Modulation and Line-Coding Techniques in Wireless LANs

The emerging wireless LAN (WLAN) technologies offer a wide range of modulations, coding schemes, and technologies. ^[1] WLANs hold the promise of ubiquitous access to network resources without the physical limitations of the wired network. In WLANs, ( otherwise popularly known as wi-fi [wireless-fidelity]) users can move freely about their offices or access network resources from conference rooms, lobbies , cafeterias, and campus buildings . WLANs use radio frequency (RF) waves instead of a cable infrastructure, and ensure mobile, cost-effective solutions that significantly reduce the network installation cost per user .

Infrared

Infrared (IR) radio-channel falls under the visible band of radiation that exists at the lower end of the visible spectrum. It is most effective when a clear line-of-sight can be achieved between the transmitter and the receiver. The technology has two available solutions: diffused-beam and direct-beam (or line-of-sight). Currently, direct-beam WLANs offer a faster data rate than diffused-beam networks, but direct-beam is more directional because diffused-beam technology uses reflected rays to transmit/receive a data signal. IR achieves lower data rates in the 1- to 2-Mbps range. IR optical signals are often used in remote control device applications.

Photonic Wireless Transmission

The only implementation of photonic WLANs use IR light transmission at the 850 to 950 Nm band of IR light with a peak power of 2 watts. The physical layer supports 1- to 2-Mbps data rates. Although photonic wireless systems potentially offer higher transmission rates than RF-based systems, they also have some distinct limitations:

• IR light is restricted to line-of-sight operations; however, the use of diffuse propagation can reduce this restriction by allowing the beam to bounce off passive reflective surfaces.

• The power output (2 watts) is low to reduce damage to the human eye; however, it limits transmissions to about 25 meters .

• Sensors (receivers) need to be laid out accurately otherwise the signal might not be picked up.

Photonic-based WLANs are inherently secure and are immune (as are optical fiber networks) from electromagnetic interference, which can interfere with cable and RF-based systems.

Diffused IR

Diffused IR communications are described as both indirect and non-line-of-sight. The diffused IR signal, which is emitted from the transmitter, fills an enclosed area like light and does not require line-of-sight transmission. You can point the IR adapters at the ceiling or at an angle and the signal bounces off the walls and ceiling. Changing the location of the receiver does not disrupt the signal. Many diffused IR products also offer roaming capabilities, which enables you to connect several access points to the network, and connect your mobile computer to any of these access points or move between them without losing your network connection. Usually diffused IR provides a radius of 25 to 35 feet and a speed of 1 to 2 Mbps.

IR Summary

Overall, the advantages of IR include no government regulations controlling use and its immunity to electromagnetic interference (EMI) and RF interference. Disadvantages of IR include that it is generally a short-range technology (30-50 ft radius under ideal conditions), signals cannot penetrate solid objects, and signals can be affected by light, snow, ice, fog, and dirt. Because of its significant limitations, IR is not a commonly used technology for WLANs.

Ultra -High Frequency Narrowband Technologies and WLAN

The term narrowband describes a technology in which the RF signal is sent in a narrow bandwidth, typically 12.5 kHz or 25 kHz. Power levels range from 1 to 2 watts for narrowband RF data systems. This narrow bandwidth combined with high power results in larger transmission distances than are available from 900-MHz or 2.4-GHz spread spectrum (SS) systems, which have lower power levels and wider bandwidths. UHF wireless data communication systems have been available since the early 1980s. These systems normally transmit in the 430- to 470-MHz frequency range, with rare systems using segments of the 800-MHz range. The lower portion of this band, 430 to 450 MHz, is often referenced as the unprotected (unlicensed) band and 450 to 470 MHz is referred to as the protected (licensed) band.

In the unprotected band, RF licenses are not granted for specific frequencies and anyone is allowed to use any frequency in the band. In the protected band, RF licenses are granted for specific frequencies, which gives customers some assurance that they will have complete use of that frequency. Other terms for UHF include narrowband and 400 MHz RF. Because independent narrowband RF systems cannot coexist on the same frequency, government agencies allocate specific radio frequencies to users through RF site licenses. A limited amount of the unlicensed spectrum is also available in some countries . To have many frequencies that can be allocated to users, the bandwidth given to a specific user is small.

Synthesized Radio Technology

The term synthesized radio technology refers to the crystal-controlled products in legacy UHF products, which require factory installation of unique crystals for each possible channel frequency. Synthesized technology uses a single, standard crystal frequency. The required channel frequency is calculated by either dividing or multiplying the standard crystal frequency. Synthesized UHF-based solutions provide the ability to install standard equipment without replacing the hardware, which provides less complexity and the ability to tune each device.

Multiple Frequency Operation

Modern UHF systems allow access points to be individually configured for operation on one of several pre-programmed frequencies. Wireless stations can be programmed with a list of all the frequencies used in the installed access points, which allows them to change frequencies when roaming. To increase throughput, access points can be installed with overlapping coverage but using different frequencies.

Advantages include a longer range, and it is considered a low cost solution for large sites with low to medium data throughput requirements. The disadvantages include low throughput, no interoperability, and a higher potential for interference. License requirements for protected bands and larger radio and antennas that increase wireless client size are also limiting factors.

Ultra Wideband and WLANs

The origin of ultra wideband (UWB) technology stems from work in time-domain electromagnetics that started in 1962 as a simple concept. Instead of characterizing a linear time-invariant (LTI) system by the more conventional means of a swept frequency response (which is amplitude and phase measurements versus frequency), an LTI system is alternatively characterized by its impulse response. The FCC approved the technology in February 2002, and this could have a significant impact on the WLAN industry. It could transform the industry from mainly limited radar or global positioning systems (GPSs) to more business communications. Wireless technologies, such as the 802.11 family and short-range BlueTooth radios, might be replaced by a UWB technology that has a throughput tens of thousands of times greater than the 802.11b standard.

UWB energy pulses operate in the same frequency spectrum as electronic noise that is typical of printers, chips, and widespread personal electronic equipment. The implications are significant:

• The UWB does not use a carrier and consequently does not require a designated band from any of the overcrowded spectrums .

• It is less expensive and easier to build such devices.

• The electromagnetic noise requires little power (in the order of milliwatts, which is thousands of times less than cellular phones). There is less power, radiation, and distance, but there are available techniques to increase the range.

• A high level of security exists because it is almost impossible to filter pulse signals from the regular noise.

Disadvantages of UWB that should be considered include the following:

• Concerns among academics and the industry about devices operating under the 2.4-GHz band. These pulses can interfere with existing broadcasts such as GPS and public safety nets .

• A high dependence on the way transmitters are tuned , timed, and powered .

UWB is an RF wireless technology, and as such is still subject to the same laws of physics as every other RF technology. Obvious tradeoffs can be made in signal-to-noise ratio versus bandwidth, and in range versus peak, depending on power levels.

WLAN Modulations and Coding Techniques

WLANs are significantly driving the industry. The short-range BlueTooth and HiperLAN/2 are among the key contributors. The standards defined by the IEEE 802.11 Task Force are making the most significant impact.

At the physical layer, IEEE 802.11 defines three physical techniques for WLANs:

• Diffused IR

• Frequency hopping spread spectrum (FH or FHSS)

• Direct sequence spread spectrum (DS or DSSS)

Although the IR technique operates at the baseband, the other two radio-based techniques operate at the 2.4-GHz band. They can operate WLAN devices without the need for end-user licenses. For wireless devices to be interoperable, they must conform to the same physical layer standard. All three techniques specify support for 1-Mbps and 2-Mbps data rates.

Spread Spectrum RF Transmissions

Spread Spectrum (SS) RF systems are true WLANs, which use radio frequency (RF wireless) transmission as the physical layer medium. Two major subsystems exist: FHSS and DSSS. DSSS is primarily an inter-building technology, and FHSS is primarily an intra-building technology. The actual technique of SS transmission was developed by the military in an attempt to reduce jamming and eavesdropping. SS transmission takes a digital signal and expands, or spreads , it so as to make it appear more similar to random background noise rather than a data signal transmission. Coding takes place either by using FSK or PSK. Both methods increase the size of the data signal and the bandwidth. Although the signal appears louder (more bandwidth) and easier to detect, the signal is unintelligible and appears as background noise unless the receiver is tuned to the correct parameters.

FHSS

FHSS is analogous to FM radio transmission as the data signal is superimposed on, or carried by, a narrowband carrier that can change frequency. The IEEE 802.11 standard provides 22 hop patterns, or frequency shifts, to choose from in the 2.4-GHz ISM band. Each channel is 1 MHz and the signal must shift frequency, or hop, at a fixed hop rate (U.S. minimum is 2.5 hops/sec). This technology modulates a radio signal by shifting it from frequency to frequency at near-random intervals. This modulation protects the signal from interference that concentrates around one frequency. To decode the signal, the receiver must know the rate and the sequence of the frequency shifts, thereby providing added security and encryption.

FHSS products can send signals as quickly as 1.2 to 2 Mbps and as far as 620 miles. Increasing the bandwidth (up to 24 Mbps) can be achieved by installing multiple access points on the network. In FS, the 2.4-GHz band is divided into 75 1-MHz subchannels. To minimize the probability that two senders are going to use the same subchannel simultaneously , frequency hopping provides a different hopping pattern for every data exchange. The sender and receiver agree on a hopping pattern, and data is sent over a sequence of subchannels according to the pattern. FCC regulations require bandwidth up to 1 MHz for every subchannel that forces the FHSS technique to spread the patterns across the entire 2.4-GHz band, which results in more hops and a high amount of overhead.

FHSS is considered an economic solution because it provides lower cost ratios, is half the cost of a DSSS system per-node, and can scale above 10 Mbps by adding more access points. Another good point regarding FHSS is its ability to overcome noisy environments, such as metro areas. Because of hopping, it can deal with interference better.

DSSS

SS was first developed by the military as a secure wireless technology. It modulates (changes) a radio signal pseudo- randomly so it is difficult to decode. This modulation provides some security; however, because the signal can be sent great distances, you do risk interception. To provide complete security, most SS products include encryption.

DSSS works by taking a data stream of 0s and 1s and modulating it with a second pattern, the chipping sequence. The sequence is also known as the Barker code, which is an 11-bit sequence (10110111000). The chipping, or spreading, code generates a redundant bit pattern to be transmitted, and the resulting signal appears as wideband noise to the unintended receiver. One of the advantages of using spreading codes is that even if one or more of the bits in the chip are lost during transmission, statistical techniques embedded in the radio can recover the original data without the need for retransmission. The ratio between the data and width of the spreading code is called processing gain. It is 16 times the width of the spreading code and increases the number of possible patterns to 2 ¹⁶ (64 k), which reduces the chance of cracking the transmission.

The DSSS signaling technique divides the 2.4-GHz band into 14 22-MHz channels, of which 11 adjacent channels overlap partially and the remaining three do not overlap. Data is sent across one of these 22-MHz channels without hopping to other channels, which causes noise on the given channel. To reduce the number of retransmissions and noise, chipping converts each bit of user data into a series of redundant bit patterns called chips. The inherent redundancy of each chip, combined with spreading the signal across the 22-MHz channel, provides error checking and correction functionality to recover the data.

SS products are often interoperable because many are based on the IEEE 802.11 standard for wireless networks. DSSS is primarily an inter-building technology, and FHSS is primarily an intra-building technology. DSSS products can be fast and far reaching.

DSSS is best suited for large coverage areas, ensures higher data rates, requires fewer access points, and the total system cost is lower.

IEEE 802.11bThe Next Step

All previously mentioned, coding techniques for 802.11 provide a speed of 1 to 2 Mbps, which is lower than the widespread IEEE 802.3 standard speed of 10 Mbps. The only technique (with regards to FCC rules) that is capable of providing a higher speed is DSSS, which was selected as a standard physical layer technique that supports 1 to 2 Mbps and two new speeds of 5.5 and 11 Mbps.

The original 802.11 DSSS standard specifies the 11-bit chipping, or Barker sequence, to encode all data sent over the air. Each 11-chip sequence represents a single data bit (1 or 0), and is converted to a waveform, called a symbol, that can be sent over the air. These symbols are transmitted at 1 MSps (1 million symbols per second) by using a sophisticated technique called binary phase-shift keying (BPSK). In the case of 2 Mbps, you use the more sophisticated implementation, QPSK, which doubles the data rate available in BPSK with improved efficiency in the use of the radio bandwidth.

To increase the data rate in the 802.11b standard, in 1998, Lucent Technologies and Harris Semiconductor proposed to IEEE a standard called Complementary Code Keying (CCK). Rather than the two 11-bit Barker code, CCK uses a set of 64 8-bit unique code words, thus up to 6 bits can be represented by any code word (instead of the 1 bit represented by a Barker symbol). As a set, these code words have unique mathematical properties that allow them to be correctly distinguished from one another by a receiver, even in the presence of substantial noise and multi- path interference (such as interference caused by receiving multiple radio reflections within a building).

The 5.5-Mbps rate uses CCK to encode 4 bits per carrier, and the 11-Mbps rate encodes 8 bits per carrier. Both speeds use QPSK as the modulation technique and signal at 1.375 MSps. QPSK uses four rotations (0, 90, 180 and 270 degrees) to encode 2 bits of information in the same space as BPSK encodes 1. The trade-off is that you must increase power or decrease range to maintain signal quality. Because the FCC regulates the output power of portable radios to 1 watt Effective Isotropic Radiated Power (EIRP), range is the only remaining factor that can change. Thus, for 802.11 devices, as you move away from the radio, the radio adapts and uses a less complex (and slower) encoding mechanism to send data, resulting in the lower data rates. Table 2-1 identifies the differences.

Table 2-1. 802.11b Standard Options

Data Rate	Code and Code Length	Modulation	Symbol Rate	Bits/Symbol
1 Mbps	11 (Barker sequence)	BPSK	1 MSps	1
2 Mbps	11 (Barker sequence)	QPSK	1 MSps	2
5.5 Mbps	8 (CCK)	QPSK	1.375 MSps	4
11 Mbps	8 (CCK)	QPSK	1.375 MSps	8

802.11a and 802.11gThe New 5-GHz Band

The 802.11b standard for a coding technique is based on DSSS, a technology developed by the military for secure wireless transmission.

Unlike 802.11b, 802.11a was designed to operate in the more recently allocated 5-GHz Unlicensed National Information Infrastructure (UNII) band. Unlike the ISM band, which offers about 83 MHz in the 2.4-GHz spectrum, IEEE 802.11a uses almost four times that of the ISM band because the UNII band offers 300 MHz of relatively interference free spectrum. And unlike 802.11b, the 802.11a standard uses a FDM technique, which is expected to be more efficient in inter-building environments. The FCC allocates 300 MHz of spectrum for UNII in the 5-GHz block, 200 MHz of which is at 5150 MHz to 5350 MHz, with the other 100 MHz at 5725 MHz to 5825 MHz, as shown in Figure 2-4.

Figure 2-4. 5-GHz Frequency Band

The first advantage of 802.11a over 802.11b is that the standard operates in the 5.4-GHz spectrum, which gives it the performance advantage of the higher frequencies. But frequency, radiated power, and distance together are in an inverse relationship, so moving up the 5-GHz spectrum from 2.4 GHz results in shorter distances and requirements for more power. That is why the 802.11a standard increases the EIRP to the maximum of 50 mW. The 5.4-GHz spectrum is split into three working domains and every domain has restrictions for maximum power.

The second advantage relies on the coding technique used by 802.11a. ^[2] 802.11a uses an encoding scheme called coded orthogonal FDM (COFDM or OFDM). Each subchannel in the COFDM implementation is about 300-kHz wide. COFDM works by breaking one high-speed data carrier into several lower-speed subcarriers, which are then transmitted in parallel. Each high-speed carrier is 20-MHz wide and is broken up into 52 subchannels, each approximately 300-kHz wide (see Figure 2-5).

Figure 2-5. Orthogonal Frequency Division Multiplexing (OFDM)

COFDM uses 48 of these subchannels for data, while the remaining four are used for error correction. COFDM delivers higher data rates and a high degree of signal recovery, thanks to its encoding scheme and error correction. Each subchannel in the COFDM implementation is about 300-kHz wide. To encode 125 kbps, the well-known BPSK is used, which yields a 6000-kbps data rate. Using QPSK, it is possible to encode up to 250 kbps per channel, which combined achieves a 12-Mbps data rate. By using 16-level QAM encoding 4 bits per hertz, and achieving data rates of 24 Mbps, the standard defines basic speeds of 6.12 and 24 Mbps that every 802.11a-compliant product must support. Data rates of 54 Mbps are achieved by using 64 QAM, which yields 8 to 10 bits per cycle, and a total of up to 1.125 Mbps per 300-kHz channel. With 48 channels, this results in a 54-Mbps data rate; however, the maximum theoretical data rate of COFDM is considered to be 108 Mbps.

NOTE

802.11a is not backward compatible with 802.11b. As a result, the 802.11a deployment in an already deployed 802.11b environment can be complicated. One possible solution here is the Cisco 1200 product, which supports both standards.

802.11g is an extension of 802.11b, and will broaden 802.11b's data rates to 54 Mbps by using the same OFDM technology as 802.11a within the 2.4-GHz band. Because of backward compatibility, an 802.11b radio card interfaces directly with an 802.11g access point (and vice versa) at 11 Mbps or lower depending on the range. Similar to 802.11b, 802.11g operates in the 2.4-GHz band, and the transmitted signal uses approximately 30 MHz, which is one third of the band. This limits the number of non-overlapping 802.11g access points to three, which is the same as 802.11b. This means that the same difficulties exist with the 802.11g channel assignment as with the 802.11b when covering a large area with a high density of users. Another big issue here is the considerable RF interference from other 2.4-GHz devices. The 802.11g standard is still under development and the release of 802.11g radio cards and access points can be expected by late 2002 or early 2003.

NOTE

The newly accepted IEEE 802.11e adds multimedia, quality of service (QoS), and enhanced security to the current IEEE 802.11 wireless standard. IEEE 802.11e is a Media Access Control (MAC) level enhancement that works with both 802.11b (2.4 GHz) and 802.11a (5 GHz) physical layers , and with the upcoming IEEE 802.11g physical layer specification. Significant elements of ShareWave's patented Whitecamp network protocol have been incorporated into the IEEE 802.11e specification. Recently, ShareWave announced, "The Bodega Wireless LAN Platform Features Whitecap2 Technology, which Adds Wi-Fi (802.11b) Compliance to Multimedia and QoS Capabilities."