Spread Spectrum Technology DSSS FHSS OFDM
Spread spectrum techniques involve spreading the bandwidth needed to transmit the data.
Spreading the bandwidth has several advantages.
The sender spreads the signal means converts the narrowband signal into a broadband signal.
The energy needed to transmit the signal is the same, but it is now spread over a comparatively larger frequency range.
The power level of the spread signal can be lower than that of the original narrowband signal without any loss of data.
Depending on the generation and reception of the spread signal, the power of the user signal can be as low as the background noise there.
It makes now difficult to differentiate between the user signal and the background noise and therefore difficult to detect the original signal.
What is spread spectrum
During the transmission process, narrowband and broadband interference added to the signal level.
The sum of interference and the original signal is obtained and received. The receiver knows how to dispreads the signal. The receiver converts the spread original signal into a narrowband signal again, while spreading the narrowband noise interference and leaving the broadband interference.
The receiver node applies a bandpass filter to cut off frequencies of left and right narrowband signals.
In the end, the receiver restructures the original data because the power level of the original signal is high, which means the signal is much stronger than the remaining noise interferences.
Spreading the spectrum is done via two ways as shown in the following two sections.
What is a direct sequence spread spectrum
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Direct sequence spread spectrum (DSSS)
Direct sequence spread spectrum (DSSS) system takes a user bitstream and performs an (XOR) function. This process is called chipping sequence as shown in the following figure –
The result is either the sequence of – 0110101 (if user bit equals to 0) or its complement 1001010 (if the user bit equals to 1).
The DSSS receiver process is more complex than the transmitter process. The receiver only performs the inverse functions of the two transmitter modulations steps.
However, noise and multi-path propagation processes need some additional processes to rebuild the original data signals.
The first step in the receiver side is demodulating the received signal. This is done by using the same carrier as the transmitter reversing the modulation and results in a signal with approximately the same bandwidth as the original spread spectrum signal. Some more filtering process can be applied to produce this signal.
What is Frequency hopping spread spectrum
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Frequency-hopping spread spectrum
For frequency hopping spread spectrum (FHSS) process, the total present Bandwidth is split into many channels of smaller bandwidth and guard spaces between the channels.
Transmitter and receiver stay on one of these channels for a definite time and then hop to the channel.
This process uses FDM and TD techniques.
The pattern of channel usage is called the hopping sequence, and the time spend on a channel with a particular frequency is called the dwell time.
FHSS comes in two variations that are, slow hopping and fast hopping
What is Orthogonal frequency division multiplexing
Introduction to 802.11a OFDM
Orthogonal frequency division multiplexing (OFDM) is a technique of digital signal modulation. In this technique, a single data stream is split across several separate narrowband channels at different frequencies in order to reduce interferences and crosstalk.
The original data stream bits sent serially (one after the other) and they are transmitted in parallel means several at once on separate channels, but at a slower speed, each sub-streams relative to the original signal.
It means symbols sent in the sub-streams are comparatively longer and spaced farther apart.
In the original stream, each bit might be represented by a 1-nanosecond (ns) segment of the signal, with 0.25 ns spacing between bits.
Splitting the signal across the 4 component streams allows each bit to be represented by 4 ns of the signal with 1ns spacing between bits.
This reduces interference among symbols and makes it easy to receive each symbol correctly and also maintaining the same throughput.
OFDM technology
OFDM technology was first implemented in the 1960s and 1970s during the research in order to minimize the interference or noise among the channels that are near to each other in frequency.
It is applied to achieve accurate data transmission in situations that are prone to interferences and signal corruption when more conventional modulation schemes are used.
- OFDM is used in the following-
- Wi-Fi,
- DSL internet access,
- 4G wireless communications,
- Digital television and radio broadcast services
The 802.11a Wireless Local Area Network (LAN) revision to the original 802.11 standard was improved in 1999.
The 802.11a Wireless LAN standard uses the same core protocol as the original standard.
It operates in a 5 GHz band and uses a 52-subcarrier orthogonal frequency division multiplexing (OFDM) system with a maximum raw data rate of 54 Mbit/s.