Following The WLAN Alphabet To Lower Power
The quest for low power in electronic devices is one that shows no sign of abating any time soon. Pressure for it comes from many different sources, such as the continual drive to pack more functionality into ever smaller, mobile electronic devices.
To try and maintain a decent battery life for today’s power-hungry “road-warriors,” engineers have to reduce power consumption. To achieve that, they must spend more time and care in choosing the components, materials and techniques they will employ to drive down power consumption. They also must look closely at the wireless communications protocols they choose to employ.
One protocol that has received a great deal of attention of late is IEEE 802.11n—one in a long line of standards to emerge from the alphabet soup that is the Wireless Local Area Network (WLAN). Its improved performance and enhanced reliability over previous WLAN standards is thought to be the critical component in finally enabling WLANs to function as predictably as their wired counterparts. It’s no surprise than that a recent study from ABI Research predicts that by 2012 shipments of 802.11n technology will account for a full 60 percent of the market. Today, virtually 84 percent of the WLAN market stems from 802.11 a/g technology accounts.
Unfortunately, as is typical with any new technology, 802.11n faces a slew of challenges. In the Wi-Fi enabled, battery-sensitive market, for example, a key challenge to 802.11n’s widespread proliferation is power consumption. Meeting this challenge demands an ultra low-power technology, but can 802.11n deliver the low power necessary to take the battery-sensitive mobile by storm? Let’s take a closer look.
802.11a, b, g…n?
Essentially, IEEE 802.11n is designed to enable Wi-Fi networks to do more, faster and over a larger area. Well suited for both enterprise and home networks, it has the potential to deliver up to twice the range and five times the throughput of traditional WLANs (e.g., 802.11a, b or g). To date, draft 2.0 of the 802.11n standard has been approved and forms the basis for Wi-Fi CERTIFIED 802.11n draft 2.0 products. Certification of such products began in June 2007 and is done by the Wi-Fi Alliance (www.wi-fi.org). The final 802.11n standard is expected in September 2009.
802.11n technology builds on previous 802.11 standards by adding 40MHz operation to the physical (PHY) layer (enabled in either the 5- or 2.4-GHz mode), and frame aggregation to the MAC layer. It is based on Multiple-Input-Multiple-Output (MIMO) technology, which employs multiple receiver and transmitter antennas to transport two or more data streams simultaneously in the same frequency channel. This allows MIMO to coherently resolve more information than possible using a single antenna.
Is low-power 802.11n possible?
While the use of MIMO gives 802.11n significantly increased data rates, its multiple transmit-and-receive chains also increase power consumption—turning MIMO-based products into potential power hogs and dramatically impacting battery life. That fact alone has raised many concerns. A full-featured 802.11n access point (AP) will typically consume much more power than a legacy 802.11a, b or g AP, although a device’s actual power consumption will depend heavily on the implementation and vendor involved.
To address this power concern, IEEE 802.11n has extended the power management capability of the 802.11 MAC to include the following mechanisms:
Power Save Multi-Poll (PSMP). This mode is an extension of the Automatic Power Save Delivery (APSD) approach specified in the 802.11e standard for improved Quality of Service (QoS). With PSMP, the client schedules the frames that it transmits as the trigger for delivering downlink frames. This reduces the contention between clients and between the client and the AP, which in turn dramatically improves power conservation in the clients. As a dynamic method, PSMP immediately adjusts to changes in traffic demand by the clients using it. While this mode is often touted as a way for VoIP clients to save power, it is generally best used only in situations with relatively heavy traffic loads.
Spatial Multiplexing (SM) Power Save. In contrast to PSMP, SM Power Save allows an 802.11n client to power down all but one of its radios and can operate in either dynamic or static mode. In dynamic mode, all but one of the client’s radios is turned off. The client can quickly turn on radios, as needed, when it receives a frame. After the frame reception is complete, the client can return to a low-power state by again disabling all but one radio.
In static SM Power Save mode, the client behaves as an 802.11 a or g client by turning off all but a single radio. The client’s AP is notified that the client is operating in the static single-radio mode and that it must send only a single spatial stream to the client until otherwise notified.
802.11n also specifies an optional power save mode—Dynamic MIMO Power Save. This mechanism essentially allows 802.11n devices to dynamically change the number of transmit-and-receive chains that are active when traffic loads are light, such as by downshifting from 3×3 to 1×1 MIMO.
The wave of 802.11n products
By employing the mechanisms previously specified, low-power operation of 802.11n is possible. Of course, it doesn’t end there. Today, 802.11n Draft 2 chip developers like Atheros, Broadcom, RedPine Signals, and Qualcomm, just to name a few, are employing their own advanced techniques and process technology to minimize power consumption. Their continued pursuit to develop low-power 802.11n chips is bolstered by announcements like Apple’s use of 802.11n in its next-generation iPhone and iPod Touch models.
The Apple devices are said to use the Broadcom BCM4329 wireless chip, a complete IEEE 802.11 a/b/g/n system (MAC/baseband/radio) with Bluetooth 2.1 + Enhanced Data Rate (EDR), and FM radio receiver and transmitter (Figure 1). The chip not only adds support for 802.11n features, including the ability to find and join 5-GHz networks, but also incorporates new power savings, such as advanced design techniques and process technologies to reduce active and idle power consumption and extend battery life.
Figure 1. Broadcom’s BCM4329 wireless chip supports a variety of 802.11n optional features such as SpaceTime Block Coding (STBC), Short Gual Interval (SGI), A-MPDU aggregation, Block Ack, Greenfield, and RIFS. During WLAN operation, it achieves low active transmit and receive power consumption and ultra-low power in standby and idle modes.
Perhaps one of the most significant low-power 802.11n offerings to come to market recently hails from Qualcomm (www.qualcomm.com). Its new WCN1320 N-Stream WLAN chip is the industry’s first dual-band 802.11n standards-based WLAN solution with 4×4 MIMO technology (Figure 2). Based on 65-nanometer CMOS process technology, the chip combines an embedded applications processor, media-access controller, digital baseband, radio-frequency transceiver, and system power-management in a single compact 12×12 mm package. With its 4×4 MIMO technology, it uses four spatial streams to distribute multiple streams of concurrent voice, video and data in either the 5- or 2.4-GHz radio bands. The WCN1312 chip incorporates advanced power-management techniques to minimize sleep, standby, and active power consumption.
Figure 2. With performance of 600 Mbps, Qualcomm’s WCN1320 chip enables the distribution of multiple simultaneous streams of high-definition video, voice and data throughout the home. Sophisticated algorithms take advantage of the chip’s multiple transmitters and receivers to increase data throughput, extend range and overcome interference with a spectrally-efficient solution.
IEEE 802.11n is a technology whose time has now come. Expected to be fully approved later this year, it will open the door to a wealth of high-performance mobile applications like HD video, high-resolution imaging and voice over wireless LAN (VoWLAN). Realizing this goal will require special attention to reducing power consumption. Many of the current 802.11n Draft 2 chips achieve this goal through use of advanced design techniques and process technologies, but they also take advantage of the standard-specified power saving modes. Future chips based on the final 802.11n standard will need to follow suit. Doing so will help ensure the success of 802.11n, while also driving continued growth of the wireless connectivity market.
Cheryl Ajluni is a contributing editor and freelance technical writer. She is the former Editor in Chief of Wireless Systems Design (WSD) Magazine