Frequency-Hopping Transmission

Frequency hopping depends on rapidly changing the transmission frequency in a predetermined, pseudorandom pattern, as illustrated in Figure 11-1. The vertical axis of the graph divides the available frequency into a number of slots. Likewise, time is divided into a series of slots. A hopping pattern controls how the slots are used. In the figure, the hopping pattern is {2,8,4,6}. Timing the hops accurately is the key to success; both the transmitter and receiver must be synchronized so the receiver is always listening on the transmitter's frequency.

Figure 11-1. Frequency hopping

Frequency hopping is similar to frequency division multiple access (FDMA) but with an important twist. In FDMA systems, each device is allocated a fixed frequency. Multiple devices share the available radio spectrum by using different frequencies. In frequency-hopping systems, the frequency is time-dependent rather than fixed. Each frequency is used for a small amount of time, called the dwell time.

Among other things, frequency hopping allows devices to avoid interfering with primary users assigned to the same frequency band. It works because primary users are assigned narrow frequency bands and the right to transmit at a power high enough to override the wireless LAN. Any interference caused by the secondary user that affects the primary user is transient because the hopping sequence spreads the energy out over a wide band.[*] Likewise, the primary user only knocks out one of the spread-spectrum device's slots and looks like transient noise. Figure 11-2 shows the result when frequency slot 7 is given to a primary user. Although the transmission in the fourth time slot is corrupted, the other three transmissions succeed.

[*] If the primary user of a frequency band notices interference from secondary users, regulators can (and will) step in to shut down the secondary user, hence the low power used by spread-spectrum modulation techniques.

If two frequency-hopping systems need to share the same band, they can be configured with different hopping sequences so they do not interfere with each other. During each time slot, the two hopping sequences must be on different frequency slots. As long as the systems stay on different frequency slots, they do not interfere with each other, as shown in Figure 11-3. The gray rectangles have a hopping sequence of {2,8,4,7}, as in the previous figures. A second system with a hopping sequence of {6,3,7,2} is added. Hopping sequences that do not overlap are called orthogonal. When multiple 802.11 networks are configured in a single area, orthogonal hopping sequences maximizes throughput.

Figure 11-2. Avoiding interference with frequency hopping

Figure 11-3. Orthogonal hopping sequences

802.11 FH Details

802.11 divides the microwave ISM band into a series of 1-MHz channels. Approximately 99% of the radio energy is confined to the channel. The modulation method used by 802.11 encodes data bits as shifts in the transmission frequency from the channel center. Channels are defined by their center frequencies, which begin at 2.400 GHz for channel 0. Successive channels are derived by adding 1-MHz steps: channel 1 has a center frequency of 2.401 GHz, channel 2 has a center frequency of 2.402 GHz, and so on up to channel 95 at 2.495 GHz. Different regulatory authorities allow use of different parts of the ISM band; the major regulatory domains and the available channels are shown in Table 11-1.

The dwell time used by 802.11 FH systems is 390 time units, which is almost 0.4 seconds. When an 802.11 FH PHY hops between channels, the hopping process can take no longer than 224 microseconds. The frequency hops themselves are subject to extensive regulation, both in terms of the size of each hop and the rate at which hops must occur.

802.11 Hop Sequences

Mathematical functions for deriving hop sets are part of the FH PHY specification and are found in clause 14.6.8 of the 802.11 specification. As an example, hopping sequence 1 for North America and most of Europe begins with the sequence {3, 26, 65, 11, 46, 19, 74, 50, 22, ...}. 802.11 further divides hopping sequences into nonoverlapping sets, and any two members of a set are orthogonal hopping sequences. In Europe and North America, each set contains 26 members. Regulatory authorities in other areas have restricted the number of hopped channels, and therefore each set has a smaller number of members. Table 11-2 has details.

Joining an 802.11 Frequency-Hopping Network

Joining a frequency-hopping network is made possible by the standardization of hop sequences. Beacon frames on FH networks include a timestamp and the FH Parameter Set element. The FH Parameter Set element includes the hop pattern number and a hop index. By receiving a Beacon frame, a station knows everything it needs to synchronize its hopping pattern.

Based on the hop sequence number, the station knows the channel-hopping order. As an example, say that a station has received a Beacon frame that indicates that the BSS is using the North America/Europe hop sequence number 1 and is at hop index 2. By looking up the hop sequence, the station can determine that the next channel is 65. Hop times are also well-defined. Each Beacon frame includes a Timestamp field, and the hop occurs when the timestamp modulo dwell time included in the Beacon is 0.

ISM Emission Rules and Maximum Throughput

Spectrum allocation policies are the limiting factor of frequency-hopping 802.11 systems. As an example, consider the three major rules imposed by the FCC in the U.S.:[*]

[*] These rules are in rule 247 of part 15 of the FCC rules (47 CFR 15.247).

1. There must be at least 75 hopping channels in the band, which is 83.5-MHz wide.
2. Hopping channels can be no wider than 1 MHz.
3. Devices must use all available channels equally. In a 30-second period, no more than 0.4 seconds may be spent using any one channel.

Of these rules, the most important is the second one. No matter what fancy encoding schemes are available, only 1 MHz of bandwidth is available at any time. The frequency at which it is available shifts continuously because of the other two rules, but the second rule limits the number of signal transitions that can be used to encode data.

With a straightforward, two-level encoding, each cycle can encode one bit. At 1 bit per cycle, 1 MHz yields a data rate of 1 Mbps. More sophisticated modulation and demodulation schemes can improve the data rate. Four-level coding can pack 2 bits into a cycle, and 2 Mbps can be squeezed from the 1-MHz bandwidth.

The European Telecommunications Standards Institute (ETSI) also has a set of rules for spread-spectrum devices in the ISM band, published in European Telecommunications Standard (ETS) 300-328. The ETSI rules allow far fewer hopping channels; only 20 are required. Radiated power, however, is controlled much more strictly. In practice, to meet both the FCC and ETSI requirements, devices use the high number of hopping channels required by the FCC with the low radiated power requirements of ETSI.

Effect of Interference

802.11 is a secondary use of the 2.4-GHz ISM band and must accept any interference from a higher-priority transmission. Catastrophic interference on a channel may prevent that channel from being used but leave other channels unaffected. With approximately 80 usable channels in the U.S. and Europe, interference on one channel reduces the raw bit rate of the medium by approximately 1.25%. (The cost at the IP layer will be somewhat higher because of the interframe gaps, 802.11 acknowledgments, and framing and physical-layer covergence headers.) As more channels are affected by interference, the throughput continues to drop. See Figure 11-4.

Figure 11-4. Throughput response to interference in FHSS systems