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In this paper, the technology of Spread Spectrum is discussed, along with the pros and cons of Spread Spectrum. Further this article analyses the comparison between Frequency Hopping and Direct Sequence Spread Spectrum Systems.
Spread Spectrum methods are techniques, where a signal (such as an electrical or electromagnetic) created in a particular bandwidth is intentionally expanded in the frequency domain, thus a signal with a extensive bandwidth is resulted. These methods are used for number of motives, such as to develop and stabilize safety communications, enhancing the resistance against natural intrusions and trafficking, to avoid exposure and to reduce power flux density (for instance satellite downlinks).
Spread Spectrum was firstly intended for use by Armstrong during 1920 - 1930 with wideband FM. The actual need for Spread Spectrum was coming with World War II. Nevertheless, Hedy Lamarr (Hollywood movie actress) and George Antheil (forward thinking composer) were granted the first public patent. (Xiaoying, N/A). At the start, Spread spectrum signals for digital communications were built up and applied for communications of military to offer resistance to aggressive trafficking, to conceal the signal by transferring at lower power or to formulate it in such a way which will make it easier for the multiple users to communicate via the same channel. It was only in 1980s that the Spread Spectrum was used for commercial purposes. Nowadays, it is used to grant dependable communications in a mixture of commercial applications. (Schilling et al, 1990).
The usual radio signal is known as a narrow-band signal. Spread spectrum is a method which acquires a narrow-band signal and extends that over a wider segment of the radio-frequency band. This leads to Low Probability of Intercept and anti-jam characteristics being the reason why the military has used Spread Spectrum for so many years. Signals are deliberately made to be much broader band than the information they are carrying enabling them to be more noise-like. Spread spectrum when performing, the keyed in data is captured in a predefined process by the transmitter. Each recipient must recognize such predefined process and the signal must be de-spread before the data can be read. To be eligible to be a spread spectrum signal, two criteria need to be met. One is the transmitted signal bandwidth must be greater than that of the information bandwidth. And the other is the functions except for the information being transmitted are employed to determine the resultant transmitted bandwidth.
(The ABCs of Spread Spectrum - A Tutorial, 2008)
The need for Spread Spectrum arises in situations where a communication signal be hard to detect, and still tough to demodulate even if it is detected. Also during other instances where a signal is required that is not easy to interfere with.
There are two techniques in performing such spreading: frequency hopping and direct sequencing.
Direct Sequence Spread Spectrum
Direct Sequence Spread Spectrum performs by accumulating unused data named 'chips', to the signal, at a minimum of 10 chips per bit are accumulated to the signal. The code used to transform the transferred data is termed as the spreading code and only receivers which understand the spreading code can decode the signal. This distinctive spreading code is the one which permits numerous direct sequence transmitters to function in the same area. Since the transmission is stretched across a broad frequency band, transmission power is lesser than the narrowband transmissions enabling it to be used in the ISM band. To other users the direct sequence transmission seems to be low power background noise. Because of the low power signal and it is spread across a wide frequency, it is exposed to noise. But, in the cases of signal corruption the unused data helps to recoup the original signal, the number of chips is directly proportional to the immunity from interruption. Direct Sequence provides a superior throughput and is more protected to interference than frequency hopping. Unluckily it uses two to three times more power and likely to be comparatively expensive. The wireless LAN industry seems to be split equally between Frequency Hopping and Direct Sequence. AT&T is one user of DSSS and are a major producer of wireless LAN products.
There are number of benefits of Direct Sequence Spread Spectrum. One of which is each symbol is transmitted over multiple frequencies at the same time. It is thought to be very efficient. Also it has higher speed than Frequency Hopping at comparable distances. Further the system capacity is higher than Frequency hopping. The Speciality of Direct Sequence Spread Spectrum is that it has the intelligence to find the nearest and clear channel. It also must be noted that it is resistant to deliberate or unintentional jamming. Also condensed signal / background noise level will obstruct interception
(Spread Spectrum Technology/N/A)
Figure 1 demonstrates the most common type of direct sequence modulated spread spectrum signal.
Figure 1 - A Spectrum Analyzer Photo of a Direct Sequence Spread Spectrum signal.
The idea of frequency hopping was first brought in by Nikola Tesla in July 1900. Tesla came up with the concept after the demonstration in 1898 of the world's first radio-controlled submersible boat, when it was obvious that the wireless signals which were controlling the boat had to be secured from interfered or interrupted. His patents covered two basically dissimilar methods for accomplishing resistance to interference; both were functioning by adjusting the carrier frequency or other special characteristic. The first was having a transmitter that performed concurrently at two or more difference frequencies and a receiver in which each of the individual transmitted frequencies had to be tuned in, in order for the control circuit to react. The second method used a variable-frequency transmitter controlled by an encoding wheel that adjusted the transferred frequency in a predetermined way. These patents describe the basic standards of frequency hopping, frequency-division multiplexing, also the electronic and gate logic circuit.
Frequency Hopping does just what its name implies. i.e., it "hops" from one frequency to another over a wide band. The precise order in which frequencies are in use is a function of a code sequence, and the pace of hopping from frequency to frequency is a function of the information rate. Figure 2 illustrates an output spectrum of a frequency hopping system. The bandwidth of a frequency hopping signal is just w times the number of frequency slots obtainable, where w is the bandwidth of individual hop channel.
Frequency Hopping devices utilize comparatively less power and are commonly cheaper, However, the performance of Direct Sequencing systems is normally better and more dependable.
The biggest advantage of frequency hopping rests in the coexistence of multiple access points in the same area, which is not possible in direct sequence.
Frequency hopping has two benefits. Electrical noise will only have an impact on a small part of the signal. Also, the impact from any other type of radio communications functioning in narrow bands of the spectrum will be reduced. Any such interruption that takes place will lead only to a slightly decreased quality of voice transmission or a tiny loss of data. Since data networks accept successful receipt of data, any missing pieces will generate a request to transmit the lost data.
Figure 2 - A Spectrum Analyzer Photo of a Frequency Hopping Spread Spectrum signal.
Operational Advantages of Spread Spectrum Modulation
Operational benefits provided by Spread Spectrum modulation are as follows:
Low Probability of Intercept (LPI): LPI means that any anonymous person cannot easily snoop or spy on the conversation or has to use high-priced ways to achieve this. A benchmarked communications receiver selects a demodulation circuitry, such as an amplitude demodulator, frequency demodulator or phase demodulator, depending on the modulation system employed at the transmitter. In a Spread Spectrum system, the receiver demodulates the transmitted power via some correlation procedure and successfully combines several elements within a broader bandwidth. A single-channel receiver will hence spot a small portion of the transmitted signal which will be too weak for usual detection, and even if it could be exaggerated to a detectable level would be imperfect and thus impossible to understand.
Low probability of position fix: Usual radio transmitters are effortlessly identified by trouble-free and low-priced direction finders. The spectral-spreading conceptions make this job much more demanding as greater processing energy and time to integrate will be required within each resolution.
Low probability of signal abuse: This refers to the possibilities which are present within a communication circumstance to take advantage of the communication link by some manoeuvring of the waveforms used to take the message. These could be, demolition of synchronization messages; devastation or adjustment of the message contents; undetectable or hidden accumulation of data bits.
High resistance to congestion and intervention: Spread Spectrum systems are intrinsically more vigorous against congestion and intervention than systems not using spreading techniques. This property of Spread Spectrum modulation is used in military systems where the main design objective is to develop a system which is able to deliver a message through a very hostile and hidden mean. Spread Spectrum modulation techniques offer an added factor which is not seen in usual systems. This is because a code is used in the spreading system and unless the interferer gets hold of such code the effect of the congestion/intervention is minimized by a considerable amount. There are two vital aspects of Spread Spectrum with respect to congestion and intervention. First is relating to the real protection provided by an Spread Spectrum code. The Spread Spectrum receiver gives a post-detection signal-to-noise and signal-to-interference improvement. This means that if the Spread Spectrum signal can be obtained with substantial clarity and power and without intervention signals, exceptional jammers may re-route the same waveform with more force and create troubles for the detection process in the data link. This is a realistic restriction since a limited number of spreading codes are used in a Spread Spectrum modem and hence the transmission tends to repeat the same codes a many number of times. The susceptibility rests in the fact that the code may be exposed if it sticks out clearly only once for instance if it is received at a very short distance from the transmitter. This indicates the very important requirement of Spread Spectrum systems, that the spectral-spreading system must be substantially responsive and utilize regular change of code structures. The next essential facet of Spread Spectrum modulation with regard to congestion and intervention is that Spread Spectrum modulation offers a sensible way of dealing with frequency-band congestion.
High time resolution/reduction of multipath effects: Multipath effects are one of the inevitable effects in radio communication. Multipath means that the signal attaining the receiver antenna has passed through by two or more paths. Because these ways unavoidably are of various lengths, the time delays of signals that have come along the relevant paths are dissimilar and the signal will fade in or out with small dislocation of the transmitter and receiver. Techniques exist whereby the signal is coded such that the signals reaching the receiver through different ways adjoin in phase at the receiver. Adaptive techniques capitalizing on the broader bandwidth of Spread Spectrum waveforms make it possible to utilize radio communication under exceedingly severe multipath conditions. For instance, wireless local area networks occupied inside rooms or buildings where the transmission conditions are very poor.
Cryptographic capabilities: The coding features of Spread Spectrum modulation have repercussions for probable safekeeping function in a communication link. The spreading codes can be selected such that they serve the dual purpose of spreading the frequency spectrum of the transmitted signal and making it complicated to decode the message. This necessitates the code to provide the essential spectral signature, has superior anti-cryptographic property, and is substantially unused. As these criteria set rather harsh requirements on the coding strategy, a general approach is to execute Spread Spectrum coding and mix up the code for cryptography separately and usually in sequence.