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Spread spectrum in Wireless Communication

Dec, 22 2020, 11:40 am [IST]
Spread spectrum in Wireless Communication

Spread spectrum

As the name implies, spread spectrum techniques involve spreading the bandwidth needed to transmit data. Spreading the band with it has several advantages. The major benefit of this method is the resistance to narrowband interference.

Spread spectrum helps to deal with narrowband interference for a single channel, it can be used for several channels. spread spectrum can increase resistance to narrowband interference. The same technique is now after to do all narrowband signals. All narrowband signals are now spread into broadband signals using the same frequency range. Know more frequency planning is needed, and all senders use the same frequency band.

To separate different channels, CDM is now used instead of FDM. Spreading of a narrow band signal is achieved using a special code. Each channel is allotted its own code, which the receivers have to apply to recover the signal. Without knowing the code the signal cannot be recovered and behaves like background noise. This is a security effect of spread spectrum if a secret code is used for spreading.

Features that make spread spectrum and CDM very attractive for military applications are the coexistence of several signals without co-ordination, robustness against narrowband interference, relative high security, and characteristics like background noise.

Spectrum technologies also show disadvantages. One drawback is the increased complexity of receivers that have to dispread a signal. Another problem is the biggest frequency band that is required due to the spreading of the signal. Although spread signals seem more like noise, they still boost the background noise level and may interfere with other transmissions if no special care is taken.

Spreading The spectrum can be obtained in two distinct ways:

  1. Direct sequence spread spectrum
  2. Frequency-hopping spread spectrum

Direct sequence spread spectrum

Direct sequence spread spectrum (DSSS) sisters take a user bitstream and perform an XOR with a so-called chipping sequence. The chipping sequence consists of smaller pulses, called chips. If the chipping sequence is generated properly it appears as random noise, Thus this sequence is also sometimes called pseudo-noise sequence.

The DSSS receiver is more complicated than the transmitter. However, noise and multipath propagation need an additional tool to reconstruct the original data. The first step in the receiver includes demodulating the received signal. This is done using the same carrier as the transmitter reverse in the modulation and results in a signal with roughly the same language as the original spread spectrum signal. Additional filtering can be implemented to create the signal.

While demodulation is quite familiar from common radio receivers, the next steps develop a real challenge for DSSS receiver, contributing to the complexity of the system. The receiver has to understand the original chipping sequence, i.e., the receiver fundamentally creates the same pseudo-random sequence as the transmitter.

Frequency-hopping spread spectrum

For a frequency hopping spread spectrum (FHSS) system, the total possible bandwidth is divided into multiple channels of less bandwidth Plus guard spaces between the channels. Transmitter and receiver stand on one of these channels for a particular time and then anticipate to another channel. This system thus achieves FDM and TDM. The design of channel usage is called the hopping sequence.  FHSS comes in two variances:

  • Slow hopping
  • Fast hopping

In slow hopping, the transmitter uses one frequency for several bit periods. Slow copying systems are typically cheaper and have relaxed tolerances, but they are not as immune to narrowband interference as fast hopping systems. Slow frequency hopping is an option for GSM.

For fast hopping systems, the translator changes The frequencies several times during the transmission of a single bit. 

The receiver of an FHSS system has to know the hopping sequence and must stay synchronized. It then performs the inverse operations of the modulation to reconstruct user data. Additionally, several filters are needed.

Compared to DSSS, spending is simpler using FHSS systems. FHSS systems only use a portion of the total band at any time while DSSS systems always used the total bandwidth available. DSSS systems on the other hand are more resistant to Fading and multipath effects.


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