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At the heart of this new wireless system is the 802.11n radio, which is responsible for transmitting and receiving all of the information shared between devices. To make 802.11n work, this radio must overcome specific technical challenges, linking devices such as laptops, GameBoys, TVs, PDAs, and Apple iPods together quickly and efficiently. In addition, it must avoid interference with other nearby wireless technologies, such as cellular phones, Bluetooth headsets, and other users of the unlicenced Industrial, Scientific and Medical (ISM) radio band (2.400"2.500 GHz).
How WLANs Work
In all forms of wireless communication, a carrier wave is modulated with the information that the system wants to transmit from point A to point B. One radio in this system is called the access point (which controls multiple radios) and the other is called the station (STA) or terminal.
When a system needs to support multiple users, "multiple access" techniques must be used to divide the communications resources. There are several standard techniques commonly used for multiple access. The first technique uses time division multiple access (TDMA), where all users share the same frequency, but each user listens or talks at a different time. This technique is commonly used in 802.11g radios, and was the technology of choice for many second-generation digital cellular radios. A second technique is to share the frequency. Here, multiple users listen or talk at the same time, but use different frequencies. The third technique uses orthogonal codes to separate multiple users. Here, users use the same frequency at the same time, but special coding is used to separate the multiple users. This technique is the core of many third-generation cellular networks, and is known as code division multiple access (CDMA). Finally, many new standards use a hybrid combination of time and frequency division. For example, WiMAX, the new high speed, fourth-generation cellular standard currently being rolled out in many countries around the world, uses a technique known as orthogonal frequency division multiple access (OFDMA), where users are assigned unique slots in both time and frequency.
The Evolution of a WLAN Standard
With the rise of the wireless industry and the idea of wireless networks, standards were required to guarantee interoperability across platforms and portable devices. The IEEE originally formed a committee on local area networks (LANs) in 1980, and since then the group has created more than fifty IEEE 802 standards [1].
The standards that support WLAN connections are known as the IEEE 802.11 standards and a variety of versions have been released or are in development. The IEEE 802.11 working group was initially charged with developing specifications for wireless networks with a 50 to 150-ft. range. A good way to think of it is to imagine a wireless Ethernet, which allows computers and devices that are equipped with wireless networking cards to share files, print, and access the Internet. WLAN technology is now widely used in many applications, including PCs, handheld game systems, and game consoles.
The first 802.11 specification was released in 1997 for 2.4 to 2.5 GHz systems. Its theoretical maximum data rate was 2 Mb/s. However, in order to make wireless data networks more robust, overhead (such as guard bands, training sequences, and data redundancy) exists, which reduces this rate, so that a typical user experienced a throughout of approximately 0.7 Mb/s. In 1999, 802.11a was released to address frequencies in the 5-GHz band, offering 20 Mb/s typical throughput and a maximum data rate of 54 Mb/s, using OFDM. The operating frequencies of 802.11a in the US fall into the universal national information infrastructure (U-NII) bands: 5.15-5.25GHz, 5.25-5.35GHz, 5.47-5.725 GHz, and 5.725-5.825GHz. Similar bands also exist in Europe and in Japan. The same year, 802.11b again addressed the 2.4 to 2.5 GHz range, boosting typical throughput to 4.5 Mb/s with a maximum data rate of 11 Mb/s. In 2003, the IEEE updated the 2.4 to 2.5 GHz specification yet again with 802.11g, with typical throughput of 20 Mb/s and maximum data rates of 54 Mb/s. 802.11g is virtually identical to 802.11a, but because it operates at a lower frequency, it was less expensive to implement, and 802.11g has been the WLAN technology of choice for the past four years.
Since the development of the 802.11a protocol (1999), WLANs have used OFDM. In this technique, the usable bandwidth is precisely divided into a large number of smaller bandwidths, or "subcarriers," that are mathematically orthogonal. The high-speed information is then divided onto these multiple lower-speed signals that are transmitted simultaneously on different frequencies in parallel (Figure 1).
1. Today's WLANs predominantly use OFDM modulation, which divides available spectrum into subcarriers that are mathematically orthogonal.
Wireless networks are much more challenging to implement compared to wired networks. One of the main reasons for this is an impairment known as multipath fading. Multipath occurs when signals bounce off of objects in the environment. Multiple versions of the same signal arrive at the receiver at slightly different times using different paths. If the arrival times of the copied versions are delayed enough, they can interfere destructively with the desired signal, and the signal is destroyed.
A major advantage of OFDM is its use of multiple subcarriers, which makes it tolerant to multipath fading. Because small amounts of information are carried on individual subcarriers, if one or two are lost, they can be recovered with error correction codes. Furthermore, each of the subcarriers operates at a low speed, so small timing differences caused by multipath have less effect.
In addition to its use in WLANs, OFDM has become a common technique in communications, and is currently being used in DSL, very-high-speed DSL (VDSL), and digital video broadcasting (DVB) systems. In fact, many of the techniques in use in the latest WLANs are not altogether new! OFDM, for instance, was first used notably in the 1960s for military radio applications. Despite its early promise, OFDM was not widely deployed because of complex circuit design challenges. The advent of modern CMOS (complimentary metal oxide semiconductor, the basis of the microprocessor) "silicon" chips helped to make OFDM cost-effective in consumer applications.
WLAN networks allow device mobility in the home, office, or campus, eliminating the need to run Ethernet wiring. However, the typical data rates of wired Ethernet are 10 Mb/s for standard 10Base-T Ethernet, 100 Mb/s for Fast Ethernet, and 1000 Mb/s for Gigabit Ethernet. With a maximum data rate of 54Mbps, current generation WLANs have comparatively low data rates and throughput as a tradeoff for enhanced flexibility. That was the impetus for the latest specification development effort, 802.11n, which aims to use new technology in order to achieve typical throughput of 74 Mb/s and data rates up to 248 Mb/s for systems operating in the 2.4 GHz and/or 5 GHz bands.
802.11n Advantages
802.11n departs from the time, frequency, or code division schemes that came before it; this protocol aims to send data at the same time on the same frequency. Normally, these signals would interfere with each other. However, 802.11n can take advantage of a concept called "spatial diversity," which separates the data streams by using multiple antennas. Spatial diversity uses multiple paths (and ironically, multipath was previously an impairment) to effectively increase throughput. Thus, it is ideal for the cluttered indoor and urban environment. The name given to this technology is MIMO, which stands for multiple-input, multiple-output. Because MIMO systems use multiple radios to achieve higher throughput, these systems can consume more power and have more circuitry compared with single radio (SISO) 802.11g systems.
The essence of MIMO is shown in Figure 2. Data is sent simultaneously on transmit antennas 1 and 2. Data from the transmit antenna 1 travels to receive antenna 1 and 2 using different paths (spatial diversity). Mathematically,
Rx1=h11*Tx1 + h21*Tx2
Rx2=h21*Tx1 + h22*Tx2
The channel matrix (H = h11, h12, h21, and h22) is measured by transmitting a known tone from antennas 1 and 2. Once this is known, the receiver measures Rx1 and Rx2, and since it knows the H matrix, it can calculate the actual transmitted data.
Note that due to spatial diversity, MIMO systems are able to transmit data at twice the rate of the standard single antenna system used in 802.11g radios. Of course, this higher data rate comes at a cost; MIMO systems require a minimum of two complete radios, so to first order, power consumption is doubled.
2. Schematic diagram of a MIMO system
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