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Cutting the cord: how the world’s engineers built Wi-Fi


Spreading the spectrum to avoid industry, science, and medicine

The whole point of a local area network (LAN) is that everyone gets to build their own local system without having to coordinate with anyone else. But for radio spectrum, tight regulation is the norm in order to avoid undue interference between different users. Wi-Fi sidesteps the need for such regulation, and the need to pay for radio spectrum, by shacking up with microwave ovens, cordless phones, Bluetooth devices, and much more in the unlicensed 2.4GHz industrial, scientific, and medical (ISM) band.

The original 802.11 specification published in 1997 worked at speeds of 1 and 2Mbps over infrared—though this was never implemented—or by using microwave radio on the 2.4GHz ISM band. Users of the ISM band are required to use a spread spectrum technique to avoid interference as much as possible. Most modulation techniques put as many bits in as few Hertz of radio spectrum as possible, so the available radio bandwidth is used efficiently. Spread spectrum does exactly the opposite: it spreads the signal out over a much larger part of the radio band, reducing the number of bits per Hertz. The advantage of this is that it effectively dilutes any narrow-band interference.

Frequency hopping

The simplest spread spectrum modulation technique is frequency hopping—obsolete now in Wi-Fi—which is exactly like it sounds: the transmission quickly hops from one frequency to another. Frequency hopping was invented several times over the first half of the 20th century, most notably by actress Hedy Lamarr and composer George Antheil. Through her first husband (of six), Lamarr became interested in the problem of how to guide torpedos in a way that resists to jamming and even detection by the enemy. Frequency hopping can help resist jamming, but quickly changing frequencies only works if the sender and receiver operate in perfect sync.

Antheil, who had created the composition Ballet Mécanique for a collection of synchronized player pianos, contributed the synchronization based on a piano roll. A 1942 patent for frequency hopping was granted to Lamarr and Antheil, but the significance of the invention wouldn't be recognized until the 1990s, when the wireless revolution got underway.

802.11 direct-sequence spread spectrum

The 1997 version of the 802.11 standard allows for 1 or 2Mbps direct-sequence spread spectrum (DSSS) modulation. Rather than moving a narrowband transmission across the 2.4GHz band, DSSS transmits a signal that occupies a much bigger portion of the band, but does so continuously. The underlying data signal in a DSSS transmission is of relatively low bandwidth compared to the DSSS radio bandwidth. Upon transmission, the data signal gets spread out and upon reception the original data signal is recovered using a correlator, which reverses the spreading procedure. The correlator gets rid of a lot of narrowband interference in the process.

It's a bit like reading a book at the beach. Sand on the pages will make the text harder to read. To avoid this, you could bring a book with big letters and then hold it at a larger distance than usual. The letters then appear at the normal size, but the sand grains are smaller relative to the letters, so they interfere less.

IEEE 802.11-1997 uses a symbol rate of 1MHz, with one or two bits per symbol encoded through differential binary or quadrature phase-shift keying (DBPSK or DQPSK), where the phase of a carrier signal is shifted to encode the data bits. ("Differential" means that it's not the absolute phase that matters, but the difference relative to the previous symbol. See our feature on DOCSIS for more details about how bits are encoded as symbols.)

The 1MHz DBPSK/DQPSK signal is then multiplied with a pseudorandom signal that has 11 "chips" for each symbol (for a chiprate of 11MHz). When transmitted over the channel, the spectrum is flattened because the chiprate is higher than the bitrate. When such a wide signal suffers narrowband interference, the impact is low since most of the transmitted signal remains unaffected. The receiver takes the incoming sequence of chips and multiplies it by the same pseudo-random sequence that was used by the transmitter. This recovers the original PSK-modulated signal.

802.11b: complementary code keying

Being able to transfer data wirelessly at speeds topping out not much higher than 100 kilobytes per second is impressive from the perspective that it works at all, but for real-world applications, some extra speed would be helpful. This is where 802.11b comes in, which specified 5.5Mbps and 11Mbps bitrates in 1999.

Simply increasing the symbol rate and, along with it, the DSSS chiprate, would allow for higher bitrates but would use more radio spectrum. Instead, 802.11b encodes additional data bits in the DSSS chipping sequence, using a modulation scheme called complementary code keying (CCK). The 5.5Mbps rate uses four 8-bit chipping sequences (encoding two bits), while the 11Mbps rate uses 64 (six bits). Together with the two DQPSK bits, this adds up to four or eight bits per symbol with a 1.375MHz symbol rate.

802.11a: orthogonal frequency-division multiplexing

For 802.11a (1999), the IEEE selected a very different technology than DSSS/CCK: orthogonal frequency division multiplexing (OFDM). DSSS is a single-carrier approach since there is a single high-rate (which means wideband) signal modulating the carrier. This has some drawbacks, as a wide-band channel presents different behaviors at different frequencies and needs to be equalized. It is far more convenient to send a large number of carriers and use low-rate (narrowband) signals to modulate each of them. It wasn't until powerful digital signal processors (DSPs) were available that the inverse fast fourier transform (IFFT) algorithm could be used to pack a large number of subcarriers very closely (in frequency) without too much mutual interference.

Rather than have a single signal occupy a whopping 22MHz of radio spectrum as with DSSS, 802.11a transmits 48 data-carrying subcarriers over 20MHz. Because there are so many subcarriers, the bit and symbol rates for each of them are quite low: the symbol rate is one every 4 microseconds (250kHz), which includes a 800-nanosecond guard interval between symbols. The low symbol rate and the guard time for each subcarrier are OFDM's secret weapons. These allow it to be isolated from reflections or multipath interference. 2.4GHz radio waves have a tendency to bounce off of all kinds of obstacles, such that a receiver will generally get multiple copies of the transmitted signal, where the reflections are slightly delayed. This is a phenomenon some of us remember from the analog TV days, where it showed up as ghosting.

A problem with radio transmission is that signals invariably "echo" across the radio band, with copies of the intended transmission also appearing on higher and lower frequencies, causing interference for neighboring transmissions. This makes it extremely challenging to transmit signals close together in frequency—which is of course exactly what OFDM does. It gets away with this because the carrier frequencies and symbol rates are chosen such that the spikes in the frequency band are "orthogonal": just where one transmission is most powerful, all other transmissions are magically silent. It's similar to how WWI era military aircraft were able to shoot through the arc of the propellor; both the propellor and the machine gun bullets potentially occupy the same space, but they're carefully timed not to interfere.

With techniques like BPSK, QPSK, 16QAM, or 64QAM (1, 2, 4, or 6 bits per symbol), this adds up to between 250 kbps and 1.5Mbps per subcarrier. The total raw bitrate for all 48 subcarriers is thus 12 to 72Mbps.

Unlike 802.11-1997 and 802.11b, 802.11a uses the 5GHz ISM band rather than the 2.4GHz ISM band. Although it took a while to sort out 5GHz availability around the world—which didn't exactly help 802.11a deployment—the 5GHz band has a lot more room than the cramped 2.4GHz band, with fewer users. 5GHz signals are more readily absorbed so they don't reach as far. This is often a downside, but it can also be an advantage: neighboring networks don't interfere as much because the signals don't pass through walls and floors that well.

Although 802.11a saw some adoption in the enterprise space, it wasn't a runaway success. What was needed was a way to bring 802.11a speeds to the 2.4GHz band in a backward compatible way. This is exactly what 802.11g (2003) does by using 802.11a-style OFDM in the 2.4GHz band. (though it adds additional "protection" mechanisms [discussed later] for 802.11b coexistence.)

802.11n: much, much, much more speed

Surprisingly, 802.11g didn't satiate the need for more speed. So the IEEE went back to the drawing board once again to find new ways to push more bits through the ether. They didn't come up with any new modulations this time around, but rather used several techniques that add up to much higher bitrates, and, more importantly, much better real-world throughput. First, the number of OFDM subcarriers was increased to 52, which brought the raw bitrate up to 65Mbps.

The second speed increase came from MIMO: multiple-input/multiple-output. With MIMO, a Wi-Fi station can send or receive multiple data streams through different antennas at the same time. The signal between each combination of send and receive antennas takes a slightly different path, so each antenna sees each transmitted data stream at a slightly different signal strength. This makes it possible to recover the multiple data streams by applying a good deal of digital signal processing.

All 802.11n Wi-Fi systems except mobile phones support at least two streams, but three or four is also possible. Using two streams increases the bitrate to 130Mbps.

The third mechanism to increase speed is an option to use "wide channels." 40MHz rather than 20MHz channels allow for 108 subcarriers, which brings the bitrate to 270Mbps with 2x2 MIMO (two antennas used by the sender and and two antennas used by the receiver). Optionally, the guard interval can be reduced from 800 to 400 nanoseconds, adding another 10 percent speed increase, bringing the total bitrate to 300Mbps for 2x2 MIMO, 450Mbps for 3x3 MIMO, and 600Mbps for 4x4 MIMO. However, as the 2.4GHz band is already very crowded, some vendors only implement wide channels in the 5GHz band.

Last but not least, 802.11n makes it possible to transmit multiple payloads in one packet and use block acknowledgments, reducing the number of overhead bits that must be transmitted.

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