Major advantages of 802.11ac: multi-user MIMO

Major advantages of 802.11ac: multi-user MIMO

The one thousand Mbit/s Wi-Fi technology of 802.11ac means faster speeds for Wi-Fi clients than 802.11n. 11ac is actually composed of several important technologies, including the multi-user MIMO technology. 11ac is divided into two phases: Wave 1 and Wave 2. It is expected that 11ac will implement Wave 2 in the second half of this year, while multi-user MIMO technology is part of Wave 2.

The 802.11ac standard has added and extended many technologies, which not only greatly improved the overall data transmission rate and throughput, but also protected and maintained these gains through more stable signals. This high-reliability signal supports larger throughput over a longer distance. To maintain the stability of the connection, it can also reduce the throughput gently.

In 11ac, the channel width is increased to 80 MHz (the channel width in 11n is 20 MHz). In Wave 2, the channel width is further increased to 160 MHz. 11ac supports four spatial streams in the Wave 1 phase. In the Wave 2 phase, this number will be increased to eight. Compared with the 11n 64 QAM, 11ac has a higher density modulation scheme, reaching 256 QAM. In the Wave 2 stage, 11ac will adopt multi-user MIMO technology with the emission beam forming function.

The basic concept of MU-MIMO (multi-user MIMO) is very simple: A 11ac access point can send data to four clients at the same time (the client must also support MU-MIMO ), at the same time, an independent "Space stream" is created for each client ". In contrast, the current 3-space stream 11n access point can only communicate with one client at a time, so multiple clients must communicate one by one in turn. Basically, MU-MIMO can make the Wi-Fi network more efficient and ensure that all users have a good "wireless experience" when accessing a large number of users ". Currently, access points and wireless router chips can be purchased on the market. Network manufacturers hope to produce the first batch of network and client products later this year.

Technically, multi-user MIMO is an option of 11ac, and vendors are not required to provide support for it. However, most observers believe that in the short term, it will become a standard feature of almost all 11ac products.

MIMO was first introduced in the 802.11n standard. It was introduced to solve a previous problem, but it significantly increased the data transmission rate of Wi-Fi. The previous problem is called the "multi-path" problem, that is, when a wireless signal is transmitted, It will be reflected by the surrounding objects, this allows the signal to arrive at the receiver along multiple paths at different times and angles. The "multi-path" problem may cause signal interference and slow down.

MIMO makes full use of the characteristics of multi-path, and adds an antenna to both the sending and receiving ends of the signal. Multi-antenna technology improves the signal-to-noise ratio during transmission through spatial diversity, and enhances the system reliability through anti-attenuation. Through space reuse, users can transmit more data through two, three, or four spatial streams using different paths between antenna groups.

According to Bahador Amir, Quantenna's Director of Technical marketing, the company's chips can talk to four clients at the same time, or divide the clients into four groups for parallel transmission. Then, in each group consisting of five or six or more clients, the Quantenna chip uses the traditional Wi-Fi specification to communicate with one client at a time, completes the conversation with each client in sequence.

Amir said that in the past, in this case, the Access Point will gather the client with the nearest client. as the distance from the Access Point increases, the client must slow down the transmission rate, increase the network conversation time, leading to unnecessary occupation of network bandwidth. However, with multi-user MIMO technology, clients that are far away from the cloud will be centralized to share the capacity, while other clients that are close to the cloud can enjoy a faster rate.

The transmitting beam is optional in 11n, but required in 11ac. Matthew Gast from Aerohive said that the launch beam is a key supporting technology for multi-user MIMO. This technology enables access points to "centralize" or "manipulate" Radio Frequency Energy in specific directions of specific clients. To achieve this function, you first need to constantly talk to each client to collect measurements of a specific channel. the access point can fine-tune the signal phase based on the collected measurements. This adjustment gives higher effective power to specific client signals. With multi-user MIMO technology, the access point can simultaneously optimize the signal of up to four clients.

Multi-user MIMO has now reached a critical period. Thanks to the popularity of smartphones and tablets, the number of Wi-Fi clients is growing rapidly. End users want to free themselves from the cost of cellular networks and require online and upgrade of video, music, and continuous social networks. In the vicinity of a Wi-Fi access point, you may not have a Windows or Mac laptop, but almost everyone wants to connect one, two, or more mobile devices to nearby Wi-Fi devices.

Amir of Quantenna said: "As the number of clients increases, the access point must be able to cope with the above situation. When using single-user MIMO technology, you will be in trouble when the access point is saturated. Because the Access Point must communicate with 100 clients in turn. Your time will be insufficient, resulting in low bandwidth services. Multi-user MIMO technology can greatly improve this situation ."

Most smartphones and tablets with 11ac devices may be single-stream clients (with only one antenna), and their [note] rate may be 433 Mbps. 4-stream 11ac access points using beam technology are ideal for such clients.

Craig Mathias, head of the Farpoint Group of the wireless consulting company and blogger of the "Network World" website in the United States, said: "A big practical problem with multi-user MIMO is how well they can work? The answer is that no one knows yet. I only saw it in the lab. There are no products available for deployment, so it is not clear what benefits users can obtain ."

Mathias points out that Wi-Fi networks are always filled with variables. Although the theoretical capacity of the 11ac access point using an 80-MHz channel is 1.3 Gbps, "this value is not achieved in implementation," he said ". "The Benchmark Test Results of vendors are uneven. The radio performance changes greatly, and firmware, Chip, circuit board design, drivers, antennas, and radio environments also have an impact. Therefore, we cannot predict what performance the user will get ."

"My main advice to users is to wait and evaluate the new 11ac products after they appear ." Mathias said. "Users should spend time watching how multi-user MIMO technology will work, and most importantly, how they will work in your particular environment ."

As mentioned above, the widely deployed 11n access points, routers, and clients all use MIMO technology. Most Access Points support three spatial streams. However, if three clients-laptops, smartphones, or tablets-are connected to the 11n Access Point of the 3-space stream, only one device can communicate with the access point at a time. In fact, all three space streams "Lock" one client. The time slice algorithm allows equal access to all clients connected to the access point. Based on the number of clients (for example, a laptop has three antennas and a smartphone has only one antenna), and the maximum throughput they support, the capacity of most access points may be idle for a given period of time.

Matthew Gast, author of The 802.11ac Survival Guide and emerging technical director of Aerohive networks, said: "A common misunderstanding is that multi-user MIMO technology can make the network faster. This is not the case. They only achieve multi-logical connection with devices at the same time, thus improving the network efficiency ." Because data can be transmitted to four devices at a time, the 11ac access point can fully utilize its Gigabit available capacity at any time.

Currently, Quantenna has released a 11ac chip named QSR1000 to the market. The chip has four RF Frequencies and four receiving antennas, supporting 4-space stream and multi-user MIMO technology. This is the only chip that has implemented these features so far. However, Qualcomm, Broadcom, and other competitors have promised to launch multi-user MIMO products to the market. Quantenna said it would launch a 11ac Chip Supporting 8x8 MIMO technology in 2015.