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The network structure includes the adapters of interface of network without wire and the base stations that send and receives the signals radio. In a network without wire, the adapters of interface of network in every converted one of computer and numerical given base stations to the signals radio, that they transmit to the other devices on the same network, and they receive and convert signals radio entering other elements of network of return to the numerical data.
Connecting workstations to a local network by radio offers several advantages over connecting the same computer through a wired connection. First, wireless provides convenient access for portable computers; it's not necessary to find a cable or network data outlet. And second, it allows a user to make a connection from more than one location and to maintain a connection as the user moves from place to place. For network managers, a wireless connection makes it possible to distribute access to a network without the need to string wires or cut holes through walls. In practice, access without cables means that the owner of a laptop or other portable computer can walk into a classroom, a coffee shop, or a library and connect to the Internet by simply turning on the computer and running a communication program.
The techniques that are used to modulate numerical the news for that it can be transmitted through the microphone wave, the satellite or down below a pair of cable is different thereto analogical transmission. The data transmitted through the satellite or the microphone wave is transmitted as an analogical signal. The techniques that are used to transmit analogical signals are used to transmit numerical signals. The problem is to convert the numerical signals to a form that can be treated as an analogical signal that is then in the fitting form or to be transmitted down below a pair of twisted or applied cable to the step of RF where is modulated to a frequency that can be transmitted through the microphone wave or the satellite.
The equipment that is used to convert digital signals into analogue format is a modem. The word modem is made up of the words "modulator" and "demodulator". A modem accepts a serial data stream and converts it into an analogue format that matches the transmission medium. There are many different modulation techniques that can be utilised in a modem. These techniques are,
Amplitude shift key modulation (ASK)
Frequency shift key modulation (FSK)
Binary phase shift key modulation (BPSK)
Quadrature phase shift key modulation (QPSK)
Quadrature amplitude modulation (QAM)
Some new popular keying techniques include Gaussian minimum shift keying (GMSK) and differential quadrature phase shift keying (DQPSK). GMSK is a type of FSK modulation that uses continuous phase modulation, so it can avoid abrupt changes. It is used in GSM (Group Special Mobile) systems, and DECT (digital enhanced cordless telecommunications). DPSK is a type of phase modulation, which defines four rather than two phases. It is used in TDMA (time division multiple access) systems in the United States.
A significant drawback of traditional radio frequency (RF) systems is that they are quite vulnerable to sources of interference. Spread spectrum modulation techniques resolve the problem by spreading the information over a broad frequency range. These techniques are very resistant to interference. Spread spectrums techniques are used in code division multiple access (CDMA) systems.
Modulation Techniques and How They Work
There has been a swift move toward higher data rates in the wireless local area network (WLAN) industry. Companies are quickly developing WLAN systems that provide data rates hitting 11 Mb/s or higher. To achieve these higher speeds, direct sequence spread spectrum (DSSS) WLAN design engineers must evaluate and implement different modulation techniques. Currently, DSSS WLAN systems employ binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) modulation techniques. Although these modulation schemes are sufficient in 1 and 2 Mb/s systems, they do not meet the demands of higher data rate transmission schemes.
To develop higher data rate DSSS WLAN systems, engineers need to replace BPSK and QPSK schemes with more complex modulation techniques. Some of the most common complex techniques used in high speed DSSS WLANs include M-ary orthogonal keying (MOK), complementary code keying (CCK), cyclic code shift keying (CCSK), pulse position modulation (PPM), quadrature amplitude modulation (QAM), orthogonal code division multiplexing (OCDM), and orthogonal frequency division multiplexing (OFDM).
In short, the process of modulation is the varying in a signal or a tone called a carrier signal. Data is then added to this carrier signal in a process known as encoding. Imagine that you are singing a song. Words are written on a sheet of music. If you just read the words, your tone is soft and does not travel far. To convey the words to a large group, you use your vocal chords and modulation to send the words farther. While you are singing the song, you encode the written words into a waveform and let your vocal cords modulate it. People hear you singing and decode the words to understand the meaning of the song.
The modulation is which Wireless networks use to send data. It renders capable the sending of encoded data that use signals radio. The Wireless networks use the modulation as a wave carrier that means that the modulated tones carry data. A form of modulated wave consists in three parties:
The volume of the signal
The timing of the signal between peaks
The pitch of the signal
Wireless networks use a few different modulation techniques, including these,
Multiple Input Multiple Output (MIMO)
The sections that follow cover these modulation techniques in further details;
DSSS is the modulation technique that 802.11b devices use to send the data. In DSSS, the transmitted signal is spread across the entire frequency spectrum that is being used. For example, an access point that is transmitting on channel 1 spreads the carrier signal across the 22-MHz-wide channel range of 2.401 to 2.423 GHz. To encode data using DSSS, you use a chip sequence. A chip and a bit are essentially the same thing, but a bit represents the data, and a chip is used for the carrier encoding. Encoding is the process of transforming information from one format to another. To understand how data is encoded in a wireless network and then modulated, you must first understand chipping codes.
In BPSK, 1 bit per symbol is encoded. This is okay for lower data rates. QPSK has the capability to encode 2 bits per symbol. This doubles the data rates available in BPSK while staying within the same bandwidth. At the 2-Mbps data rate, QPSK is used with Barker encoding. At the 5.5-Mbps data rate, QPSK is also used, but the encoding is CCK-16. At the 11-Mbps data rate, QPSK is also used, but the encoding is CCK-128.
OFDM - Orthogonal Frequency Division Multiplexing
OFDM is not considered a spread spectrum technology, but it is used for modulation in wireless networks. Using OFDM, you can achieve the highest data rates with the maximum resistance to corruption of the data caused by interference. OFDM defines a number of channels in a frequency range. These channels are further divided into a larger number of small bandwidth subcarriers. The channels are 20 MHz, and the subcarriers are 300 kHz wide. OFDM provides multiple frequency channels at regular spacing, each modulated by PSK. MIL-STD 188C has specified OFDM for decades for use in wire line and radio modems. It is commonly radiated over single sideband radios since it is very tolerant to spectral notches caused by multipath fading. You end up with 52 subcarriers per channel. Each of the subcarriers has a low data rate, but the data is sent simultaneously over the subcarriers in parallel. This is how you can achieve higher data rates.
OFDM is not used in 802.11b because 802.11b devices use DSSS. 802.11g and 802.11a both used OFDM. The way they are implemented is a little different because 802.11g is designed to operate in the 2.4-MHz range along with 802.11b devices.
Some form of diversity is necessary to make OFDM work in a WLAN environment since a narrowband fade can remove one or more of the carriers. By spreading symbol energy over multiple frequencies, a robust link can be made. OFDM makes best use of the spectrum with the channel filled edge to edge somewhat uniformly. This creates the least interference between users.
Despite these advantages, there are some problems with OFDM modulation schemes in WLAN applications. The long symbols of OFDM are said to make it more multipath resistant. But, the summing of 16 independent carriers can produce large amplitude modulation. This makes the transmitter difficult to design and rules out offering limiting capabilities in the WLAN receiver. Processing is another problem in OFDM-based WLAN systems. The processing of OFDM is traditionally done with Fast Fourier transforms (FFTs) and inverse FFTs. FFTs are generally more complex and power hungry than the simple correlation techniques used by the other waveforms in high speed WLAN systems. This increases power consumption in WLAN systems, which can be costly to today's wireless manufacturers.
OFDM - Orthogonal Frequency Division Multiplexing
OFDM - for Broadband Wireless Communications
Adaptive Modulation and Coding TechniquesÂ
Physical Interfaces of IEEE 802.11 (Wi-Fi), IEEE 802.16 (Wi-Max and Mobile Wi-Max) and 3G-LTE
OFDMA as a Multiple Access Technique
SC-FDMA (Single-Carrier FDMA-3G-LTE)
MIMO is a technology that is used in the new 802.11n specification. Although at press time, the 802.11n specification had not yet been ratified by the IEEE, many vendors are already releasing products into the market that claim support for it. Here is what you need to know about it, though. A device that uses MIMO technology uses multiple antennas for receiving signals (usually two or three) in addition to multiple antennas for sending signals. MIMO technology can offer data rates higher than 100 Mbps by multiplexing data streams simultaneously in one channel. In other words, if you want data rates higher than 100-Mbps, then multiple streams are sent over a bonded channel, not just one. Using advanced signal processing, the data can be recovered after being sent on two or more spatial streams.
With the use of MIMO technology, an access point (AP) can talk to non MIMO capable devices and still offer about a 30 percent increase in performance of standard 802.11a/b/g networks.
MOK modulations techniques are well known and have been shown to be a good solution for developing high data rate WLAN systems.MOK techniques were extensively studied in the 60s in analog implementations. MOK techniques, however, did not catch on in these early analog systems because of their complexity. But now, with integrated digital implementations, engineers can effectively use MOK modulation schemes and gain the benefits of this waveform. In WLAN applications, a variation of MOK, called M-ary bi orthogonal keying (MBOK), is being implemented in higher data rate systems. MBOK allows one more bit per symbol essentially free. In addition, it lets WLAN systems provide multi-channel operation in the ISM band by virtue of keeping the total spread bandwidth the same as the existing IEEE 802.11 standard.
The MBOK spectrum is filtered to meet a spectral mask of -35 dB at +/-11 MHz and -50 dB at +/-22 MHz using a filter with a 17 MHz 3 dB. This lets WLAN systems offer three non-interfering channels in the 2.4 to 2.483 GHz ISM band while accounting for spectral energy reduction at the band edges.
In the MBOK modulation scheme, the spread function is picked from a set of M orthogonal vectors by data word. Since the in-phase (I) and quadrature (Q) channels can be considered independent when coherently processed, both can be modulated this way. Bi-orthogonal keying adds one more bit to each of the I and Q channels by using both true and inverted versions of the spread function (i.e. BPSK modulation). This allows MBOK-based systems to pack 8 bits into each symbol. To offer the same bandwidth as existing IEEE 802.11 DSSS modulation specifications, MBOK-based WLAN systems employ an 11 Mchips/s chipping rate and an increased symbol rate of 1.375 MSamples/s. This allows the WLAN to offer an overall 11 Mb/s data rate in the WLAN device while still providing interoperability with current IEEE 802.11 preamble and header specifications. In WLAN applications, MBOK modulation has also been shown to deliver slightly better Eb/N0 through its embedded coding properties. This allows the MBOK-based systems to tolerate more interference than other systems, such as BPSK- and QPSK-based devices.
MBOK modulation was promoted to the IEEE standard committee for inclusion into the standard for high-speed WLANs. The standard committee did not adopt this modulation, but did adopt a similar modulation technique known as CCK.
Jointly developed by Harris and Lucent Technologies, CCK is a form of MOK modulation where the code symbols are four phase modulated. Since CCK's symbols are QPSK in nature, they simultaneously occupy both the I and Q channels. The code set of complementary codes, however, is much richer than the set of Walsh codes, so a much higher M can be used in the M-ary process. Thus, by using a set of 64 codes, CCK-based WLANs can modulate 6 bits on the M index and 2 bits on QPSK to create an 8 b code symbol that, in effect, has 16 b of complexity. This functionality allows CCK-based WLAN systems to offer 11-Mb/s data rates.
CCK's complementary codes are created as complex symbols so that an 8 chip symbol has, in effect, 16 b of complexity. One of the main benefits of CCK is its ability to handle multipath interference. In multipath conditions, the absence of simultaneous orthogonal signals in CCK minimizes cross rail interference. This allows CCK-based devices to be less susceptible to multipath interference, which in turn allows these WLAN devices to provide better system performance.
MOK modulation can also be accomplished using a form of PPM called CCSK. CCSK is simpler to demodulate than MBOK because it only correlates on one sequence. As a result, it is starting to find strong acceptance in high-speed WLAN designs. One problem with CCSK is that it is not quite as efficient as MBOK because its symbols are not entirely orthogonal. This efficiency problem, however, can be easily handled. By using cyclically shifted Barker words, CCSK based WLAN designs can achieve the same Eb/N0 as MBOK-based approaches.
CCSK is very similar to PPM. The main difference between the two arises during the modulation process. PPM techniques modulate the whole symbol while CCSK techniques only modulate the correlation pulse. In CCSK, this results in lower amplitude modulation in the transmitted waveform and therefore a lower power amplifier cost. The main problem with any variety of PPM, including CCSK, is their susceptibility to interference. In CCSK based WLANs, multipath delay spread can be more than a chip's length, causing interference problems in the WLAN system.
PPM is another popular waveform used to increase data rates in WLAN products. By definition, PPM is a form of pulse time modulation in which the position in time of a pulse is varied.
In general, higher data rate WLAN systems employ a PPM waveform that consists of DSSS symbols with 11 chip Barker words, which are time shifted to provide up to 3 bits in the time shift. These symbols can be BPSK or QPSK modulated to give 1 or 2 more bits per symbol. In addition, when using PPM, both the I and Q channels can be modulated independently using BPSK to make a total of 8 bits per symbol. This allows PPM based WLAN systems to achieve a total bit rate of 8 Mb/s.
When using PPM, engineers must remember one important property. In PPM, adjacent symbols are overlapped or have gaps between them. This property can be troublesome in WLAN applications. It creates an amplitude modulation of 6 dB and makes the transmit power amplifier less efficient.
Quadrature modulation occurs when two carrier components, differing in phase by 90 deg., are each modulated by a different signal. QAM is a form of quadrature modulation where some form of pulse amplitude modulation is used for both inputs. Some companies have evaluated QAM for high-speed WLAN designs. But QAM has not proven to be an effective solution in these high-speed systems. QAM with spreading is straightforward in concept, but suffers from low efficiency. Since this modulation scheme requires a very clean, undistorted signal, it is very sensitive to multipath interference.
Eb/N0 performance also causes problems in QAM-based WLAN systems. The Eb/N0 performance of QAM is not as good as MOK because it has both phase and amplitude components. As a result, QAM-based WLAN systems are much more sensitive to distortion and require an equalizer to operate properly. The equalizer causes two headaches for WLAN designers. First, the equalizer requires a training sequence. This training sequence increases the length of the preamble and the complexity of the WLAN. Second, the equalizer adds cost to the overall system design. This is always a problem in the cost-sensitive wireless market. Due to these problems, QAM has not proven to be an efficient and effective solution for higher data rate WLAN systems.
Some designers have also looked at OCDM modulation for high speed WLAN systems. To increase data rates, this modulation scheme simultaneously provides multiple spread channels on the same frequency. As a result, this method sends multiple streams of data on the same orthogonal channel. OCDM based WLAN systems send multiple streams of data on the same orthogonal channel.
Sharp is using OCDM modulation for its 10 Mb/s WLAN modem. Through the OCDM modulation technique, Sharp says it has increased data rates by using CCSK Barker words for the orthogonal pseudorandom noise (PN) spread channels. In essence, Sharp's approach uses multiple CDMA channels to send more data. Golden Bridge is also using OCDM modulation techniques in its WLAN products. In Golden Bridge's WLAN devices, Walsh codes are used for signal spreading. Under this approach, 16 channels of 16 chip orthogonal symbols are BPSK modulated and summed in an analog sense. There are a few problems with Golden Bridge's approach. First, OCDM produces a high degree of amplitude modulation as it sums 16 independent channels, forcing the engineer to implement a very linear, and very costly, power amplifier to meet the spectral mask. Second, OCDM draws more power and eliminates the ability to provide limiting capabilities in the receiver.
802.11's Modulation Techniques
The original 802.11 standard specified two different spread spectrum transmission techniques: DSSS and FHSS. All radio equipment use the 2.4 GHz ISM band, and systems based on the original 802.11 standard provide data rates up to 2 Mbps. This is possible because DSSS utilizes an 11 bit chipping code called the Barker Sequence for signal spreading with modulation being achieved using either binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK) techniques. (For FHSS, a modulation technique called Gaussian frequency shift keying or GFSK is employed.) Furthermore, in the U.S. DSSS deployments provide 11 independent channels by using different predefined chipping codes. (FHSS based implementations provide for 78 different logical channels through different hopping patterns, although in reality fewer channels would be actually usable due to frequency separation requirements.)
FHSS was dropped from the 802.11b specification because it was felt that "direct spread" could handle the tradeoffs between wireless devices coexisting with other users, while extracting the greatest capacity from systems that are both power and band limited. FCC 01 158 Amendment of Part 15 of the Commission's Rules Regarding Spread Spectrum Devices, Wi LAN, Inc. et al. that it would consider relaxing the spread spectrum requirement on the ISM band in order to abandon the peaceful "coexistence of equipment" requirement in favour of support for greater wireless network capacity. Therefore, for high bit rates above 2 Mbps 802.11b's purely spread spectrum techniques have been supplanted by CCK modulation so as to provide 4 or 8 bits per transmission symbol. The combination of QPSK and CCK is what enables 802.11b's maximum data rate of 11 Mbps. Lower data rates are accommodated through a dynamic rate shifting scheme. Also, the reader should note that 802.11g supports CCK modulation so as to provide backwards compatibility with 802.11b.