The Cisco® Aironet® 3700 Series Access Point refines several innovations that result in industry-leading performance. This white paper explains why that matters, and how we do it.
1. The Cisco Aironet 3700 Series Access Point introduces ClientLink 3.0.
2. ClientLink 3.0 expands and enhances the existing ClientLink family by extending proven transmit beamforming (TxBF) and maximal radio combining (MRC) techniques for new features added via 802.11ac (80-MHz channel width and 256 quadrature amplitude modulation [QAM]). This provides benefit for all 802.11ac client devices.
3. At any given range, TxBF improves downstream performance and throughput to clients, while MRC improves upstream performance and throughput from clients. TxBF and MRC improve the overall signal quality between the clients and the access point: With TxBF the client “hears” the access point with a better signal quality at range; with MRC the access point “hears” the client with a better signal quality at range. This improvement is achieved on a per-packet basis for all client device types (802.11g, 802.11a, 802.11n, and 802.11ac).
4. ClientLink 3.0 also adds support for standards-based Explicit Compressed Beamforming Feedback (ECBF) for clients that implement it (ECBF is an optional feature in 802.11ac). With ECBF the client provides estimates of the wireless channel conditions to the access point. This can be effective but comes at the expense of overhead (airtime is consumed for the ECBF messaging, which reduces throughput and capacity).
Note: ECBF is also commonly (but incorrectly) shortened to Explicit Compressed Beam Forming).
5. The enhancements introduced in ClientLink 3.0 are primarily for 5 GHz, since 802.11ac is 5 GHz only. Theproven no-client-involvement, zero-overhead approach is applicable to and benefits all 802.11ac clients.
6. For all 802.11a/n clients at 5 GHz, and all 802.11g/n clients at 2.4 GHz, the proven no-client-involvement, zero-overhead ClientLink/ClientLink 2.0 methods continue to be supported in ClientLink 3.0 and the Cisco Aironet 3700 Series.
7. ClientLink 3.0 is a key component in the High-Density Experience (HDX) solution suite.
In summary, beamforming is most practical using techniques that don't expect assistance from the client, such as Cisco ClientLink 3.0 (and its predecessors). ClientLink 3.0 solves the problem of channel estimation without depending on client assistance and continues to add genuine value. ClientLink 3.0 still helps (1) legacy 802.11a/n clients, (2) those 802.11ac clients that do not support 802.11ac sounding, and (3) clients at 2.4 GHz. It also avoids the overhead of standards-based explicit sounding.
Market and Technology Overview
As three-spatial-stream 802.11ac devices come to market, and as iPads and other one- and two-spatial-stream 802.11ac devices proliferate, it is critical to maximize performance for all devices. Not doing so could result in a slower network and in slower application performance for all devices.
The Cisco Aironet 3700 Series Access Points were designed to give best-in-class performance to all devices, allowing optimal network performance and investment protection so enterprises can effectively accommodate any device allowed on the network, regardless of how many spatial streams it has.
The keys to the best-in-class performance of the Aironet 3700 Series are:
● The first enterprise-class four-transceiver 802.11ac multiple-input multiple-output (MIMO) design (4x4:3)
● Cisco ClientLink 3.0 beamforming that works with all 802.11ac and 802.11n clients
● Cisco CleanAir® technology to manage interference
How does it all work? It’s worth starting with a bit of history. The first generation of 802.11n devices that came to market, beginning a few years ago, were able to support a maximum data rate of 300 Mbps. This data rate could be achieved by running two spatial streams, each carrying 75 Mbps of data per 20 MHz of spectrum, over a double-wide 40-MHz channel. The formula was 75 Mbps x 2 streams x 2 channels = 300 Mbps. Note that in order to support two spatial streams bidirectionally, a minimum of two MIMO transceivers were required at both ends of the link (access point and client).
Next, a newer generation of 802.11n devices emerged in the market, and these devices supported up to three spatial streams. This means that the theoretical maximum data rate that can be achieved is now 75 Mbps x 3 streams x 2 channels = 450 Mbps. To achieve this maximum data rate bidirectionally, both ends of the link (client and infrastructure) must support three spatial streams, which in turn requires that both ends have at least three MIMO transceivers.
Very recently, a new generation of 802.11ac devices have emerged. Not only can these devices support three spatial streams, but they also support an 80-MHz channel (twice that of 802.11n and four times that of 802.11a) and may also support up to 256-QAM (up to a 30 percent improvement over the modulation capabilities of 802.11a/n). This means that the theoretical maximum data rate that can be achieved is now 75 Mbps x 3 streams x 4 channels x 1.3 = 1170 Mbps. It is also worthwhile to note that 1170 Mbps also assumes an 800-nanosecond (ns) guard interval (GI). If one assumes a 400-ns GI, the theoretical maximum data rate is 1300 Mbps.
Maximum speed is important, but we also need to consider how often it will be achieved. It turns out that getting to the 1300-Mbps maximum data rate is not easy and requires careful design, as we’ll see later in this white paper. For now, keep in mind that current solutions that use 3x3:3 architecture (three transceivers, three spatial streams) on both sides of the link will rarely achieve 1300 Mbps in real-world applications and will have significant performance degradation beyond a short distance. In real-world scenarios, in which clients are not all 10 feet from the access point, the fourth transceiver on one end of the link is required to provide the necessary reliability for three spatial streams to work. And the logical place to put the fourth transceiver is naturally on the access point, where size and power are of less concern than on battery-powered clients.
In the end, the fourth transceiver on the access point gives extra decibels of link margin, which translates to better performance. As this paper will explain, in the uplink direction (client to access point), the extra receiver allows for MIMO equalization gain. This means that 1300 Mbps can be achieved at twice the distance of existing solutions (30 feet instead of 15 feet).
In the downlink direction, the extra transmitter allows for beamforming to the client. The Cisco implementation of beamforming, known as ClientLink 3.0, was designed to work with all 802.11ac clients-both those that support standards-based beamforming and those that don’t. This includes support for beamforming to clients that support three, two, or even one spatial stream.
So, while a 3x3:3 architecture offers only a nice peak-data story, the 4x4:3 Cisco Aironet 3700 Series offers both a high peak-data rate and real-world utility. Of course, building a 4x4:3 solution requires a custom chipset and additional engineering, so it takes a customer-responsive vendor such as Cisco to deliver it.
For real-world benefit, three-spatial-stream devices must work with usable range for a typical enterprise use. To get this, you really need the Cisco Aironet 3700 Series. Competitive designs deliver data rates of 1300 Mbps only at a range of 15 feet or less. And since access points are usually ceiling mounted, a range of 15 feet does not cover 15 feet of floor space. Because the signal must also travel from the ceiling to the typical height of a device, the area covered at the maximum rate by current designs is closer to 12 feet from the access point. With the Aironet 3700 Series, the usable range for three spatial streams at 1300 Mbps goes to 30 feet. This doubling in range actually results in a 500 percent increase in the coverage area that can achieve 1300 Mbps.
The features in the Cisco Aironet 3700 Series also yield excellent benefits with older 802.11n clients and with 802.11ac devices that support fewer than three spatial streams. These benefits are very important, because it will take a long time for all clients to support three spatial streams, and many phones will likely stay at one spatial stream (even though they are 802.11ac and can support 256-QAM).
The Gory Details: Uplink
Although three MIMO receivers are theoretically sufficient to handle three spatial streams, the problem is that this configuration provides no redundancy against channel fading or any of the inevitable hardware impairments: its operation is disappointingly short range or erratic in practice. For this reason, in the Cisco Aironet 3600 Series, Cisco leapfrogged the products that depend on pure spatial multiplexing and delivered hybrid spatial multiplexing and diversity with the addition of a fourth receiver. Cisco continues this uncompromising approach in the Aironet 3700 Series.
Cisco’s solution is akin to GPS positioning: to determine the latitude and longitude of a GPS receiver, three GPS satellite signals are necessary. More than three GPS satellites are required for superior location accuracy, and even entry-level GPS products can track 12 satellites or more.
The benefits of adding the fourth receiver to support three spatial data streams come from redundancy gain and diversity gain.
The signal captured by the extra antenna provides some redundancy gain. As in linear algebra, the fourth receiver provides four equations while there are only three unknowns. The extra equation provides an additional dimension of freedom when resolving the transmitted signals. On its own, redundancy gain increases range by about 10 percent.
Diversity gain is more important than redundancy gain. Due to channel fading, one antenna may see a deep fade, resulting in a very low signal level received on that antenna. For three spatial data streams, one needs a minimum of three strong signals so that three data streams can be resolved. If just one of them experiences deep fading, the MIMO detection will fail. By comparison, with 4x4 MIMO, if one antenna experiences a deep fade, there are still three good signals from which three data streams can be resolved. In actual implementations, the signals from all four antennas are carefully weighted according to quality, ensuring that the signals with better quality are utilized. 4x4 MIMO has markedly better performance in fading channels due to its balanced hybrid of spatial multiplexing for speed and diversity gain for robustness.
The technical term for the way the fourth receiver gets used is MIMO equalization. MIMO equalization is the comprehensive means to make best use of the received signal, whether it is transmit beamformed, space-time block-coded, spatially expanded, or unimproved. For MIMO equalization, having more receive chains helps the most, with the biggest gain coming from one extra receive chain, and diminishing returns thereafter. Accordingly, a 3x3 access point is a reasonable choice for receiving two spatial streams, but not for three spatial streams.
The Gory Details: Downlink
To get a parallel benefit in the downlink direction (from the fourth transmitter), Cisco created ClientLink 3.0, which is a scheme to fully exploit the four MIMO transmit chains. It combines beamforming with spatial multiplexing to improve the speed and reliability of downlink traffic. Even when clients transmit frames with fewer spatial streams than antennas, ClientLink 3.0 uses its advanced, patent-pending algorithms to provide full beamforming gain, whether clients help sound out the channel or not. Furthermore, Cisco access points are fully engineered to avoid any dependence on clients for calibration assistance. However, if such client assistance is available, ClientLink 3.0 is ready to take advantage of it. (802.11ac defines a single, though optional, protocol for an 802.11ac device to assist an 802.11ac access point. The protocol selected closely follows the 802.11n Explicit Compressed Beamforming Feedback protocol. See the section “Standards-Based Beamforming” for additional information.)
Consider the case of a client limited to three transceivers and trying to receive three spatial streams. Detection is very complicated and sensitive to poor signal quality, so any help from the access point is very welcome. As Figure 1 illustrates, using their four transmit chains, Cisco Aironet 3700 Series Access Points have the degrees of freedom to form three spatial beams (where each beam carries one stream of data) and direct them to each of the receive chains at the client. These beamformed signals add up in-phase at the receiver and thereby combat channel fading.
Figure 1. Beamformed Signal Projection with
ClientLink 3.0 Benefits
The benefits of ClientLink 3.0 are twofold. The proof of the benefits can be seen in Figure 2, which shows the rate at different ranges for a competitive access point. One can see that with a 3x3:3 client sending and receiving to a 3x3:3 access point, 1300 Mbps can be achieved only at short distances from the access point.
On the other hand, by adding a fourth receive chain at the access point, 1300 Mbps becomes usable out to 30 feet. This behavior is well matched to the expected coverage range of access points in typical enterprise deployments.
The second benefit is that all four transmit chains are used, even when the number of spatial streams is fewer than four. This results in some gain in the signal level due to more transmit power. Also, since each stream is sent from all four transmit chains collectively, there is significant diversity gain (one stream will not be completely wiped out if one or two antennas are in a deep fade).
As Figure 2 shows, the result is evident in the improvement to the achieved data rate at range, which also correlates closely with the reliability of each rate at that range. Assuming a typical enterprise environment, and at a typical enterprise range of 30 feet, a 4x4:3 architecture operates at 1300 Mbps, whereas 3x3:3 can’t do better than 975 Mbps.
Figure 2. Coverage Comparison of the Cisco Aironet 3700 Series with ClientLink 3.0 to a Competitive Access Point
Figure 2 shows the gains from the extra transmit chain. The Cisco Aironet 3700 Series can deliver 1300 Mbps as far as 25 feet (24 feet horizontally). We see that just one extra dimension of redundancy, backed by sophisticated signal processing and RF expertise, is a huge boost.
Table 1 summarizes the results of Figure 2. Forty-one measurements were made in a cubicle environment typical of an enterprise. Note that the Aironet 3700 Series achieved 256-QAM for 100 percent of the measurements in the given environment, whereas the competitive access point achieved only it in only 51 percent (and achieved only 1300 Mbps 7 percent of the time).
Table 1. Summary of Results of Competitive Comparison
Competitive Access Point %Connections per Data Rate
Cisco Aironet 3700 Series %Connections per Data Rate
A 3x3:3 architecture (with 256-QAM) offers a peak rate of 1300 Mbps, but nothing more. To make that peak rate achievable over a useful range, the Cisco Aironet 3700 Series provides a higher level of engineering and the custom silicon that only a larger vendor like Cisco can offer. On the uplink, four receive antennas and an optimized MIMO equalizer extend the range of 1300 Mbps out to 25 feet. Even at longer ranges, the additional diversity provides a significant advantage over a 3x3:3 design. On the downlink, the barrier to success is even higher. An additional transmit chain is necessary, but in addition client-neutral beamforming, such as ClientLink, ClientLink 2.0, or ClientLink 3.0, is essential to deliver the higher rates over the important ranges. For these reasons, ClientLink 3.0 is a key component in Cisco’s High Density Experience (HDX) solution suite.
Appendix: Primer on MIMO and Associated Performance-Enhancing Techniques
The amendment 802.11n introduced a set of new features that significantly improved WLAN range, reliability, and throughput. Among the new features, the most profound is the rich multiple-input multiple-output (MIMO) technique that allows multiple data streams to be sent simultaneously (Figure 3).
Figure 3. How MIMO Systems Work
MIMO is a flexible technique that can be used in different ways. At one extreme is pure transmitter and/or receiver diversity, while at the other extreme is pure spatial multiplexing. In the middle is a hybrid mode that offers both diversity and spatial multiplexing.
In the case of pure transmitter and receiver diversity, the same data streams are transmitted and received with multiple antennas. Since the same data is transmitted and received as multiple copies, the possibility of errors occurring in the signal is significantly reduced. Thus, the benefit of the transmitter and receiver diversity is to enhance the link’s robustness.
In the case of pure spatial multiplexing, multiple data streams are transmitted and received simultaneously over the same bandwidth. The data is different between the data streams. Given the same frequency bandwidth, and over the same time period, the data throughput increases N-fold, where N is the number of data streams. Thus, the benefit of spatial multiplexing is to increase the data throughput.
The hybrid MIMO mode offers the best of both worlds: additional diversity and spatial multiplexing, to improve both link robustness and data throughput. The hybrid MIMO mode is possible whenever there are more transceivers than data streams. For example, one could transmit and receive two data streams between two 4x4 MIMO devices.
For the transmitter diversity and hybrid modes, getting maximum benefits does require the vendor to go beyond the basics, however. Transmit beamforming is the premier technology, followed by space-time block coding (STBC) and spatial expansion. Let’s look at each of these in turn.
If multiple copies of the same data are sent from multiple transmit antennas, after going through the wireless channel, the multiple copies of the data have different attenuations and phases at the receive antennas. (Think of phase as like the sign of a number, such as +1 or-1, yet also allowing shades of gray.) This is because each copy of the data starts from a different antenna and gets reflected by different walls and furniture as it traverses its path to the receive antennas. If there are two paths arriving at a location with equal attenuation, but one path has the opposite phase of the other, they cancel each other out, which is not good if you’re trying to receive the signal at that location.
This headache motivates counter-measures. If the phase for each data stream is known, by precompensating the phase at the transmit antennas, the multiple copies of the data should have the same phase at the receive antennas. Instead of canceling, they constructively combine with each other. This scheme to precompensate the phases so that multiple copies of the data arrive at the receive antennas with the same phase is called transmit beamforming.
Transmit beamforming allows the multiple copies of the data arriving at the receive antennas to have the same phase so they add up with maximum reinforcement. Transmit beamforming significantly improves the signal quality received at the receive antennas. Transmit beamforming doesn’t just accept the wireless link it is handed-it also changes and improves it.
Transmit beamforming does require information about the phases (and oftentimes attenuations too) between the transmit chains and receive chain(s)-in other words, channel information. The 802.11n amendment introduced three ways to obtain this information; two depend on client hardware support and one can work with any client. The 802.11ac amendment specifies only one method depending upon client hardware support.
Standards-Based Beamforming for 802.11n
There are two types of standards-based beamforming: explicit beamforming and implicit beamforming.
In explicit beamforming, information about the wireless channel is fed back to the transmitter by the receiver. In order for the receiver to measure the wireless channel, the transmitter first sends a special sounding packet from all transmit antennas. The receiver examines the sounding packet at each receive antenna, extracts the wireless channel information, and sends that information back to the transmitter.
Explicit beamforming is an optional mode defined in 802.11n and requires the support of the client (receiver). Thechannel sounding protocol incurs some overhead. Still, explicit beamforming provides the most accurate knowledge about the whole channel, from all transmit chains to all receive chains. When available, it should be used judiciously.
Implicit beamforming does not require that a sounding packet be sent. Instead, the channel information is obtained by using the symmetry or reciprocity of the channels that is characteristic of Wi-Fi systems. The transmit and receive chains at the access point share the same set of antennas, so when an access point receives the uplink signal from the client, the access point extracts the channel information from the client’s transmit chains to the access point’s receive chains. Further, due to channel reciprocity, the same channel information applies equally well to the downlink transmit beamforming.
Standards-based implicit beamforming augments these capabilities to address two issues. First, implicit beamforming is most straightforward when a MIMO client transmits out of all its antennas, which is not always the case. When the client transmits out of fewer antennas, the access point cannot measure the wireless channel fully and cannot maximize beamforming gain. Second, implicit beamforming requires that a device’s transmit hardware and receive hardware be well matched. According to implementations, this may be possible during manufacturing, or via internal calibration in the field, or in the worst case may require over-the-air assistance from the client. For these reasons, 802.11n defined optional hardware modes where:
● The client can send additional sounding information from its additional transmit chains even if they weren’t being used to send actual data. This mode, when supported, has very low overhead.
● The client can assist with calibration of the access point.
Standards-Based Beamforming for 802.11ac
In theory, any device with multiple antennas can beamform to any other device at any time. What 802.11ac specifies is the opportunity for the receiver to help the beamforming transmitter do a better job of beamforming. This is called “sounding,” and it enables the beamforming transmitter to precisely align its transmitted energy toward the receiver. 802.11ac defines a single, though optional, protocol for one 802.11ac device to sound other 802.11ac devices. The protocol selected closely (but not completely) follows the 802.11n Explicit Compressed Beamforming Feedback (ECBF) protocol, as described below.
A device, typically an access point (more on this to follow), sends a Very High Throughput (VHT) Null Data Packet (NDP) Announcement frame whose only purpose is to contain the address of the access point (the beamformer) and of the target client recipients (the beamformees). The VHT NDP Announcement frame is immediately followed by a VHT NDP intended for those target recipients. Each intended recipient measures the RF channel from the access point to itself using the preamble of the VHT NDP and compresses the channel. The first intended recipient responds immediately with the compressed channel information in a VHT Compressed Beamforming frame. Other recipients respond subsequently when they are polled by the access point. The VHT NDP Announcement frame, the VHT NDP, and the VHT Compressed Beamforming frame are all similar to features in 802.11n. However, because of some subtle differences, the 802.11ac sounding is not backward compatible with 802.11n devices.
ECBF is known to provide the most precise estimate of the channel that takes into account all the imperfections at transmitter and receiver.
However, ECBF comes with a lot of overhead: the VHT NDP Announcement frame, the VHT NDP itself, and the frames carrying the compressed feedback from each recipient. For an access point with four antennas, the compressed feedback varies from 180 to 1800 bytes, depending on the number of client antennas and level of compression. Sounding just one single-antenna 80-MHz client takes about 250 microseconds. When devices can transmit at 433 Mbps, this has an impact, since that same time could have instead been used to send an extra 13,000 bytes.
Also, although any device with multiple antennas can beamform to any other device at any time, the practical limitation on the effectiveness of ECBF is that the target recipient ideally has fewer antennas than the device intending to beamform. Since many client devices have only a single antenna, they cannot beamform to the access point, and those clients that have multiple antennas are at a disadvantage, since the access point typically has more antennas.
Therefore, technologies that solve the problem of sounding without being limited by client implementation and/or depend on client assistance (such as Cisco's ClientLink technologies) continue to add genuine value. They help (1) legacy 802.11a/n clients, (2) those 802.11ac clients that do not support 802.11ac sounding, and (3) clients at 2.4 GHz. They also avoid the overhead of standards-based explicit sounding.
Introducing Cisco ClientLink 3.0 Beamforming
802.11a/g clients cannot support standards-based beamforming, either explicit or implicit, and many 802.11n and 802.11ac clients do not support standards-based beamforming either. For this reason, it is vital that vendors aiming at a comprehensive solution provide beamforming modes that work for any client. As with Cisco ClientLink and ClientLink 2.0, ClientLink 3.0 beamforming does just that.
ClientLink was designed for single-antenna clients or clients using one spatial stream. The access point can measure the wireless channel any time the client transmits, even if it’s just one packet, and the access point then uses that information to maximally reinforce the data send-back to the client. For multiple-antenna clients, vendors must use innovative technology such as ClientLink 2.0. and 3.0. Single-antenna clients also benefit from ClientLink 2.0 and 3.0.
Therefore, as with ClientLink and ClientLink 2.0, ClientLink 3.0 beamforming offers high gain, works with every 802.11a/n/ac client-even 802.11ac clients that do not support standards-based beamforming-and incurs no overhead.
In summary, beamforming is particularly valuable due to the vulnerability of low-antenna-count devices to destructive fading. 802.11n offers many incompatible flavors of beamforming, each involving client assistance, and the industry has never put its weight behind any one of them. 802.11ac improves the situation by specifying only one standardized method, but it does so at the expense of overhead. Thus, beamforming is most practical using techniques that don't expect assistance from the client, such as Cisco ClientLink 3.0 and its predecessors. For these reasons, ClientLink 3.0 is a key component in Cisco’s High Density Experience (HDX) solution suite.
On the receiving side, the menu of technologies is very short: it includes only MIMO equalization. MIMO equalization is the comprehensive means to make best use of the received signal, whether it is transmit beamformed, space-time block coded, spatially expanded, or unimproved. For MIMO equalization, having more receive chains helps the most, with the biggest gain coming from one extra receive chain, and diminishing returns thereafter. Accordingly, a 3x3:3 access point is a solid choice for receiving two spatial streams. So too is a 2x3:2 access point, since it provides the same number of receive antennas and thus identical gain.