ATHEROS COMMUNICATIONS, INC. Maximizing MIMO Effectiveness by Multiplying WLAN Radios x3 By Winston Sun, Ph.D. Member of Technical Staff May 2006
Introduction The recent approval of the draft 802.11n specification has opened the way for fully leveraging multiple input/multiple output (MIMO) technologies in wireless LANs (WLANs). Surprisingly, few industry observers anticipated that MIMO options would immediately expand beyond the conventional 2x2 (two transmit by two receive) MIMO architecture. But in fact, single-chip integration makes it cost-effective to expand the number of complete transmit/receive chains to 3x3 for dramatic performance improvements-far beyond the traditional practice of simply increasing the number of antennas per radio. The 11n draft specification specifies an optional mechanism for intelligent antenna selection as well as maximal ratio combining (MRC) from fixed receive chains. Although antenna selection can improve performance, using the MRC algorithm with the same antenna configuration out-performs the simpler switched antenna scheme. In a typical access point (AP), where antennas can be spaced 4 to 6 cm apart, you can expect 2 to 3 db improvement using antenna selection. However, using fixed antennas in conjunction with maximal ratio combining (MRC) guarantees the same improvement without having to switch antennas. Furthermore, using a third antenna with MRC gives as much as 1.7 db additional improvement. Thus, more antennas are useful, but the big gains possible with 802.11n come from increasing the number of radios and using MRC. As the tests described in this article show, WLAN users with a 3x3 MIMO architecture can expect about a 50 percent increase in throughput at any given point across the average home compared to 2x2 MIMO. This comparison is for 2x2 and 3x3 systems that use the same number of unique spatially multiplexed data streams (more on spatial multiplexing later). The additional radios in the 3x3 architecture do not carry more unique data streams, but the radios do improve WLAN throughput, range and robustness. The key goal is to give home users trouble-free, highthroughput service under virtually any circumstances. With this capability, WLAN products will be much more readily acceptable in the consumer market, especially as applications and media content demand greater bandwidth. Theoretically, there are no limits to the number of radios that a MIMO system could use. The question is: How many radios are cost-effective in terms of silicon price and power consumption? This article answers that question with test data for various MIMO architectures and an analysis of the trade-offs. 2 3x3 MIMO Brings Triple-Radio Performance to Wireless LANs
MIMO Uses the Space Available The 802.11n draft specifies a number of optional techniques for improving performance, but the primary MIMO technology is spatial multiplexing, a technique for delivering multiple data streams via different spatial channels. The theoretical work behind MIMO began in 1984, and spatial multiplexing was among the earliest MIMO concepts. At that time, multiple transmitters and receivers were relatively expensive, MIMO was not widely implemented. Today, the availability of highly integrated radios-on-a-chip make MIMO highly practical, even to the point of integrating three complete radios on a single chip. The basics of spatial multiplexing are easy to understand by looking at two transmitters spaced far apart, each transmitting to two receivers spaced equally far apart. If you arrange the transmitters and receivers across enough area and use directional antennas, the two sets of transmitters/receivers do not interfere with each other, so they can send two unique data streams at the same frequency. The two data streams are multiplexed across the available space. Each receiver detects a unique data stream that travels via a unique physical path, resulting in a doubling of throughput. In a realistic indoor environment, radio signals reflect from many objects, causing the signals to travel by multiple paths. You can no longer expect multiple unique data streams to travel straight to multiple receivers, even with highly directional antennas. Signal processing techniques can detect and differentiate the multiple data streams, however, so long as the streams travel via unique paths. In an office or home environment, the multiple unique data streams do not travel straight to multiple receivers, but the streams do take unique multi-path routes to the receivers, where signal processing can sort out the streams. This capability turns multi-path effects from an annoyance to a useful feature of the environment that WLANs can exploit for higher performance. The more unique paths an environment provides, the more potential there is to increase throughput. By integrating multiple radios in a single chip, WLAN devices exploit the multiple spatial channels in a cost-effective way. Home and office environments easily support two unique spatially multiplexed streams. How many radios will provide the best performance for those two streams? The system does not have to have a one-to-one correspondence between transmitters, receivers and unique data streams. As shown by the tests described in the following section, increasing the number of transmitters and receivers has a significant effect on performance. MIMOx3 The first WLAN aspect to consider is the downlink direction from AP to client. The following analysis compares the performance of 2x2 and 3x3 access points. Both setups use two unique data streams. Because 3x3 devices are only now becoming available, this analysis uses simulations for the comparison. These simulations are based on straightforward radio models and include the realistic operating characteristics given in the International Telecommunication Union's statistical description of typical home operating conditions (the ITU1238 model): 11g band (2.4 GHz), 20-MHz mode, 50-ns rms delay spread, 3.1 db path loss, 15 dbm power per transmit chain, 3 db antenna gain, 5 db system receiver noise, 12 db shadowing margin from multi-path, and 4 db signal attenuation through a floor. The simulations show that the 3x3 configuration outperforms the 2x2 configuration in the downlink direction (AP to client) by more than 40 percent in the 20-100 foot range (see Figure 1). This result has been verified by initial real-environment tests of a recently announced Atheros XSPAN 3x3 device, featuring Signal-Sustain Technology. The tests with initial silicon show an average 44 percent performance advantage over the 2x2 configuration at 50 feet, 51 percent at 100 feet, and 62 percent 150 feet. On average, the 3x3 system provides about a 50 percent advantage. 2006 Atheros Communications, Inc. 3
Figure 1. Comparison of Downlink (AP to Client) Throughput for 2x2 and 3x3 WLAN Systems, Both Using Two Data Streams (Simulation Results) Of course, these same advantages are available in the uplink direction (client to AP) if client devices use the 3x3 configuration. The AP typically has the ability to transmit at a higher power than client devices, however, because clients are often battery-powered devices. The AP also tends to have greater flexibility for accommodating multiple antennas. Thus, a full 3x3 configuration may not be appropriate for clients. What about a 2x3 system (transmitters x receivers) for clients? To find out, the simulations can be changed to compare the 2x3 system with the 2x2 configuration using the same ITU1238 model. Again, both systems use two spatially multiplexed streams. Figure 2 shows the simulation results. The 2x3 system improves average uplink performance by about 20 percent compared to the 2x2 system, so the additional downlink transmitter significantly improves performance coverage. Figure 2. Uplink (Client to AP) Throughput for 2x3 and 2x2 Systems Using Two Data Streams (Simulation Results) 4 3x3 MIMO Brings Triple-Radio Performance to Wireless LANs
The hardware for testing a 2x3 configuration is more readily available, so chamber testing can confirm the comparison of the 2x3, 2-stream system and 2x2, 2-stream system. As Figure 3 shows, the chamber tests reveal the same average 2x3-system improvement as the simulations. Tests in a real-world environment further confirm the advantage for the 2x3 system (see Figure 4). Figure 3. Chamber Measurements Confirming the Simulation Results in Figure 2 Figure 4. Real-World-Environment Measurements Confirming the Results in Figure 2 2006 Atheros Communications, Inc. 5
Realistic Data Rates Using traditional 20-MHz channels, a single-band 3x3 device can achieve 130-Mbps raw data rate for ~65-Mbps real end-user throughput. This performance surpasses any existing standard or proprietary WLAN solutions. Further, the draft 802.11n includes the option of 40-MHz-channel mode, which enables WLAN systems to get 300-Mbps raw data rate for ~150-Mbps TCP/IP throughput (see Figure 5). Figure 5. Data Rates for a System Using 40-MHz Channels (Simulation Results) The Best Trade-Off Are additional MIMO transmitters and receivers cost-effective? A cost/benefit comparison shows that a 2-stream MIMO system's performance and cost track almost linearly up to a 3x3 configuration (see Figure 6). The cost grows considerably beyond that point, however, while performance improves only a little. Figure 6. Relative Cost/Performance of MIMO Systems 6 3x3 MIMO Brings Triple-Radio Performance to Wireless LANs
Performance does not increase linearly because the performance improvement contributed by each additional transmitter or receiver tends to diminish past a certain number. One reason is that spatial multiplexing only works when the signal streams are statistically uncorrelated and only so many uncorrelated signal paths are available to a real-world device. The closer the spacing between antennas, the more correlated the signals get. At the 2.4-GHz carrier frequency, for example, the wavelength of the radio signals is ~12 cm. Typical APs are 16 to 20 cm wide. If the separation between each receive antenna is less than 6 cm (half a wavelength), the signals at those antennas will likely have a large degree of correlation, which limits the benefits of spatial multiplexing. As for the cost, additional transmitters or receivers have both a dollar and a power cost, so the cost in terms of both dollars and power dissipation correlate closely. Each transmit chain requires an antenna, power amplifier (PA), RF mixer, RF filter, IF mixer, IF filter, baseband filter, DAC, and some incremental digital circuitry. Each receive chain requires an antenna, low-noise amplifier (LNA), RF filter, RF mixer, IF filter, IF mixer, baseband filter, ADC, and digital circuitry to process the spatially multiplexed signals. Due to the cost of increasing the amount of on-chip circuitry involved, the total cost (dollar amount and power) is roughly a linear function of the number of transmit chains, the number of receive chains, and the number of data streams. Because the computational complexity of extracting data streams increases according to the square of the number of streams, this factor also affects cost. In formal terms, cost is proportional to f c (N, M, S, S2), where f c is a linear combination function. This overview of the trade-offs shows that simply adding more antennas and receivers does not improve the performance linearly, but rather saturates up to the number of uncorrelated signals collected. Therefore, performance is proportional to f p (S, I x [1-e-(N+M)]), where S = number of streams, I = number of uncorrelated signals measured by the receive chains, N = number of Tx chains, and M = number of Rx chains. As Figure 6 shows, a 3x3, 2-stream system represents an ideal configuration for the typical WLAN. Integrating the additional transmit and receive chains on a single chip keeps the incremental cost low, yet considerably enhances performance. In fact, a cost analysis model estimates that the bill of materials for a 3x3 AP can be about the same as a conventional 2x2 device for reasons involving the WLAN chip's level of integration and other factors. For 2-stream spatially multiplexed MIMO systems, both the 2x3 and 3x3 architectures offer significant performance benefits over the conventional 2x2 configuration for little additional cost. Thus, for access points with larger power budgets, 3x3 is the optimal configuration, while client solutions benefit the most with a 2x3 configuration. 2006 Atheros Communications, Inc. 7
COMPANY CONFIDENTIAL - Subject to Change without Notice The information in this document has been carefully reviewed and is believed to be accurate. Nonetheless, this document is subject to change without notice. Atheros assumes no responsibility for any inaccuracies that may be contained in this document, and makes no commitment to update or to keep current the contained information, or to notify a person or organization of any updates. Atheros reserves the right to make changes, at any time, to improve reliability, function or design and to attempt to supply the best product possible. Atheros Communications, Incorporated 5480 Great America Parkway Santa Clara, CA 95054 t: 408/773-5200 f: 408/773-9940 www.atheros.com