| Quadrupling Wi-Fi speeds with 802.11n |
by James M. Wilson (Aug. 9, 2004)
Foreword
The current 802.11a/b/g WLAN standards offer the convenience of wireless connections with adequate performance for most of today's wireless networking applications, However, as next-generation wireless applications emerge, higher WLAN data throughput will be required. In response to this anticipated need, both the IEEE TGn and the Wi-Fi Alliance have set goals for the next generation of WLAN performance.
This whitepaper by a technical marketing engineer in Intel's Communications Technology Lab introduces Intel's vision for an IEEE 802.11n implementation that exceeds the IEEE TGn's expectations, which are roughly quadruple that of 802.11a and g. In addition, 802.11n will support all major platforms, including consumer electronics, personal computing, and handheld platforms, and will be usable throughout all major environments, including enterprise, home, and public hotspots.
The Next Generation of Wireless LAN Emerges with 802.11n
By James M. Wilson
Overview: Higher Performance WLAN
In response to growing market demand for higher-performance wireless local area networks (WLANs), the Institute of Electrical and Electronics Engineers - Standards Association (IEEE-SA) approved the creation of the IEEE 802.11 Task Group N (802.11 TGn) during the second half of 2003. The scope of TGn's objective is to define modifications to the Physical Layer and Medium Access Control Layer (PHY/MAC) that deliver a minimum of 100 megabit-per-second (Mbps) throughput at the MAC SAP (top of the MAC; see Table 1, below).
This minimum throughput requirement represents an approximate 4x leap in WLAN throughput performance compared to today's 802.11a/g networks. TGn's purpose for this next step in WLAN performance is to improve the user experience with existing WLAN applications while enabling new applications and market segments. At the same time, TGn expects a smooth adoption transition by requiring backward compatibility with existing IEEE WLAN legacy solutions (802.11a/b/g).
Wireless LAN Throughput by IEEE Standard
| IEEE WLAN Standard | Over-the-Air (OTA) Estimates | Media Access Control Layer, Service Access Point (MAC SAP) Estimates | | 802.11b | 11 Mbps | 5 Mbps | | 802.11g | 54 Mbps | 25 Mbps (when .11b is not present) | | 802.11a | 54 Mbps | 25 Mbps | | 802.11n | 200+ Mbps | 100 Mbps |
Table 1. Comparison of different 802.11 transfer rates. (Source: Intel Labs)
The Wi-Fi Alliance has also shown interest in TGn's work toward 802.11n. Industry representatives have come together under the Wi-Fi Alliance - High Throughput Marketing Task Group to define and publish a Marketing Requirements Document (MRD). The Wi-Fi Alliance MRD specifies performance expectations that will enhance the end-user experience in regard to increased throughput, increased range, more robustness to interference and a more reliable user experience throughout the Basic Service Set (BSS).
Intel is contributing to the success of 802.11n in many ways. First, Intel chaired the TGn committee responsible for developing the core documents that will be used to guide TGn in the development of the 802.11n standard, and has submitted contributions to these foundational documents, including channel models, usage models, functional requirements and comparison criteria.
Intel has also been responsible for technical submissions to TGn on MAC and PHY technologies, performance measurement methodologies, and simulation methodologies. Intel helped coauthor the Wi-Fi Alliance MRD for High Throughput WLANs, and continues to provide industry leadership by conducting ongoing discussions with WLAN industry leaders. Through all of these efforts, Intel and other industry leaders will jointly develop and submit a complete IEEE TGn proposal for the IEEE 802.11n standard.
Achieving Next-Generation WLAN Performance
Intel believes that simply demonstrating 100 Mbps under some conditions will not be enough to ensure a robust user experience with emerging applications. Intel's vision for the IEEE 802.11n standard will achieve and even exceed the IEEE TGn target goal of 100 Mbps at the MAC SAP. Intel expects the 802.11n WLAN technology will support consumer electronics (CE), personal computing, and handheld communications platforms throughout all major enterprise, home and public hotspot environments.
The broad scope of this vision advocates practical and cost-effective implementations that will scale robustly from low-end devices to high-throughput applications using technical approaches that may be developed and implemented within the time frames outlined in IEEE TGn.
Intel believes 802.11n should employ an evolutionary philosophy reusing existing technologies where practical, while introducing new technologies where they provide effective performance improvements to meet the needs of evolving applications. Reuse of legacy technologies such as Orthogonal Frequency Division Multiplexing (OFDM), forward error correction (FEC) coding, interleaving and quadrature amplitude modulation (QAM) mapping should be maintained to keep costs down and ease backward compatibility.
PHY Protocol Data Unit (PPDU) packets should be decodable without prior knowledge of transmission method. Legacy devices must be able to partly decode and avoid transmitting over new high-throughput packets, even if these packets are not fully intelligible by legacy devices. At the same time, seamless legacy (802.11a/g) interoperability (legacy devices operating in an 802.11n high-throughput network) must be supported without unreasonable performance penalties to high-throughput operation.
There are three key areas that need to be considered when addressing increases in wireless LAN performance. First, improvements in radio technology will be needed to increase the physical transfer rate. Second, new mechanisms implementing the effective management of enhanced PHY performance modes must be developed. Third, improvements in data transfer efficiency are needed to reduce the performance impacts of PHY headers and radio turnaround delays that would otherwise reduce the improvements achieved with increases in physical transfer rate.
At the same time, while developing new approaches to achieve performance, coexistence with existing 802.11a/b/g legacy devices is required. All of these areas must be addressed when considering practical and effective implementations for cost-sensitive market segments.
Increasing the Physical Transfer Rate
One approach to increasing the physical transfer rate of wireless systems employs multiple antenna systems for both the transmitter and the receiver. This technology is referred to as multiple-input multiple-output (MIMO), or smart antenna systems. MIMO exploits the use of multiple signals transmitted into the wireless medium and multiple signals received from the wireless medium to improve wireless performance.
MIMO can provide many benefits, all derived from the ability to process spatially different signals simultaneously. Two important benefits explored here are antenna diversity and spatial multiplexing. Using multiple antennas, MIMO technology offers the ability to coherently resolve information from multiple signal paths using spatially separated receive antennas. Multipath signals are the reflected signals arriving at the receiver some time after the original or line of sight (LOS) signal has been received. Multipath is typically perceived as interference degrading a receiver's ability to recover the intelligent information. MIMO enables the opportunity to spatially resolve multipath signals, providing diversity gain that contributes to a receiver's ability to recover the intelligent information.
Another valuable opportunity MIMO technology may provide is Spatial Division Multiplexing (SDM). SDM spatially multiplexes multiple independent data streams, transferred simultaneously within one spectral channel of bandwidth. MIMO SDM can significantly increase data throughput as the number of resolved spatial data streams is increased. Each spatial stream requires its own TX/RX antenna pair at each end of the transmission (Figure 1). It is important to understand that MIMO technology requires a separate radio frequency (RF) chain and analog-to-digital converter (ADC) for each MIMO antenna. This increasing complexity ultimately translates to higher implementation costs as higher-performance systems are required.
Intel expects MIMO technology to play an important role in achieving the IEEE TGn goals. MIMO technology should be used in IEEE 802.11n to evolve the existing OFDM physical interface presently implemented with legacy 802.11a/g. However, practical solutions will likely require additional technological approaches. Implementations requiring more than two RF antenna chains will need to be carefully architected to keep costs down while maintaining performance expectations.
Figure 1. Basic two-antenna MIMO system with two-stream SDM example.
Another important tool that can increase the PHY transfer rate is wider spectral bandwidth channels. Increasing channel bandwidth is not a new concept. It can easily be seen from Shannon's capacity equation [C = B log2 (1+SNR)] that theoretical capacity limits "C" are directly increased when considering increases in occupied bandwidth "B" (see Figure 2).
Figure 2. Increasing capacity limits. (Source: Intel Labs)
Using a wider channel bandwidth with OFDM offers significant advantages when maximizing performance. Wider bandwidth channels are cost effective and easily accomplished with moderate increases in digital signal processing (DSP). If properly implemented, 40-MHz channels can provide greater than two times the usable channel bandwidth of two 802.11 legacy channels. Coupling MIMO architecture with wider bandwidth channels offers the opportunity of creating very powerful yet cost-effective approaches for increasing the physical transfer rate.
MIMO approaches using only 20-MHz channels will require higher implementation costs to meet the TGn requirement of 100 Mbps at the MAC SAP. Meeting the IEEE TGn requirement with only 20-MHz channels would require at least three antenna analog front ends at both the transmitter and receiver. At the same time, a 20-MHz approach will struggle to provide a robust experience with applications that demand higher throughput in real user environments.
Figure 3 illustrates simulation results (using TGn channel model D) reflecting over-the-air (OTA) throughput at different SNR values where SNR is post detection after channel impairments have been taken into account. A MAC efficiency of 70 percent is assumed to illustrate the TGn 100-Mbps Top-of-MAC requirement (140-Mbps OTA).
These results compare the performance of 20-MHz and 40-MHz implementations. We illustrate each system configuration using the following convention. A two-antenna transmitter communicating with a two-antenna receiver over a 40-MHz channel is represented by 2x2-40 MHz where 2 data streams are transferred. Also presented in these results are: - 4x4-20 MHz transferring 4 data streams
- 2x3-20 MHz transferring 3 data streams
- 2x2-20 MHz transferring 2 data streams
The primary advantage provided by a 2x3-20 MHz implementation over the 2x2-20 MHz implementation is improved signal-to-noise ratio (SNR). This is noted with improved range for given throughput capability. This data shows that a MIMO two-stream implementation does not achieve 100-Mbps Top-of-MAC requirements. Achieving the 100-Mbps goal using only 20-MHz channels will require MIMO implementations supporting at least three data streams. It is easy to see the advantage of a 2x2-40 MHz implementation in these results. Notice that even doubling the number of RF chains using a 20-MHz implementation to transmit four data streams does not achieve the performance possible with only two RF chains using a 40-MHz channel transmitting two data streams. Using 40-MHz channels allows for reduced complexity, keeping costs down while delivering throughput for a robust user experience.
Figure 3. Over-the-air (OTA) throughput with different bandwidth channels. (Source: Intel Labs)
Intel believes both MIMO technology and wider bandwidth channels will be required to reliably satisfy the higher throughput demands expected from 802.11n. Choosing conservative increases in channel bandwidth, combined with conservative approaches in MIMO technology will enable cost-effective solutions that meet such requirements. A combined approach, employing both MIMO and 40-MHz channels, will enable the IEEE 802.11n technology to reach even higher performance as Moore's Law and CMOS process technology improvements advance DSP capabilities.
The Intel vision for the IEEE 802.11n standard will provide for a lowest-common capability to ensure high-throughput networks function efficiently. The standard should support both 20-MHz and 40-MHz channels where 40-MHz channels would be the widest channel, consisting of two adjacent legacy 20-MHz spectral channels and 20-MHz channels for use where spectrum availability is limited.
All 802.11n devices should support 40-MHz, where government or regulatory rules allow. Support of 40-MHz channels by all 802.11n devices is needed to prevent inefficiencies associated with channel width multiplexing between 20-MHZ and 40-MHz high-throughput devices. This would enable the highest possible performance within an 802.11n network. Environments limiting channel width to 20 MHz would be burdened by the added cost of complex MIMO implementations to achieve required performance. Intel expects that regulatory restrictions in those environments presently not allowing 40-MHz channels will align to support 40 MHz as 802.11n devices become ubiquitous.
The 802.11n standard should also require support for at least two MIMO spatial data streams using Spatial Division Multiplexing (SDM). Specifying support for at least two spatial data streams provides for architecture designs that can efficiently interoperate in high-throughput networks. Supporting at least two spatial data streams will require a minimum of two transmit antennas on all 802.11n implementations. Support for more than two transmit antennas, or two spatial streams, should be optional, with the maximum number limited to four for practical reasons.
Advanced features that may maximize throughput for those applications requiring the highest performance may be implemented optionally. Intel expects advanced features of this type to be specified in the 802.11n standard to ensure interoperability but to be optional for implementation only where they make sense. This would include features like more than two transmit antennas, channel adaptive beam-forming, and advanced FEC coding approaches (those features are not addressed in this article).
Managing PHY Performance Modes
When maximizing data throughput, intelligent mechanisms will be required to manage the selection of PHY Layer performance modes. Although the MAC Layer does not contribute directly toward increasing the physical transfer rate, it will play a key role in effectively optimizing selection of the PHY Layer performance modes.
Intel believes fast channel adaptation should be managed at the PHY Layer without MAC interaction. Once the initial adaptation is established, using over-the-air signaling in a timely fashion, the MAC layer will need to establish and maintain adaptation to wireless channel conditions. This will include managing the selection of modulation coding schemes, code rates, antenna configurations, channel bandwidths, and channel selection where optimization of TX/RX relationships can maximize throughput.
Improving Transfer Efficiency
A large contributor to overall throughput at the MAC SAP will be new MAC features that maximize throughput efficiency. It is important to understand that PHY header and radio turnaround delays significantly limit achievable throughput. These overheads are not reduced with the same ratio that the PHY payload rate is increasing. In fact, the PHY headers need to be longer to support the new advanced PHY Layer modes described earlier. Understanding that headers will need to grow in size, total connection overhead must be minimized.
An important approach to improving transfer efficiency is provided with new aggregate exchange sequences. An aggregate exchange is where multiple MAC Protocol Data Units (MPDUs) are aggregated into a single PHY Protocol Data Unit (PPDU). Aggregate exchange sequences are made possible with a protocol that acknowledges multiple MPDUs with a single block acknowledgement (Block ACK) in response to a block acknowledgement request (BAR). This protocol effectively eliminates the need to initiate a new transfer for every MPDU. If attempting to use the existing MAC protocols without aggregation, a PHY rate of 500 Mbps would be required to achieve the TGn throughput goal of 100 Mbps at the MAC SAP.
Additional opportunities exist with new MAC mechanisms to transfer data in both directions also without initiating a new transfer. This approach allows a responder to aggregate MPDUs in a reverse direction in response to an initiating station transfer. Mechanisms are also possible that minimize turnaround times between the initiator and the responder while ensuring contention protection within the BSS.
To more effectively transfer data and reduce connection overhead, Intel believes aggregated PPDUs containing multiple MPDUs from a single source to a single destination are needed. Maximizing efficiency for this kind of capability will require PPDUs longer in length than the current standard allows (4095 bytes).
Intel expects aggregate PPDUs will also be able to transfer data to multiple destinations using new MPDU formats. This would be valuable for applications such as Voice over Internet Protocol (VoIP). This approach could provide a high BSS capacity for many stations needing access, each having relatively low throughput per station requirements.
802.11 Legacy Coexistence
The IEEE TGn requires backward compatibility with 802.11a/b/g devices. Intel expects legacy 802.11b devices will coexist, and legacy 802.11a/g devices will interoperate with 802.11n devices when operating in the same band and channel. This means 802.11n will need to support 20-MHz channels for backward compatibility.
The MAC will be responsible for managing backward compatibility with existing legacy 802.11a/b/g devices. This will include coexistence for all legacy devices (802.11a/b/g) entering an 802.11n BSS. The MAC will also provide interoperability with supported modulation schemes (such as OFDM) in matching spectral environments (for example, 2.4-GHz ISM or 5.0-GHz U-NII as implemented). Coexistence mechanisms will need to manage channel bandwidth mismatches in mixed BSS environments and ensure that mixed mode operation is supported with low-overhead between 802.11n and legacy 802.11a or 802.11g.
Summary
Presently, 802.11a/b/g WLANs provide adequate performance for today's networking applications where the convenience of a wireless connection can provide the user value. As next-generation wireless applications emerge, higher WLAN data throughput will be required. In response to this need, both IEEE TGn and the Wi-Fi Alliance have set expectations for the next generation of WLAN performance.
The Intel vision for the IEEE 802.11n standard will achieve and exceed the IEEE TGn expectation of 100 Mbps at the MAC SAP (Top-of-MAC). The 802.11n technology will support all major platforms, including consumer electronics, personal computing and handheld platforms, throughout all major enterprise, home and public hotspot environments. The broad scope of this vision advocates practical implementations that will operate robustly using technical approaches that may be developed and implemented cost effectively within the time frames targeted by IEEE TGn.
Key considerations in architecting the next generation of WLAN are costs and robust performance. Intel believes both MIMO technology and wider bandwidth channels will be required to reliably satisfy the higher throughput demands of next-generation applications. At the same time, overall throughput at the MAC SAP will be enabled with new MAC features maximizing throughput efficiency.
More info
Learn more about Intel's work with wireless technologies: Read more on the development and industry status of 802.11:
 About the author: James M. Wilson is a technical marketing engineer in the Communications Technology Lab, part of the Corporate Technology Group. In his 21 years at Intel, he has worked on a variety of projects, including mixed signal component development, component test engineering, PC and server platform development and product introductions, and several wireless technologies including: Bluetooth, Home RF, 802.11 and ultra wideband. He is presently working on radio interconnect technologies for future applications. Wilson received his B.S.E.E.T. in electrical engineering from DeVry Institute of Technology.
Copyright © Intel Corporation 2003-2004. All rights reserved. Reproduced by DeviceForge.com with permission. This article was originally published in Intel's Technology@Intel Magazine.
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