Direct Sequence Spread Spectrum Computer Science Essay

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In telecommunications, direct-sequence spread spectrum (DSSS) is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency.

Features of Direct-sequence spread spectrum

1.It phase-modulates a sine wave pseudorandomly with a continuous string of pseudonoise (PN) code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate.

2.It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.

Transmission method of Direct-sequence spread spectrum

Direct-sequence spread-spectrum transmissions multiply the data being transmitted by a "noise" signal. This noise signal is a pseudorandom sequence of 1 and −1 values, at a frequency much higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band.

The resulting signal resembles white noise, like an audio recording of "static". However, this noise-like signal can be used to exactly reconstruct the original data at the receiving end, by multiplying it by the same pseudorandom sequence (because 1 Ã- 1 = 1, and −1 Ã- −1 = 1). This process, known as "de-spreading", mathematically constitutes a correlation of the transmitted PN sequence with the PN sequence that the receiver believes the transmitter is using.

For de-spreading to work correctly, the transmit and receive sequences must be synchronized. This requires the receiver to synchronize its sequence with the transmitter's sequence via some sort of timing search process. However, this apparent drawback can be a significant benefit: if the sequences of multiple transmitters are synchronized with each other, the relative synchronizations the receiver must make between them can be used to determine relative timing, which, in turn, can be used to calculate the receiver's position if the transmitters' positions are known. This is the basis for many satellite navigation systems.

The resulting effect of enhancing signal to noise ratio on the channel is called process gain. This effect can be made larger by employing a longer PN sequence and more chips per bit, but physical devices used to generate the PN sequence impose practical limits on attainable processing gain.

If an undesired transmitter transmits on the same channel but with a different PN sequence (or no sequence at all), the de-spreading process results in no processing gain for that signal. This effect is the basis for the code division multiple access (CDMA) property of DSSS, which allows multiple transmitters to share the same channel within the limits of the cross-correlation properties of their PN sequences.

As this description suggests, a plot of the transmitted waveform has a roughly bell-shaped envelope centered on the carrier frequency, just like a normal AM transmission, except that the added noise causes the distribution to be much wider than that of an AM transmission.

In contrast, frequency-hopping spread spectrum pseudo-randomly re-tunes the carrier, instead of adding pseudo-random noise to the data, which results in a uniform frequency distribution whose width is determined by the output range of the pseudo-random number generator.

Benefits of Direct-sequence spread spectrum

Resistance to intended or unintended jamming

Sharing of a single channel among multiple users

Reduced signal/background-noise level hampers interception (stealth)

Determination of relative timing between transmitter and receiver

Uses of Direct-sequence spread spectrum

The United States GPS and European Galileo satellite navigation systems

DS-CDMA (Direct-Sequence Code Division Multiple Access) is a multiple access scheme based on DSSS, by spreading the signals from/to different users with different codes. It is the most widely used type of CDMA.

Cordless phones operating in the 900 MHz, 2.4 GHz and 5.8 GHz bands

IEEE 802.11b 2.4 GHz Wi-Fi, and its predecessor 802.11-1999. (Their successor 802.11g uses OFDM instead)

Automatic meter reading

IEEE 802.15.4 (PHY and MAC layer for ZigBee)

Multi-carrier code division multiple access

Multi-Carrier Code Division Multiple Access (MC-CDMA) is a multiple access scheme used in OFDM-based telecommunication systems, allowing the system to support multiple users at the same time.

MC-CDMA spreads each user symbol in the frequency domain. That is, each user symbol is carried over multiple parallel subcarriers, but it is phase shifted (typically 0 or 180 degrees) according to a code value. The code values differ per subcarrier and per user. The receiver combines all subcarrier signals, by weighing these to compensate varying signal strengths and undo the code shift. The receiver can separate signals of different users, because these have different (e.g. orthogonal) code values.

Since each data symbol occupies a much wider bandwidth (in hertz) than the data rate (in bit/s), a signal-to-noise-plus-interference ratio (if defined as signal power divided by total noise plus interference power in the entire transmission band) of less than 0 dB is feasible.

One way of interpreting MC-CDMA is to regard it as a direct-sequence CDMA signal (DS-CDMA) which is transmitted after it has been fed through an inverse FFT (Fast Fourier Transform)

Rationale of MC-CDMA

Wireless radio links suffer from frequency-selective channels. If the signal on one subcarrier experiences an outage, it can still be reconstructed from the energy received over other subcarriers.

Downlink of MC-CDMA

In the downlink (one base station transmitting to one or more terminals), MC-CDMA typically reduces to Multi-Carrier Code Division Multiplexing. All user signals can easily be synchronized, and all signals on one subcarrier experience the same radio channel properties. In such case a preferred system implementation is to take N user bits (possibly but not necessarily for different destinations), to transform these using a Walsh Hadamard Transform, followed by an I-FFT.

Variants of MC-CDMA

A number of alternative possibilities exist as to how this frequency domain spreading can take place, such as by using a long PN code and multiplying each data symbol, di, on a subcarrier by a chip from the PN code, ci, or by using short PN codes and spreading each data symbol by an individual PN code - i.e. di is multiplied by each ci and the resulting vector is placed on Nfreq subcarriers, where Nfreq is the PN code length.

Once frequency domain spreading has taken place and the OFDM subcarriers have all been allocated values, OFDM modulation then takes place using the IFFT to produce an OFDM symbol; the OFDM guard interval is then added; and if transmission is in the downlink direction each of these resulting symbols are added together prior to transmission.

An alternative form of multi-carrier CDMA, called MC-DS-CDMA or MC/DS-CDMA, performs spreading in the time domain, rather than in the frequency domain in the case of MC-CDMA - for the special case where there is only one carrier, this reverts to standard DS-CDMA.

For the case of MC-DS-CDMA where OFDM is used as the modulation scheme, the data symbols on the individual subcarriers are spread in time by multiplying the chips on a PN code by the data symbol on the subcarrier. For example, assume the PN code chips consist of {1, -1} and the data symbol on the subcarrier is -j. The symbol being modulated onto that carrier, for symbols 0 and 1, will be -j for symbol 0 and +j for symbol 1.

2-dimensional spreading in both the frequency and time domains is also possible, and a scheme that uses 2-D spreading is VSF-OFCDM (which stands for variable spreading factor orthogonal frequency code-division multiplexing), which NTT DoCoMo is using for its 4G prototype system.

As an example of how the 2D spreading on VSF-OFCDM works, if you take the first data symbol, d0, and a spreading factor in the time domain, SFtime, of length 4, and a spreading factor in the frequency domain, SFfrequency of 2, then the data symbol, d0, will be multiplied by the length-2 frequency-domain PN codes and placed on subcarriers 0 and 1, and these values on subcarriers 0 and 1 will then be multiplied by the length-4 time-domain PN code and transmitted on OFDM symbols 0, 1, 2 and 3.[1]

NTT DoCoMo has already achieved 5 Gbit/s transmissions to receivers travelling at 10 km/h using its 4G prototype system in a 100 MHz-wide channel. This 4G prototype system also uses a 12x12 antenna MIMO configuration, and turbo coding for error correction coding.