Direct-sequence spread spectrum in Wireless Networks is a technique that transmits a data signal over a range of frequencies, spreading it uniformly across the allocated spectrum. Direct-sequence spread spectrum is used to ensure that a particular frequency band (and its corresponding range of frequencies) is kept free from interference. This technique can be related to escaping the problem of co-channel interference (like two different wireless networks transmitting on the same frequency band) and cross-talk interference. Direct-sequence spread spectrum can also be used as an alternative approach to orthogonal frequency division multiplexing, where the baseband signal is encoded and transmitted across a quantity of fixed, predetermined channels. In this situation, each channel may carry different information, data signals, or time slots for different applications within the same network. Direct-sequence spread spectrum has also been used to transmit data that is encrypted and, in some processes, it is used to transmit non-data signals like power signaling or control signals.
Direct Sequence Spread Spectrum (DSSS) is a communication system that was developed in the 1980s. It divides the bandwidth of a radio channel into wide frequency bands and transmits these signals over separate frequencies. In this frequency-hopping process, each signal is assigned a different orthogonal sequence of frequencies.
All other radios in the range must gain each signal sequentially and then transmit it, which significantly reduces the risk of interference from outside sources or jamming. The time required for this process is proportional to the number of frequencies used for transmission. When security agencies need to be ready to communicate secretly, DSSS can be implemented so that their transmissions cannot be spied upon by other parties who are monitoring broadcasts on a shorter wavelength or through tapping devices.
For Example, the NIST specification for the Advanced Encryption Standard used in the Secure Electronic Transaction program defines a system that uses eight bits of data per transmitted symbol in an eight-bit wire transmission to transmit a 128-bit cryptographic key. A receiver would need to correlate eight different symbols to calculate a hash value. If only one of these symbols were encoded with a source-synchronous code, then the receiver would need to acquire each of eight signals in order, which would take time proportional to the number of signals. By using DSSS, however, a single signal can be transmitted that is available for correlation and decryption at any moment.
Working:
In order for a direct-sequence spread spectrum to be used in wireless networks, it is necessary that each node of the network has a frequency synthesizer. This synthesizer is useful in determining the signals that are required to be transmitted and at which frequencies these signals need to be amplified in order to ensure that interference occurs. It is also important that each node has a means of receiving signals, which is usually accomplished with a corresponding means of demodulation that can cancel out the spreading function.
- Direct-sequence spread spectrum employs error correction coding at the transmitter and the receiver. It requires an external hardware device called a correlator that can compare incoming sequences with those generated by the transmitter.
- Direct-sequence spread spectrum is a particular scheme for signal-spreading, which includes two complementary schemes called direct-sequence spread spectrum and frequency hopping spread spectrum. In the direct-sequence spread spectrum, the spreading code is contained within the transmitted signal to be used for spreading.
- The spreading code can be transmitted in multiple ways like Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), or more complex modulation schemes like Quadrature Amplitude Modulation (QAM).
- Different modulation schemes are used to select the transmit power in order to achieve certain characteristics. The higher peak-to-average ratio can be achieved with lower transmit power when using a higher order QAM modulation scheme.
Key Points:
- Suppose, a wireless base station uses a direct-sequence spread spectrum to transmit data in each frequency channel. Initially, the base station uses only 1 bit to represent each symbol because of the fact that it only transmits one bit per second. The code is known as Binary Phase Shift Keying (BPSK).
- To transmit more bits per symbol, the rate of change is made from 1 baud per second to 2 baud. To synchronize all these changes they are chosen uniformly at 0.5 baud and transmitted once every 2 seconds. This will lead to this sequence: [0,1,2,…].
- Suppose, the base station wants to transmit more bits per symbol, so the rate of change is made from 2 baud to 3 baud. To synchronize all these changes they are chosen uniformly at 0.667 baud and transmitted once every 3 seconds. This will lead to this sequence: [0,1,2,…].
- The key point here is that when the number of bits per second changes instead of changing them one by one as we did above, in a direct-sequence spread spectrum the numbers of bits per second correspond to those frequencies which are used for RF transmission.
Features and Transmission method:
A point of confusion is the difference between wideband code division multiple access (WCDMA) and wideband direct sequence spread spectrum (W-DSSS). These are two different systems in the same family but with a different methods of encoding:
- Transmission: W-DSSS is a technique for transmitting data over optical fiber. It uses one fixed code that is transmitted at all times known as the “preamble” signal. A narrow band DSSS signal is transmitted on top of this known signal using a randomly generated code
- Decoding: The receiver correlates the preamble signal with each unique code to recover an accurate clock for processing the narrow band DSSS signal. W-DSSS is the more efficient system of the two. WCDMA uses a more traditional spread spectrum technique where after some number of symbols have been transmitted, the signal is regenerated using a pseudo-random sequence. The receiver correlates this signal with a different pseudo-random sequence, which allows for jamming ability.
Uses and benefits:
- The main advantage of DSSS technology is the ability to protect signals from interference and jamming.
- DSSS is a wide-spectrum modulation technique where two (or more) data sequences are multiplied together, modulated onto a radio frequency carrier, transmitted through an antenna, and received by an antenna. The two data sequences are usually referred to as I and Q channels.
- They may be independently generated, or they may be derived from the same data sequence, e.g., I=cos(2πfct) and Q=sin(2πfct), where c is the fundamental frequency of transmission (the symbol rate). In the case of binary phase shift keying (BPSK), I and Q are the same.
- However, in quadrature amplitude modulation (QAM), I and Q carry different data (i.e., I might be a digital 0, while Q is a digital 1).
- The primary advantage of the direct-sequence spread spectrum is that security is enhanced without any additional hardware.
- Any jamming resulting from radio to radio communication through what was otherwise a secure channel, such as the ISM band, will be mitigated the moment a jammer operates in the same frequency band.
- In addition, since each symbol is encrypted with its specific code, there are no known transmission characteristics that can be used to extrapolate encoded information. The lack of correlation between symbols precludes the possibility of eavesdropping or confusing two different messages or two different recipients in real-time.
Disadvantages:
Following are some of the disadvantages of DSSS system:
- The acquisition time is too large with the serial search system. Because of that DSSS system is slow.
- The sequence which is created by PN code generator output must have a high rate. SO because of that length of the such sequence needs to be long enough to make the sequence truly random.
- DSSS system required very high bandwidth.
- But this bandwidth is less than the FSSS system.
- The variable distance between the transmitter and receiver, affect the synchronization.
