Data Communications Transmission Systems Computer Science Essay

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In this essay the importance of Spread Spectrum for data communications will be discussed. Firstly the essay will cover the History of Spread Spectrum and secondly how the data is transmitted and what the advantages and disadvantages are of using the communication.

Thirdly the two key implementations of Spread Spectrum will be discussed which are Frequency Hop Spread Spectrum (FHSS) and Direct-Sequence Spread Spectrum (DSSS)

Lastly a conclusion will be discussed with why Spread Spectrum is in demand and will continue to be for the future.

History

Spread spectrum technology originates from World War II where it was developed by the military and was deployed when sending communications between allies. The technique was originally discovered using a player piano to control the frequency hops and was envisioned as a way to provide secure communications during wartime. When the signals where transmitted for communication, the spread spectrum modulation would make the signals more difficult to interfere with and be intercepted by the enemy. This was made possible by Spread Spectrum using a signal bandwidth which is wider than the information bandwidth they are carrying to make them more noise-like.

To the present day the U.S military still deploy Spread Spectrum for its satellites. It is also used for satellite positioning systems (GPS) and 3G mobile communications and for Wide Local Area Networks (WLANs) and Bluetooth technology.

How it works

Spread spectrum is a modulation technique that uses wide bandwidth and when sending data transmissions it spreads the data over the available frequency band in excess of the minimum bandwidth required to send the information. In spreading the data over wide bandwidth it makes the signal resistant to noise and interference.

It can also be used to transmit either analog or digital data, using an analog signal. It achieves this by using the same transmit power levels to narrowband transmitters which are low and is measured in watts per hertz.

It therefore allows Spread Spectrum and narrowband signals to occupy the same bandwidth with minimum interference.

In the diagram below are the key elements of a spread spectrum system. When Input is fed into a channel encoder it creates an analog signal alongside a narrow bandwidth. This signal is then further modulated using an arrangement of random digits known as a pseudorandom sequence. A pseudorandom sequence is an algorithm for generating a sequence of numbers that approximates the properties of random numbers. This modulation technique greatly increases the bandwidth therefore spreading the spectrum of the signal to be transmitted. On the receiving end the same digit sequence is used to demodulate the spread spectrum signal. Finally, the signal is fed into a channel decoder to recover the data.

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Figure Model of a Spread Spectrum System

(Figure 1 - Stallings, William. Data & Computer Communications, Fifth Edition)

Transmissions resulting from Spread Spectrum spread the signal over a range of frequencies whereas on narrowband transmission it transmits signals on one frequency which increases the risk of interception or jamming. The reason for this is the signal-to-noise performance increases when narrowband transmits making the transmission more prone to interference.

The diagram below shows a comparison between narrowband and spread spectrum transmission.

Spread-spectrum transmissions spread out the signal over a range of frequencies. This increases signal-to-noise performance making the transmission more immune to narrowband interference. On the other hand, narrowband transmissions transmit on one frequency, increasing the likelihood of interception or jamming.

The following are the two techniques deployed to spread the bandwidth which is Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS).

Frequency Hopping Spread Spectrum (FHSS)

In the Frequency Hopping technique the signal is broadcast over different carrier frequencies that are modulated by the source signal. This in turn makes the signal modulate one carrier frequency at any split-second moment and then modulate another carrier frequency at another split-second moment. This means if a receiver is hopping between frequencies in time with the transmitter it will pick up the transmission. In the other instance any person wishing to block the signal would only succeed in blocking out only a few bits of the transmission.

The diagram below shows Frequency-Hopping spread spectrum transmitting packets of data on different carrier frequencies.

Frequency-hopping spread spectrum transmits packets of data on different carrier frequencies. To unsynchronized receivers, the transmission appears as a series of noise impulses.

Another example of FHSS is show in Figure 2 below where binary data is entered into a modulator using a digital-to-analog encoding technique called frequency-shift keying (FSK).

Frequency-shift keying (FSK) allows digital information to be transmitted by shifts in the frequency of a carrier signal. In an analog wave form the data is represented by the binary numbers 0 and 1. For example when a modem sends data it converts the data into an FSK signal which is then transmitted via telephone lines and fiber optics.

Referring again to Figure 2 below the signal is then set to a random base frequency using a pseudorandom code generator which creates a k-bit pattern for every hopping period.

The frequency table then uses the pattern to find the frequency to be used for the hopping

period and passes it to the frequency synthesizer which then creates the carrier signal of the frequency.

Figure Frequency Hopping Spread Spectrum

In Figure 3 below it is illustrating there are eight hopping frequencies in the frequency table ranging from 200 kHz to 900 kHz and the k-bit pattern is 3. The pseudorandom code generator will then create eight different 3-bit patterns ranging from 101 to 100 which are mapped to eight different frequencies in the frequency table.

Figure Frequency Hopping Table

(Figure 2 & 3, Forouzan, Behrouz A. Data Communications & Networking, Fourth Edition)

When the pseudorandom pattern is repeated after eight hopping's, which means that at hopping period 1, the pattern is 101 as illustrated in Figure 3, and the frequency selected from the frequency table is 700 kHz where the source signal will modulate this carrier frequency.

Next the second k-bit pattern will be selected which is 111, which selects the

900-kHz carrier and the third k-bit pattern selected will be 001 and the frequency carrier selected will be 300 kHz. This pattern continues until the eighth pattern is 100, where the frequency selected is 600 kHz. After the eighth hopping, the pattern repeats, starting from 101 again.

Direct Sequence Spread Spectrum(DSSS)

Direct sequence is a spread-spectrum technique that encodes each bit of data and sends it as a bit sequence in the form of zeros and ones. The original information is then spread out over a wide frequency range using more bandwidth than the original data needed. For example a 10-bit spreading code spreads the signal across a frequency band that is 10 times greater than a 1-bit spreading code.

DSSS processes every message bit by modulating the carrier using Phase-Shift Keying (PSK). PSK is a method of digital communication in which the phase of a transmitted signal is varied to convey information.

Therefore for the duration of every message bit, the carrier is modulated by PSK following a specific sequence of bits (known as chips) and goes through a process known as chipping which results in the substitution of every message bit by the same sequence of chips.

In DSSS systems, the spreading code is the chip sequence used to represent message bits. For message bits processing a 0 bit, the sequence of chips used to represent the bit remains unchanged. However for message bits processing a 1 bit the process is inverted, causing the 1 bit to become a 0 bit.

Therefore when data is being transferred over a medium the message bits 0 and 1 are represented by different chip sequences and to a non-synchronized receiver, it appears as wideband noise.

Redundancy is achieved with DSSS by the presence of the message bit on each chip of the spreading code. Even if some of the chips of the spreading code are affected by noise, the receiver may recognize the sequence and take the correct course of action regarding the received message bit.

CDMA is a Direct Sequence Spread Spectrum system. The CDMA system works directly on 64 Kbit/sec digital signals. These signals can be digitized voice, ISDN channels, modem data, etc.

The diagram below shows an example of the DSSS process and how the chip sequence is used to represent message bits.

http://www-cs-faculty.stanford.edu/~eroberts/courses/soco/projects/2003-04/wireless-computing/images/wt_dsss.gif

The Mathematical Equation for Spread Spectrum

Spread-spectrum uses the Shannon and Hartley channel-capacity theorem which is:

C = B Ã- log2 (1 + S/N)

In the above equation, C is the channel capacity in bits per second (bps), which is the maximum data rate for bit-error rate (BER). BER is the percentage of bits with errors divided by the total number of bits that have been transmitted, received or processed over a given time period. The rate is typically expressed as 10 to the negative power. For example, four incorrect bits out of 100,000 bits transmitted would be expressed as 4 x 10^-5. The BER is the digital equivalent to signal-to-noise ratio in an analog system.

Referring back to the equation, B is the required channel bandwidth in Hz, and S/N is the signal-to-noise power ratio. It is to be assumed that C, which represents the amount of information allowed by the communication channel, also represents the desired performance. Bandwidth (B) decreases, because frequency is a limited resource. The S/N ratio expresses the environmental conditions (i.e., presence of jammers, interferences, etc.).

Conclusion

Taking the two different Spread Spectrum techniques into account, DSSS has the advantage of providing higher capacities than FHSS. However the disadvantage of DSSS is it's a sensitive technology, influenced by many environmental factors such as reflections. The best way to minimize such influences is to use the technology in short distance applications hence why DSSS is deployed for indoor wireless LAN's in offices or for building to building links.

FHSS is a very robust technology, with little influence from noises, reflections or other environmental factors.

These features make the FHSS technology the one to be selected for installations designed to cover big areas where a large number of converged systems is required hence why FHSS is deployed for Wireless Broadband, where the use of DSSS is impossible because of its limitations.

In conclusion the Spread Spectrum technique is still important in the present day as it was when it was first introduced in World War II for the military. The technique has allowed for reduced crosstalk interference and improved voice quality that is used for mobile communications. It has allowed for improved security when transmitting signals due to the pseudo-random sequences and lastly it has given longer operating distances compared to an analog wireless device

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