mbit/s.

Severe interference does not exist in this testing environment. Why then is the overall throughput decreased from 110 Mbit/s to 72 Mbit/s when the number of concurrent users on the AP increases from 18 to 60? The reason can be analyzed based on the Wi-Fi data processing mechanism. All terminals compete for one transmission channel. The more terminals, the higher the possibility that channel conflicts will occur. In scenarios where the user concurrency rate is 100%, an AP endures three times the access pressure than that endured in common scenarios. This is one of the greatest challenges faced in high-density scenarios.

The Wi-Fi air interface mechanism endows AP radios with multi-user performance. When 30 users connect to an AP, a single 2.4 GHz or 5 GHz radio chip can only provide the average available bandwidth of about 1.2 Mbit/s. What kind of service experience is encountered with 1.2 Mbit/s bandwidth in e-schoolbag scenarios? When teachers share screens, make pages turn automatically, or write on electronic whiteboards, the delay on all tablets averages out to about 1 second; when 480p videos are played, however, video buffering and synchronized playing will take about 10 seconds or even longer.

Nowadays, students generally need to download and play High-Definition (HD) teaching videos. It is one of the focuses in the education industry to build high-quality wireless networks in classrooms to provide higher service capabilities for terminals and effectively support teaching activities.

Multi-Radio APs: Solving Problems of High User Concurrency Rate

How can user experience be improved in scenarios with high concurrency rate? Simply deploying more APs can meet terminal bandwidth requirements on e-classroom services, but creates the following problems:

Doubled network deployment cost: When two APs are deployed in one classroom, device purchasing, construction, and power supply costs will be doubled.

Increased O&M difficulty: After the number of deployed APs is doubled, the number of management nodes and workload will also double. This places greater demands on the limited network O&M capabilities of schools.

Is there a better solution? According to the Wi-Fi air interface mechanism, the overall performance of AP radios deteriorates as the number of concurrent terminals increases. Reducing the number of concurrent terminals on a single radio can reduce the probability of terminal collisions. Subsequently, the air interface overhead caused by collisions can be lowered, and the overall performance of AP radios and performance of a single terminal can be improved. For example, a triple-radio AP can be used for the e-schoolbag service. In this case, the number of concurrent users on a single radio is reduced from 30 to 20, the average bandwidth for each user will increase from 1.2 Mbit/s to 2.5 Mbit/s. The actual average bandwidth for each user varies with different AP capabilities and site environments. Deploying one triple-radio AP instead of two dual-band APs in one classroom reduces device purchasing, installation, and auxiliary costs (including port quantity and power consumption of switches), and simplifies daily O&M of networks and devices.

Huawei offers the AP4030TN that supports three radios: 2.4 GHz/5 GHz + 2.4 GHz/5 GHz + 5 GHz. Two of them support Software-Defined Radio (SDR) technology and can flexibly switch from 2.4 GHz to 5 GHz. When all of the three radios provide coverage services, multi-user concurrent processing capabilities and single-user service performance are improved. A single AP can be connected to a maximum of 100 concurrent users and provide average available bandwidth of 4 Mbit/s per user. This bandwidth can meet the requirements for 100 terminals playing HD videos and Videos on Demand (VoDs) (including 720p and 1080p videos) at the same time.

With SDR technology, an AP can meet the requirements of different types of terminals in various scenarios and flexibly adapt to terminal changes. An AP also provides multiple 5 GHz radios for service access and has the advantages of multiple available 5 GHz channels, low signal interference, and high service bandwidth.

By Chen Siyuan and Wei Yupeng