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 the difference between IT and IS?

If referring to Information Technology and Information Systems, many outsiders perceive no difference. However, there is a major difference in the specific areas of concentration. Information Technology usually refers to the underlying hardware and networks used to provide data where and when nece ( Full Answer )

What is the difference between there and their?

There: is generally a place Their: is plural for third person Eg. John and Sue went enjoyed their prom night together. Eg. I went to the store, but they were out of eggs there.

What is the difference between as and has?

These two words are completely different. As is a conjunction meaning at the same time or in the same manner . He chews gum as he plays the guitar . Has is a verb denoting ownership . He has a guitar . would explain in better way?

What is the difference between 'were' and 'was'?

was is the SINGULAR past tense of the verb to be (except for you). eg I was a big baby. He was ill last year. She was a nice little girl. It was good. but not you : You were my best friend before today . were is the PLURAL past tense of the verb to be (plus you singul ( Full Answer )

What is the difference between has and have?

Has is the 3rd person singular present indicative of theverb to have. The third person in -s is almost all thatremains of inflection in what is the English verb. You have, he/she has. When using the present tense (with 'to have' as the main verb), orthe present perfect tense (with 'to have' as th ( Full Answer )

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)

Difference between that and which?

These two words can be confusing. Here's a good rule of thumb: Use "that" whenever the identity "that" is replacing is known to the speaker. Examples: I will read the book that is written by my favorite author. (not: "which is written..") All that I can see from here is beautiful. She didn ( Full Answer )

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 different basic carrier permutations between users in different cells. . Interferences within the cell are averaged by using a ( Full Answer )

Differences between had and has?

Had is used to suggest that something belonged to somebody, but it is no longer in their possesion, while has suggests that that something belongs to somebody at that time.

Explain ICI in ofdm systems?

Multi Carrier systems like OFDM has the problem of Inter Carrier Interference(ICI), which results from the loss of Orthogonality between the sub carriers.This happens when the FFT is considered over duration where the subcarrier is non integer number of cycles, which would be the case when multipath ( Full Answer )

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 changes the carrier frequency. Because a fixed frequency is not used, illegal monitoring of spread spectrum signals is extremely ( Full Answer )

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 stream that takes the full bandwidth of the channel. DSSS is the basis for CDMA cellphones and 802.11 Wi-Fi wireless transmission. ( Full Answer )

How ofdm eliminates multipath effects?

Because OFDM systems use orthogonal frequency(carrier),at the receiver the others copies from multipath has a different phases from the direct one, and by multiplying the generated carrier in the receiver with all received signal and integrated it over [-pi,+pi] only one the direct signal have (1) a ( Full Answer )

Advantage and Disadvantage of OFDM?

ADVANTAGES:. Makes efficient use of the spectrum by allowing overlap. . By dividing the channel into narrowband flat fading subchannels, OFDM is more resistant to frequency selective fading than single carrier systems are.. Eliminates ISI and IFI through use of a cyclic prefix. . Using ade ( Full Answer )

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, and to meet peak requirements for multimedia traffic. The book emphasizes the OFDM modem, air-interface, medium access-contr ( Full Answer )

What is the difference between and and an?

The word 'and' is a conjunction , a word used to join words,phrases, and sentences together. Eg. There was a black cat. It sat by the fire. Or: There was a black cat AND it sat by the fire. The word 'an' is an indefinite article . The indefinite articles are 'a' and 'an', which are place before a ( Full Answer )

What is the difference between difference and different?

Difference is used as in " There was a difference between her schedule, and mine" Different is used as is " Her schedule was different than mine" DIFFERENCE = a noun; DIFFERENT = an adjective. there's no difference its a synonym.

What is the difference between its and it?

"It" is a nominative pronoun like I, you, we, he, she and they. It is the third person singular neuter which means you use it when you are talking about a single thing which doesn't have a sex. E.g. "It is a coffee table" "It" is also the accusitive pronoun like me, you, us, her and him. E.g. "Put ( Full Answer )

What is loss of orthogonality means in OFDM technology?

In OFDM, sub-carrier spacing is maintained in such a way that the maximum of one sub-carrier occurs at the minimum of the successive sub-carrier, a loss of orthogonality results if this pattern is not achieved in the sub-carriers of OFDM transmission. Loss of orthogonality is due to ISI, ICS, Freq ( Full Answer )

The different between were and was?

They are different 'bits' of the same verb, to 'be', in the past tense. It just depends on what pronoun you are using, so: Singular. 1. I was 2. You were 3. He/She/It was Plural. 1. We were 2. You were 3. They were I was going... You were going... He was going... ( Full Answer )

What is different between was and were?

Was is the past tense of 'is' while 'were' is the past tense of'are'. Was is used when just one person is involved, for example, I was wearing a jacket when it was raining , or he waswearing a hat. Were is used when more than one person is involved, for example, we were excited about Christmas, ( Full Answer )

What is the difference between had and has?

Had is past tense while has is present tense. Examples: . The man HAD a cookie. --> meaning he was in possession of a cookie in the past, and perhaps doesn't have one anymore . The man HAS a cookie. --> meaning he currently is in the possession of one

What is the difference between Have had and had had?

This is a brief, non-technical answer; I'm just addressing the difference in meaning. Start with the verb to eat. I have eaten means that as of the time of the statement, my action of eating is completed. The last bite may still be moving down my esophagus as I say it. "I have eaten all of my vegeta ( Full Answer )

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 multiplexing, multiple signals, or carriers, are sent simultaneously over different frequencies between two points. However, ( Full Answer )

What is the different between you or you?

"You" and "you" are not different terms. You is used at the beginning of a sentence because of the capital Y. The word you is used in the middle or end of a sentence. Q.E.D.

What is the difference between them and they?

It's basically the same thing, it really depends on context inwhich you use the word "they" or "them". "They" is used when it isthe subject (e.g. they are stupid) whereas "them" is used as adirect object (e.g. I hate them). But, remember English is astrange language and there is almost an exception ( Full Answer )

What is difference between has and is?

"Has" is the third person present tense of the verb "to have", and "is" is the third person present tense of the verb "to be". Examples for have/has: "I have an idea" and "he has an idea". Examples for be/is: "I am a good student" and "she is a good student".

What are the difference between the has and have?

HAS = it's a 3rd person form (in the singular) - when the Subject is a singular noun or HE, SHE, IT, THIS, THAT, MINE, YOURS etc. HAVE = for the rest of the persons.

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 symbols using modulation technique, for example QPSK. Then the S/P converter takes the output 4 symbols and mixes each one wit ( Full Answer )

What is the different between at and in and on?

'At' is for general places: at the shop, at my house, at your party 'In' is for something that has space: in the room, in the car, in the box 'On' is for something that you can put at the top: on the table, on the ground

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 stream that takes the full bandwidth of the channel. DSSS is the basis for CDMA cellphones and 802.11 Wi-Fi wireless transmission. ( Full Answer )

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 relationship between the rectangular pulse and the sin(x)/x (sinc) function: A rectangular pulse in the time domain transforms to ( Full Answer )

What is the difference between 'you have not' and 'you do not have'?

The difference is that generally - but not always - in the construction "you have not" the word HAVE serves as an auxiliary, not as the main verb, whereas in the construction "you do not have" the word HAVE is always the main verb. For example You have not eaten your broccoli so you do not have a cl ( Full Answer )

How the OFDM signals are generated?

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What is meant by the term OFDM?

OFDM means Orthogonal frequency-division multiplexing. This is a method on encoding data which in digital television and radio. It is also used on 4G networks.

What is the difference between what and when?

What is asking about something -- what is your name When is asking about time date -- when were you born I have left the question marks(?) off the questions because Wiki answers doesn't like answers with questions

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 streams, put each of these streams on a slightly different frequency and dissemble all of it back into the high-rate stream a ( Full Answer )

What is the different between had and have?

"Had" is past tense and refers to possession or ownership in the past. "Have" is present tense and refers to possession or ownership at the present moment.

What is the difference between at and on?

Both are prepositions, but describe different locations between objects. For simplicity, I will 'X' and 'Y' for nouns. When one says, "X is at Y," it means that the two are the same location in space. For example, "My car is AT my house." This does not necessarily mean that X is within Y, but s ( Full Answer )

What is OFDM training symbol?

Orthogonal frequency-division multiplexing (OFDM) is a method ofencoding digital. Pilot signals and training symbols (preambles)may also be used for time.