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Ruckus Wireless | White Paper

When buying a sports car, we often focus on engine size, top speed, horsepower, and 0-60 time. But utilizing those capabili-ties requires a well-designed transmission.

In Wi-Fi, we often focus on maximum data rate, MIMO configu-ration, channel size, and fancy antennas (guilty as charged). We rarely talk about the mechanism that switches Wi-Fi gears.

Like a car, Wi-Fi devices have a transmission too. In Wi-Fi, it’s called dynamic rate adaptation (aka. rate control, rate switch-ing, or rate selection).

Rate adaptation is the function that determines how and when to dynamically change to a new data rate. When it’s tuned properly, a good adaptation algorithm finds the right data rate that delivers peak AP output in current RF conditions – unstable as they are. Though often ignored, rate adaptation is a critical component to any high performance system.

Choosing a rate: Ethernet vs. Wi-FiOn a wired Ethernet link, endpoints connect and auto-negotiate an interface speed at the fastest mutually supported signaling rate. Because Ethernet link conditions are static, the rate remains the same. Simple.

In Wi-Fi, link conditions change more often than HP changes CEOs. To find the best data rate in an undulating sea of unlicensed spectrum (mobile clients, transient devices, RF interference, temporary networks, bursty traffic, etc.), smart rate adaptation is essential. Wi-Fi systems must handle chang-ing conditions in stride, adapting communication rates based on a complex set of variables.

Finding the right balance between optimum performance and reliability with adaptive data rate algorithms

Instead of selecting the fastest mutual speed, 802.11 stations attempt to find the best speed, based on a tradeoff of reliabil-ity and performance—note that uplink and downlink conditions are different, so the AP and client have data rate autonomy.

What’s a data rate?Thanks to marketing departments, Wi-Fi speeds and feeds are fairly well known by Wi-Fi people. However, fewer people inti-mately understand why there are different data rates and why dynamically changing data rates can improve communications.

Fundamentally, it’s critical to understand that higher data rates are more “complex” than lower data rates. With lower data rates, the modulation and coding mechanisms are simplified, which makes them less efficient, but more reliable.

Each data rate is the product of some specific combination of modulation and coding—as well as other factors like channel bandwidth and spatial streams.

ModulationModulation is the process of changing the properties of a car-rier wave to represent information bits. There are three basic types of modulation: amplitude, frequency, and phase.

Figure 1 shows a simplified concept in which each modulation change represents a single bit of data (the baseband signal). We could also visualize modulation on a constellation map, as

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in Figure 2. In Figure 2, we’re focused on phase shift modula-tion, which is common in Wi-Fi. When the receiver receives a modulated signal, the phase of the signal is aligned with a constellation (i.e. the red dots) on the map and represents a specific bit pattern (e.g. 00, 01, 10, 11). More complex modula-tion types have more bits on the map.

With BPSK (binary phase shift keying), there is only one bit of information. The receiver detects the phase of the signal and matches that phase with a bit pattern (either the right or left of the constellation map): either a 0 or a 1. So the margin for error is quite large, making this a highly reliable modulation method. However, one bit per symbol is inefficient.

With QPSK (quadrature phase shift keying), there are four possible constellation points—2 bits of data. Compared with BPSK, the receiver must detect the signal’s phase with more precision. Thus, the signal quality must be higher, but the tech-nique is more efficient.

To add efficiency, the next higher order of Wi-Fi modulation uses both phase and amplitude shifts, as in Figure 3.

16-QAM (quadrature amplitude modulation)—4 bits per symbol—is shown in Figure 3. By now, I’m sure you can see the complexity and efficiency pattern emerging. Higher-order modu-lation is more efficient—more bits of data per sample. But, better signal quality is required for reliable mapping by the receiver. 64-QAM—6 bits per symbol—is also used in 802.11a/g/n, and 802.11ac will introduce 256-QAM—8 bits per symbol.

CodingAnother important mechanism that controls efficiency and reliability is the coding rate. Also known as forward error cor-rection, coding is the process of adding redundant information bits to a data stream to improve reliability over unreliable mediums. In other words, x number of data bits are converted into y number of coded bits to improve error recovery. We express this as a ratio of data bits to coded bits.

Data Bits Coded Bits Coding Rate Efficiency Reliability

1 2 1/2 Less More

2 3 2/3

3 4 3/4

5 6 5/6 More Less

As with our modulation methods, the tradeoff for coding is efficiency versus reliability. The benefit of data redundancy over noisy RF channels is a priority, but the goal is always to find the right balance.

Modulation and coding schemesWe use the term data rate to indicate the speed of a wireless connection. Data rates are determined by a number of vari-ables, but the primary elements that we can dynamically control are modulation and coding schemes. The table on the next page shows how the data rate increases or decreases based on the efficiency of the modulation and coding methods.

FIGURE 1: Basic Types of Modulation

Baseband Signal TIME

1 0 1 1 0 1 0 0

Amplitude Shift Keying (ASK)

Frequency Shift Keying (FSK)

Phase Shift Keying (PSK)

FIGURE 2: Phase Modulation with BPSK and QPSK

1 Bit — 180° Phase Shifts

b0+1

1

-1

-1 -1

-1+1

+101 11

1000+1

0

b0b1BPSKQPSK

Phase°

Q Q Q

I I I

2 Bit — 90° Phase Shifts

FIGURE 3: Phase and Amplitude Modulation with 16-QAM

Phase° Amplitu

deQ

I

b0b1b2b316-QAM Q

I-3

-3

-1

-1

+1

+1

+3

+300 10

00 11

00 01

00 00

01 10

01 11

01 01

01 00

11 10

11 11

11 01

11 00

10 10

10 11

10 01

10 00

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When we look at the guts of modulation and coding, it becomes clearer why rate adaptation is necessary. Access points are responsible for choosing the best combination of modulation and coding at any point in time for each connected device. Again, it’s always a tradeoff of efficiency (higher data rates) and reliability (lower data rates).

The 802.11 specification introduces the term dynamic rate switching and acknowledges the fundamental issue: with mul-tiple data rates, there is a need to dynamically adjust based on RF conditions. But, they don’t lend any help. So if you look at ten different Wi-Fi companies, you’ll see ten different rate con-trol algorithms.

What’s so smart about Ruckus? Wi-Fi engineers have been led to believe, and—for better or worse—site survey software validates the belief, that data rates can be reliably predicted based on a metric like RSSI or SNR. And some product manufacturers use simple metrics like these to determine the right rate.

Ruckus approaches rate selection with a unique focus. Instead of using unreliable signal measurements to hope for the best data rate, we focus on the math. Our rate selection algorithms are statistically optimized, which is our engineer-chic way of saying that we pick the best data rate based on historical, sta-tistical models of performance for each client.

Without the right algorithm, the optimal rate for any client at any given moment in time is a crapshoot. And when you’re guessing, the safest guess is to err on the side of reliability, which sacrifices throughput and capacity and causes other unwanted problems.

Let’s look at an example. The normal thought process in Wi-Fi is that frame corruption and Layer-2 errors should lead us to downshift to a more reliable data rate. It’s a reasonable assumption. However, interference is bursty and transient by nature, so the best response is not necessarily to downshift.

How do we know whether a few errors are an anomaly or a predictor of normal conditions for this network? The reality is that we don’t. Every RF environment is different, so simple thresholds are a poor answer to rate shifts. Based purely on assumptions, the best reaction to errors and retries is unknown.

In fact, the best response might be to use higher data rates because they occupy the wireless channel for a shorter period of time and are less likely to be corrupted by momentary inter-ference. Commonly enough, a data rate downshift causes more errors, which causes another downshift. And suddenly, the safe and reactionary rate switch leads to a rate shift sinkhole–gob-bling capacity as it tanks to the bottom.

In other words, purely reactive algorithms are sub-optimal in their myopia. Math is the better way. Statistics tell us more about the implications of transient interference, short-term hic-cups, and longer-term trends. Accordingly, we can adjust—or perhaps more importantly, not adjust—the data rate to opti-mize both short-term and long-term performance and capacity. This is also why spectrum analysis doesn’t help much. Identify-ing an interference source does not readily tell us how exactly that source will impact our network. Even the best heuristics aren’t as accurate as statistics.

For delay- and jitter-sensitive applications, the best data rate is also the one that consistently delivers the frame to its des-tination in the shortest amount of time. Our statistical rate selection model ensures that too.

Another unique advantage with Ruckus is our test and valida-tion rigor. Because of our custom AP hardware and software, we test and test and test everything some more. One such monotonous test is for data rate performance. Believe it or not, we test every individual MCS rate at different ranges and conditions to ensure that our performance is a bulwark of reli-ability. And this is no small feat. 802.11n MCS options are far more complex than 802.11a/g.

Data Rate (Mbps) Modulation Coding Rate OFDM Subcarriers Coded Bits per subcarrier

Coded bits per OFDM symbol

Data bits per OFDM symbol

6 BPSK 1/2 48 1 48 24

9 BPSK 3/4 48 1 48 36

12 QPSK 1/2 48 2 96 48

18 QPSK 3/4 48 2 96 72

24 16-QAM 1/2 48 4 192 96

36 16-QAM 3/4 48 4 192 144

48 64-QAM 2/3 48 6 288 192

54 64-QAM 5/6 48 6 288 216

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Ruckus Wireless, Inc.

350 West Java Drive, Sunnyvale, CA 94089 USA (650) 265-4200 Ph \ (408) 738-2065 Fx

w w w. r u c k u s w i r e l e s s . c o m

802.11n HT Rates20MHz 40MHz

GI=800 GI=400 GI=800 GI=400

Spatial Streams

Modulation Coding MCS Mbps Mbps Mbps Mbps

1 BPSK ½ 0 6.5 7.2 13.5 15

1 QPSK ½ 1 13 14.4 27 30

1 QPSK ¾ 2 19.5 21.7 40.5 45

1 16-QAM ½ 3 26 28.9 54 60

1 16-QAM ¾ 4 39 43.3 81 90

1 64-QAM 2/3 5 52 57.8 108 120

1 64-QAM ¾ 6 58.5 65 121.5 135

1 64-QAM 5/6 7 65 72.2 135 150

2 BPSK ½ 8 13 14.4 27 30

2 QPSK ½ 9 26 28.8 54 60

2 QPSK ¾ 10 39 43.4 81 90

2 16-QAM ½ 11 52 57.8 108 120

2 16-QAM ¾ 12 78 86.6 162 180

2 64-QAM 2/3 13 104 115.6 216 240

2 64-QAM ¾ 14 117 130 243 270

2 64-QAM 5/6 15 130 144.4 270 300

3 BPSK ½ 16 19.5 21.7 40.5 45

3 QPSK ½ 17 39 43.3 81 90

3 QPSK ¾ 18 58.5 65 121.5 135

3 16-QAM ½ 19 78 86.7 162 180

3 16-QAM ¾ 20 117 130 243 270

3 64-QAM 2/3 21 156 173.3 324 360

3 64-QAM ¾ 22 175.5 195 364 405

3 64-QAM 5/6 23 195 216.7 405 450

802.11a/g Rates 20MHz

Spatial Streams

Modulation Coding Closest MCS

Mbps

1 BPSK ½ 0 6

1 BPSK ¾ N/A 9

1 QPSK ½ 1 12

1 QPSK ¾ 2 18

1 16-QAM ½ 3 24

1 16-QAM ¾ 4 36

1 64-QAM 2/3 5 48

1 64-QAM ¾ 6 54

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Copyright © 2012, Ruckus Wireless, Inc. All rights reserved. Ruckus Wireless and Ruckus Wireless design are registered in the U.S. Patent and Trademark Office. Ruckus Wireless, the Ruckus Wireless logo, BeamFlex, ZoneFlex, MediaFlex, FlexMaster, ZoneDirector, SpeedFlex, SmartCast, and Dynamic PSK are trademarks of Ruckus Wireless, Inc. in the United States and other countries. All other trademarks mentioned in this document or website are the property of their respective owners. 803-71285-001 rev 01

At Ruckus, we believe in the importance of stable client connections in an unstable RF environment. In fact, our algorithms jointly adapt both the data rate and antenna pattern together to maximize reliability and throughput. But don’t take our word for it; test it for yourselves.

Economy cars are everywhere. But if you want a racecar—at economy car price—Ruckus is it. We obsess. We nitpick. We care about details. And your Wi-Fi applications will thank us.