New technologies in 802.11n
Published: 24 Jun 2008
The proposed 802.11n standard incorporates some impressive improvements to the physical hardware and firmware. In this guide, we first look at the main enhancements, then discuss MIMO and multipath environments in more detail, and finally examine why MIMO may require an added layer of intelligence to cope with real-world environments.
802.11n firmware enhancements
The physical data rate selection algorithm has the tough job of determining what data transfer rate should be used based on the measured signal strength. Whereas 802.11a/g uses 12 steps from 1Mbps to 54Mbps, 802.11n has a total of 88 incremental data rate steps, which provides a more granular drop-off when the signal strength weakens.
Whereas 802.11a/g uses transmit diversity, which is useful and logical as the device transmits from the antenna that displayed the best reception characteristics during the last receive cycle, 802.11n uses spatial multiplexing. This technique divides the information to be transmitted into independent and separately encoded data signals called streams. Each data stream is then transmitted from an independent antenna. The 802.11n standard allows up to four transmit streams. This is an interesting concept that increases data transmission capacity by multiplexing or reusing the space dimension multiple times.
To be able to transmit multiple data streams, designers had to rework OFDM (Orthogonal Frequency Division Multiplexing), which is the digital multi-carrier modulation scheme used by 802.11a/g. MIMO-OFDM, used by 802.11n devices, is the result. This development is extraordinary, as evidenced by the fact that it took almost ten years and a great deal of innovative work to perfect. MIMO-OFDM is viewed by many as the single most significant development of 802.11n.
802.11a/g networks do not work well in multipath environments: having multiple (and slightly different in phase or timing due to the environment) copies of the same transmitted RF signal arrive at the receiving antenna drives the receiver nuts, to put it politely. Since the real world is mostly a multipath environment, 802.11n was developed to make use of the slight differences exhibited by the arriving RF signals to distinguish the different data streams being transmitted. We discuss this subject in the next section of this guide.
Channel size is one determinant of how much data can be passed over a wireless link. The 802.11a/g standard uses 20MHz channels and history has proven that amount of bandwidth to be a limiting factor. In the past few years equipment developers have tried to improve the physical transfer rate of data by using proprietary technology combining adjacent channels to support greater data rates. The 802.11n standard describes how to use the much wider 40MHz channels, which are easy to implement, cost effective and only require moderate increases in digital signal processing. If properly implemented, 40MHz channels can provide greater than two times the usable channel bandwidth.
By design, TCP/IP traffic requires error-free transmission of data, and one of the controls used to regulate traffic processing is the ACK bit. The ACK bit is sent by the receiver to acknowledge receipt of each frame, which is a significant management overhead just for receipt verification. One way to improve throughput is to devise a way to acknowledge the receipt more efficiently — and that's exactly what 802.11n does with the Block-ACK. By removing the need for one acknowledgment frame for every data frame, the amount of overhead required for the ACK frames, as well as preamble and framing, is reduced.
No stone was unturned when the developers were looking at ways to improve throughput and efficiency: even the lowly guard interval was tweaked. The guard interval is used to prevent data loss from propagation anomalies as well as interference created if the following transmission starts too soon. Can that interval be reduced? It would help throughput, even if just slightly. 802.11n specifies two guard intervals 400ns and 800ns. Under optimal conditions, the 802.11n device will drop down to the 400ns guard interval to reduce what's considered unnecessary idle time.
Other improvements
For the most part, these enhancements are already designed into the pre-release hardware. Still, 802.11n equipment developers are not satisfied with just these improvements. Smart antennas, multiple radios and mesh technology are some heavy-duty technologies that are being added to enterprise 802.11n appliances, which will allow 802.11n kit to approach wired network parameters.
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