Wireless Spread Spectrum

Wireless systems transmit data over a specifi c, quite narrow frequency band that allows a transmitter and a receiver to differentiate the intended signal from background noise when the signal quality is sufficient. Narrowband interference can be avoided by filtering out any other frequencies except the designated ones at the receiver. The major advantage of narrowband signal transmission is, as the term implies, its efficient use of frequency due to only a small frequency band being used for one signal transmission. Its drawback is evident, though, as it requires well-coordinated frequency allocation for different signals, and it is quite vulnerable to signal jamming and interception.

Spread spectrum takes another approach. Instead of applying all transmitting power to a narrow frequency channel, spread spectrum converts the narrowband signals to signals of a much wider band with comparatively lower power density according to a specific signal spreading scheme. Because of the low power density (power per frequency), the converted signal appears to be background noise to others who are unaware of the spreading scheme; only the designated receiver is able to reconstruct the original signal. For data transmission with multiple channels using FM, narrowband signals in each channel can be applied with the spread spectrum technique. To be able to differentiate these channels and reconstruct those signals afterwards, each channel is assigned a code with sufficiently large distance separating it from others in the code space, and the codes are made known to the designated receiver only. The advantages of spread spectrum are as follows:

• CDM has greatly improved channel capacity (i.e., the number of signals that can be transmitted at the same time over a given frequency band) as compared to the narrowband spectrum.

• Spread spectrum offers high resistance against narrowband interference and tolerates narrowband interference because the signal is transmitted over a wide band. Even though some portion of the frequencies is distorted, the original signal can still be recovered with the error detection and correction techniques of the coding mechanism.

• Security against tapping and jamming is greater compared to narrowband spectrum techniques. Signals of spread spectrum are indistinguishable from background noise to anyone who does not know the coding scheme ( language ).

The disadvantage of spread spectrum is its relatively high complexity of the coding mechanism, which results in complex radio hardware designs and higher cost. Nonetheless, because of its remarkable advantages, spread spectrum has been adopted by many wireless technologies, such as CDMA and wireless LANs. Depending on the way the frequency spectrum is used, three types of spread spectrum systems are currently in place: directsequence spread spectrum (DSSS), frequency-hopping spread spectrum (FHSS), and OFDM.

Direct-Sequence Spread Spectrum
The DSSS spreads a signal over a much broader frequency band. It employs a “ chipping ” technique to convert a user signal into a spread signal. Given a user data bit, an XOR computation is performed with the user bit and a special chipping sequence code (digital modulation), which is a series of carefully selected pulses that are shorter than the duration of user bits. The resulting signal (the chipping sequence) is then modulated (radio modulation) with a carrier signal and sent out. On the receiver side, after demodulation, the same chipping sequence code is used to decode the original user bits. The chipping sequence essentially determines how the user signal is spread as pseudorandom noise over a large frequency band.

The ratio of the spreading (i.e., the spreading factor) varies for different spread spectrum systems. The longer the chipping sequence, the more likely a user signal can be recovered.
A transmitter and a receiver have to stay synchronized during the spreading and despreading.

Frequency-Hopping Spread Spectrum
FHSS uses a frequency-hopping sequence to spread a user signal that is known to both the transmitter and the receiver. User data is first modulated to narrowband signals, then a second modulation takes place — a signal with a hopping sequence of frequency is used as the radio carrier. The resulting spread signal is then sent to the receiver. On the receiver side, two steps of modulations are required: (1) use the same frequency-hopping sequence to recover the narrowband signal, and (2) demodulate the narrowband signal. In effect, the transmitter and the receiver follow the same pattern of synchronized frequency hopping. As a result, only if the hopping sequence is made known to the receiver can it recover the original user data bits; otherwise, the transmitted signals will appear as background noise. FHSS does not take up the entire allotted frequency band for transmission. Instead, at any given moment, only a portion of it is used for hopping. The two types of frequency-hopping systems in use are fast hopping systems and slow hopping systems. Fast hopping systems change frequency several times when transmitting a single bit, whereas in slow hopping systems each hop may transmit multiple bits.

Interestingly , the concept of frequency hopping was invented by a Hollywood actress,
Hedy Lamarr, and a composer, George Antheil, during World War II. The idea was to use a piano-roll sequence to hop between 88 channels to make decoding of radio-guided torpedo communications more difficult by enemies. Their idea was not implemented because the U.S. Navy refused to consider developing a military technology based on a musical technique. As George Antheil put it, “The reverend and brass-headed gentlemen in Washington who examined our invention read no further than the words ‘ player piano. ’ ‘ My god, ’ I can see them saying, ‘ we shall put a player piano in a torpedo. ’ ” (Source: American Heritage of Invention & Technology, Spring 1997, Volume 12/Number 4).

For both DSSS and FHSS, multiple signals with different sequence codes (either chippingsequence code or frequency-hopping code) can be multiplexed by CDM. To compare, FHSS is relatively simpler to implement than DSSS, but DSSS makes it much more difficult to recover the signal without knowing the chipping code and is more robust to signal distortion and multipath effects. Both are widely used by a large array of wireless technologies operating on the unlicensed spectrum. For example, the IEEE 802.11b standard for wireless LAN employs DSSS over the 2.4-GHz free spectrum, whereas the Bluetooth standard uses FHSS for simplicity.

Orthogonal Frequency-Division Multiplexing
OFDM is a modulation technique that utilizes multiple subcarriers in parallel to transmit user data. These subcarriers are orthogonal in that they are modulated with their own data independently. OFDM was fi rst conceived in the 1960s in an effort to minimize interference among adjacent channels in a frequency band. Because of multicarrier parallelism, OFDM offers a much higher data rate than single-carrier modulation techniques. In addition, because subcarriers are orthogonal, multipath interference can be largely reduced. In reality, some OFDM systems are actually code OFDM (COFDM), which combines error-control channel coding schemes with OFDM modulation. COFDM has some nice properties, such as being resistant against phase distortion, signal fading, and burst noise. OFDM is used in asymmetric digital subscriber line (ADSL), IEEE 802.11a/g wireless LANs, and the broadband wireless data access technology WiMax. COFDM is predominantly used in Europe for DAB and digital video broadcasting (DVB).

Source of Information :  Elsevier Wireless Networking Complete 2010


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