Difference between ofdm dsss fhss?

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Orthogonal Frequency Division Multiplexing (OFDM) is a multiple-carrier (MC) modulation technique which creates frequency diversity. A high-speed data stream is converted into multiple low-speed data streams via Serial-to-Parallel (S/P) conversion. Each data stream is modulated by a subcarrier. That way, instead of having a frequency-selective fading wireless channel, where each frequency component of the signal is attenuated and phase-shifted in different amount, we have multiple flat-fading subchannels. In other words, instead of having a signal with symbol duration smaller than the channel delay (remember that high frequency means low duration because f = 1/T), we have may subsignals with duration larger than the channel delay (to simplify things, consider this: if the symbol duration is 1 s and the channel delay is 10 s, we will have interference between 10 symbols). That way, channel distortion is compensated. The OFDM symbol is the composite signal formed by the sum of the subcarriers, so the data rate is still the same as if we transmit the original high-speed signal, but as we said, the channel distortion is compensated. OFDM compensates the Inter-Symbol Interference (ISI) caused by the fact that different signals take different paths and arrive at the receiver with different delay (multipaht propagation distortion). OFDM subchannels are not separated by a guard band, but they overlap. However, they are orthogonal at the subcarrier frequencies, and that way they don't interfere with each other. We have very good utilization of the available bandwidth due to the overlapping of the subchannels. Moreover, each subcarrier can be modulated seperatelly (usually, QAM or QPSK modulation is used, depending on the channel conditions, which are measuer using channel estimation via pilot carriers). Also, we can use adaptive modulation and conding (AMC) at each subchannel in order to accomplish error-free communication with the highest data-rate possible. Due to the orthogonality principle, we don't need a bank of modulators at the transmiter and a bank of demodulators/detectors at the receiver, but simply a chip implementing the Inverse Discete Fourier Transform (IDFT) and the DFT respectively - and that can be done easy, effectively and with low-cost using a chip running the Fast Fourier Transform (FFT) algorithm. Timing errors/phase distortion must be controlled because they may create ISI between OFDM symbols and ICI (Inter-Carrier Interference) between subcarriers. We add a Cyclic Prefix (CP) to avoid ISI between OFDM symbols and synchronization methods to avoid ICI. Also, the inherent Peak-to-Average Power Ratio (PAPR) of the OFDM signal must be reduced because forces the power amplifier of the transmitter to operate on the non-linear region of its characteristic function.

Spread Spectrum (SS) techniques convert a low-speed data stream into a high-speed data stream. That way, the bandwidth of the modulated carrier becomes much larger than the minimum required transmission bandwidth. This is like Frequency Modulation (FM): we trade transmission bandwidth with Signal-to-Noise (S/N) ratio, meaning that we can have error-free communication transmiting lower-power signals. The signal is spreaded in a huge bandwidth. That way, instead of having the noise (which is like an interference signal) concetrated to some symbols and corrupting the signal, the noise is uniformly distributed over the signal bandwidth. Moreover, the signal is not easilly detectable by a third-party because it is hided in the background noise. Finally, it has anti-jamming characteristics. There are two techniques to create a SS signal: Direct-Sequence Spread Spectrum (DSSS) multiplies the data stream with a high-data rate sequence called chip sequence or Pseudo-Noise (PN) sequence, because due to its lenght is seems as a random signal, like the noise (but of course, it is a completely deterministic signal; that's why we use the Greek term "Pseudo-", which means "it appears to be, but it is not"). DSSS creates time diversity (a "variety" in the time domain). Frequency-Hopping Spread Specturm (FHSS) uses a chip sequence to conrol the frequency hops of the carrier. The resulting signal is like a progressive-FM signal. FHSS creates frequency diversity (a "variety" in the frequency domain). SS techniques give a Spreading Gain (SG) to the transmitted signal, which is simillar to the Coding Gain (CG) of the error-control codes [remember: in Forward-Error Correction (FEC) techniques, we add additional bits to correct errors; we use this rendudancy for error control. That is, we increase the transmitted bandwidth but we can decrease the transmitted power required to have an acceptable S/N at the receiver). Moreover, SS techniques can be combined with multiple access techniques (patterns for multiple users access a network by sharing the common channel). With Code Division Multiple Access (CDMA) a code sequence is used to give an identity to each user, which than we will transmit a signal spreaded by a PN sequence. So, each user can use the whole available bandwidth for all the time, but users do not interfere because they are separated in the code domain. The orthogonality of the codes (of the signals) must be maintained, because otherwise we will have interference between the users. Finally, a RAKE receiver can be used to resolve the multiple paths and compensated the ISI caused due to the multipath propagation.

OFDM and SS techniques can be combined (MC-CDMA). OFDM can also be combined with the Frequency Division Multiple Access (FDMA) -> OFDMA. Finally, OFDM can be combined with Multiple Input-Multiple Output (MIMO) techniques. In MIMO, we have multiple transmiting and receiving antennas. So, we have N parallel channels instead of a single channel, and this creates a signal which is N times faster (oversimplified, but basically true ...). In each channel we can use OFDM to avoid ISI and frequency-selectivity, while maintaining the high-data rate. Finally, MIMO can create diversity which enables the system to receive the best-quality signal.
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What is 'ofdm'?

Answer . Orthogonal frequency-division multiplexing

What is DSSS Methodology?

D irect S equence S pread S pectrum. It's one of the many protocols for transmitting data over a wire (or other medium)

Advantages of ofdm?

Flexibility of deployment across various frequency bands with little needed modification to the air interface. . Averaging interferences from neighboring cells, by using di

What is fhss?

FHSS continuously changes the center frequency of a conventional carrier several times per second according to a pseudo-random set of channels, while chirp spread spectrum cha

Spread spectrum techniques fhss and dsss?

"Direct Sequence Spread Spectrum (DSSS) DSSS multiplies the data bits by a very fast pseudo-random bit pattern (PN sequence) that "spreads" the data into a large coded strea

What is SISO-OFDM?

SISO-OFDM is an OFDM system with one transmit and one receive antenna.

What is OFDM- based networks?

OFDM-based Broadband Wireless Networks covers the latest technological advances in digital broadcasting, wireless LAN, and mobile networks to achieve high spectral efficiency,

Why was OFDM selected for WiMax?

OFDM (orthogonal frequency division multiplexing) is a technique for increasing the amount of information that can be carried over a wireless network. In frequency-division

Why ifft in ofdm?

we use IFFT in OFDM to convert the signal from frequency domain to time domain the idea in OFDM generation, the transmitter accepts a stream of data and converts them to sym

Can you describe the fhss and dsss spread spectrum technologies?

"Direct Sequence Spread Spectrum (DSSS) DSSS multiplies the data bits by a very fast pseudo-random bit pattern (PN sequence) that "spreads" the data into a large coded strea
In The Difference Between

What is the main difference between fdm and ofdm?

FDM and OFDM both have the same overlap!. In FDM the overlap is in the time domain. In OFDM the overlap is in the frequency domain. First (you may already know this) the relat
In Electronics Engineering

How the OFDM signals are generated?

alam niyo kung sino man ang sumasagot katanungan na narito please stop kasi kami humihingi kami ng sagot yung tmang sagot sayang ang pera na aming ibinabayad so please stop.
In Electronics Engineering

What is difference between OFDM - orthogonal frequency division multiplexing and GMSK - gaussian minimum shift keying?

Orthogonal FDM is a method of passing digital data that uses multiple carriers. The basic idea is, you take a high-rate data stream, convert it into a number of low-rate data