Whitepaper n The Next Generation in Wireless Technology

Transcription

1 Whitepaper n The Next Generation in Wireless Technology

2 Introduction Wireless technology continues to evolve and add value with its inherent characteristics. First came , then a & b, followed by g, and now n. It has taken more than seven years to arrive to the final ratified proposal bringing with it several enhancements. This document describes the new capabilities and summarizes the benefits from the improvements made by n n Technology For the past seven years, the IEEE standards body has been working on a new standard that would standardize an upgrade to the radio while providing a new set of capabilities, dramatically improving reliability of communications, the predictability of wireless coverage, and increasing the overall throughput of devices, all the while keeping backwards compatibility with legacy environments. The initial n work has largely been based on the n Wi-Fi Alliance draft version 2.0 which includes the following key device requirements which the final version now includes: MIMO Describes the use of multiple-input multiple-output (MIMO) technology Enhancements Increased channel size, higher modulation rates, and lower overhead MAC Enhancements Modifies the frame formats used by n devices from those of existing devices MIMO Multiple-input multiple-output (MIMO) is the heart of n. This technical discussion of MIMO provides a basis for understanding how n can reach data rates of 600 Mbps. The nature of wireless communications is vulnerable to all sorts of interference, and distortions or noise. Similar to wired technology, signal to noise ratio (SNR) efficiency is crucial to the ability to provide efficient data transmission. The higher the SNR number the more information that can be carried on the signal and be recovered by the receiver n employs two interesting techniques to improve SNR and multipath environments: Beamforming and multipath or spatial diversity. The following sections describe their functionalities and benefits. Transmit Beamforming Beamforming is a technique used when there is more than one transmit antenna and a single receiver in an open or limited obstruction location. When there are more than one transmit antenna, each radio signal being transmitted will have a different angular phase. These differences affect the overall signal to noise ratio. By adjusting these phases so that they match at the receiver, the signal to noise ratio is dramatically increased, and thus adding to the total amount of information that can be carried by the signals and recovered by the receiver. By now, we can see that this particular methodology depends closely on a feedback mechanism between the transmitter and receiver. Information from the received signal is sent back to the transmitter allowing the transmitter to adjust its radio signals. There are several caveats that should be mentioned when implementing beamforming. 1. This particular technique is only available with n transmitters and receivers. 2. It can only be implemented when transmitting to a single receiver. 3. The feedback mechanism between receiver and transmitter is not immediate and short. If either transmitter or receiver moves, the relationship would have to be re-established. Multipath or Spatial Diversity MIMO technology makes use of multiple radio signals where each signal has its own spatial stream sent from its own antenna. In a typical wireless environment, any signal transmitted will without a doubt encounter some type of interference or reflection. As the number of transmitters increase, the number of signals being transmitted increase. As a result, signals are being received from different paths and at different times. This condition is referred as multipath. Since each antenna functions independent from the other, different data streams will source from each. At the receiving end, each data stream from each radio is combined, and after some complex processing, a cleaner or stronger signal is arrived resulting in a higher signal to noise ratio. 2

3 Enhancements Besides the introduction of a more efficient use of antennas, n has made additional changes to the radio to increase the effective throughput of the WLAN. The most important of these changes are increased channel size, higher modulation rates, and reduced overhead. The following section will describe each of these changes and the effect they have on WLAN throughput. 20- and 40-MHz Channels A brief background on spectral efficiency and channel bonding is needed in order to understand how the enhancements on radio frequency contribute to the overall increased performance. In the original standard as well as with b, their spectral efficiency or channel bandwidth is one-half the bits per hertz, with a and g, the spectral efficiency is 2.7 bits per 54Mbps. The higher the spectral efficiency the more efficient the usage of a limited frequency space can be attained. In addition to spectral efficiency, there are some proprietary WLAN systems that use a clever technique that bonds each g channel (54Mbps) into two channels known as Super G or channel bonding providing up to 108Mbps. With channel bonding, the spectral efficiency is the same as a and g, but the channel bandwidth is twice as great. This provides a simple way of doubling the data rate. One of the key enhancements found in the new n standard is its efficient usage of both 20-MHz and 40-MHz channels. Similar to other proprietary products, the 40-MHz channels in n consists of two 20-MHz channels, bonded together. When using the 40-MHz bonded channel, n takes advantage of the fact that each 20-MHz channel has a small amount of the channel that is reserved at the top and bottom, to reduce interference in those adjacent channels. When using 40-MHz channels, the top of the lower channel and the bottom of the upper channel don't have to be reserved to avoid interference. These small parts of the channel can now be used to carry information. By using the two 20-MHz channels more efficiently in this way, n achieves slightly more than doubling the data rate when moving from 20-MHz to 40-MHz channels (see figure 1). Figure MHz and 40 MHz Channels Increased Modulation Rates n uses a well known modulation technique known as orthogonal frequency division multiplexing (OFDM) which divides a radio channel into a large number of smaller channels, each with its own subcarrier signal (see Figure 1 above). Each of these carrier signals conveys information independent of all the other carrier signals. Think of a group of independent radio frequencies bunched together n increases the number of subcarriers in each 20-MHz channel from 48 to 52. This marginally increases the data rate to a maximum of 65 Mbps, for a single-transmit radio n provides a selection of eight data rates for a transmitter to use and also increases the number of transmitters allowable to four. For two transmitters, the maximum data rate is 130 Mbps. Three transmitters provide a maximum data rate of 195 Mbps. The maximum four transmitters can deliver 260 Mbps. In total, n provides up to 32 data rates for use in a 20-MHz channel. When using 40-MHz channels, n increases the number of subcarriers available to 108. This provides a maximum data rate of 135 Mbps, 270 Mbps, 405 Mbps, and 540 Mbps for one through four transmitters, respectively. Similarly, there are eight data rates provided for each transmitter, 32 in total, for the 40-MHz channel. 3

4 Lowered Overhead: Guard Interval OFDM incorporates a safety mechanism that prevents any intersymbol interference found in multipath environments. The intersymbol interference condition takes place when the beginning of a new symbol arrives at the receiver before the end of the last symbol is done (see figure 2). Figure 2. Guard Interval n uses 800 nanoseconds as the guard interval allowing for multipath difference of 800 feet. However, this guard interval can be reduced provided the multipath environment is not too rigid about the 800 feet difference between the receiver and trasnmitter n can configure the guard interval to 400 nanoseconds or 4 microseconds. This small change provides an increase in data rates. For 20-MHz channels, maximum data rates for one to four transmitters with the reduced guard interval are 72, 144, 216, and 288 Mbps, and 150, 300, 450, and 600 Mbps for a 40-MHz channel. MAC Enhancements In the previous sections we can see how improvements in radio data rates have resulted in increased performance. However, these improvements can only do so much. Every packet or frame transmitted has a certain amount of overhead. To be more precise, MAC layer protocol overhead. In addition to this overhead, interframe spaces and acknowledgements contribute in the reduction of the effective maximum throughput n has introduced several changes in the way MAC layer protocol overhead is handled. To reduce this overhead, n introduces frame aggregation. Frame aggregation is essentially putting two or more frames together into a single transmission n introduces two methods for frame aggregation: Mac Service Data Units (MSDU) aggregation and Message Protocol Data Unit (MPDU) aggregation. Both aggregation methods reduce the overhead to only a single radio preamble for each frame transmission (see Figure 3). Preamble MAC Data FCS Figure 3. Aggregation MAC Protocol Data Unit Aggregation MPDU aggregation works differently than MSDU aggregation. Rather than collecting Ethernet frames, MPDU aggregation translates each Ethernet frame to format and then collects the frames for a common destination. The collection doesn't require wrapping of another frame, since the collected frames already begin with an MAC header (see Figure 4). RP = Preamble, RH = Rapid, MH = MAC, MSDU = Ethernet Frame RP RH MH MH 1 Data 1 MH 2 Data 2 MH 3 Data 3 MH N Data N FCS Figure 4. MPDU Aggregation 4

5 MPDU is less efficient because of the extra overhead that is part of the aggregated frame. Figure 4 shows the frame format with a header followed by the data payload. In addition, this efficiency is further reduced when encryption is included. Another interesting feature that is part of MPDU aggregation is the use of block acknowledgement. With block acknowledgement, a single acknowledgement frame is produced by the recipient to the sender as a result of any frames that are not acknowledged. In environments where high number of errors are inevitable, reducing the number of acknowledgment frames is critical. MAC Service Data Unit Aggregation MSDU aggregation is the more efficient of the two types of aggregation. MSDU works by aggregating Ethernet frames with a common destination wraps them into a single frame and then transmits that wrapped collection of Ethernet frames (see Figure 5). MSDU= Ethernet Frame Preamble MAC MSDU 1 MSDU 2 MSDU 3 MSDU N FCS Figure 5. MSDU Aggregation Unlike MPDU where each frame had a header as part of the aggregated frame, MSDU has only one header consisting of the radio preamble, radio header, and MAC header. Furthermore, the aggregated frame is encrypted only once, whereas, with MPDU, each individual frame is encrypted. Similar to MPDU, there are some restrictions in that all of the constituent frames must be of the same quality-of -service (QoS) level. It is not permitted to mix voice frames with best-effort frames. Summary With the ratification of n, wireless technology continues to mature and deliver on key features that translate into increased benefits for today s and tomorrow s businesses. The enhancements brought up by n deliver the following key benefits: Increased area coverage The introduction of MIMO technology and its multipath effect reduces dramatically the chances of dead spots. The same locations that suffered from multipath effects now make use of this condition to enhance area coverage. Increased data transfer reliability Another advantage with MIMO technology is its SNR efficiency increase. This increase translates into greater data transmission and more robust to interference laden environments. Increased throughput The efficiencies made in the 20-Mhz and 40-MHz channel spectrum, channel bonding, and guard interval timing reduction have contributed to an overall increase in throughput. With the benefits that come with n, there is no reason why businesses considering deploying wireless technology should not be looking into n. This new standard has the potential to increase the WLAN capacity and throughput of every client while keeping cost low, turn-up-time short, return on investment fast, and seamless transition 5

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