In recent years there has been a wide increase in the number of wireless network users. As a result, applying technologies with the ability of flexible adaptation to varying networks has increased as well. Thus different Wireless Personal Area Networks -WPAN – and Wireless Local Area Networks – WLAN – are becoming more and more popular containing laptops, palmtops and other mobile units being able to use wireless technologies for data communication. Several technologies are known in the practice including Bluetooth, infrared – IR – and WLAN systems as well as ensuring data communication with large bandwidths and via special network units even connection to the internet can be realized within them.
If we take a look at the current digital trade, there is a big change from home appliances to the company trends ,all are changing from wired to wireless . Major part of these tools have integrated Wi-Fi, WLAN and/or Bluetooth capabilities. These devices apply wireless solutions for data transmission and for communication. Wireless methods widely used in the practice basically work at radio frequencies (RF) or the infrared range (IR) as transmission media. RF LAN and PAN devices typically operate with frequencies of the Industrial, Scientific and Medical (ISM) band (902-928 MHz, 2400-24083.5 MHz and 5725- 5850 MHz) Maximum emitted power allowed in the USA is 1 W, 100 mW in Europe and 10 mW in Japan.
WLANs data transmission quality and achievable data transmission date depends on several parameters. Some parameters are that they depend highly on modulation methods applied and on characteristics of the radio channel. Looking from the point of view of communication the wireless systems have special characteristics. On one hand the signal transmitted through the radio channel is exposed to disturbances of the transmission path to the most possible extent. On the other hand the air is a typical medium having different propagation characteristics in different regions. Therefore, in systems applying the air for communication, it is a strong need fitting of the transmittable signal features to the transmission media characteristics. Last but not least a most possible utilization of the transmission medium is of high importance, however the available resources of the medium are limited or sometimes technical tools supporting the utilization of the given range are not available in sufficient number and for a low price. Therefore radio channels are very bad communication media, because they change continuously and devices communicating through it are exposed to many disturbances.
The aim of the present paper is to investigate the various modulation techniques and procedures applied on communication abilities of wireless systems and on transmission parameters. The aim of digital modulation is to provide more information on capacity, compatibility with digital services, higher data security, better quality communications and quicker system availability.
The objective is to understand the modulations and multiple access techniques in use in modern mobile wireless, broadband access communications. To learn the performance of classic modulations such as QPSK, QAM, CPM, the space, time and frequency diversity of techniques of wireless.
The early methods of wireless transmissions were mostly amplitude modulated carrier signals that used conventional, or narrowband transmission techniques. This meant that the information to be transmitted modulated, or varied a radio frequency carrier signal’s amplitude, or strength at usually an audio rate.
By World War II radio communications had grown into a much more useful technology and efforts were beginning to concentrate on issues of wireless and security. A new transmission technique had been explored for several years by the Germans known as spread spectrum (Roberts, 2006). This produced a signal that was resilient from some of the issues with noise and interference that affected a single frequency and also was very difficult for the enemy to intercept or jam. Spread spectrum soon became a predominant Technology utilized in military radar and navigation as well (Roberts, 2006). Although much advancement was made in spread spectrum techniques over the years to come, Narrowband transmissions were the chosen standard for much of the radio spectrum. In the early 1980’s a unique development was eventually refined. This impacted the world much like radio and television had accomplished when they first came out earlier in the century. Cellular telephone introduced the combination of the already stable telephone network with a newer network of conventional radio systems that could work together to provide telephone access anywhere in the range of these radio systems. Even though the concept was nothing new, the implementation of a successful system was a milestone in radio and telephone history. The success of this technology was so good that the growth of cellular service to consumers filled the available radio spectrum allocated by the Federal Communications Commission (FCC) for its existence. The major problem with the cellular telephone system and its limitation on available was that two frequencies were needed for every conversation that was occurring at any given time.
There were also concerns about privacy in the cellular market that had plagued the FCC for years therefore a closer detailed look into the spread spectrum was in order. In 1990, the cellular systems finally adopted a digital answer to their problem. The purpose of digital modulation is to convert an information-bearing discrete-time symbol sequence into a continuous-time waveform.
COMMON MODULATION Techniques for Wireless
Over the past few years a major transition has occurred from simple analog Amplitude Modulation (AM) and Frequency/Phase Modulation (FM/PM) to new digital modulation techniques. Examples of digital modulation include • QPSK (Quadrature Phase Shift Keying) • FSK (Frequency Shift Keying) • MSK (Minimum Shift Keying)• QAM (Quadrature Amplitude Modulation) Another layer of complexity in many new systems is multiplexing.
Two principal types of multiplexing are TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access). These are two different ways to add diversity to signals allowing different signals
FM | AMPS
There are only three characteristics of a signal that can be changed over time: amplitude, phase, or frequency. However, phase and frequency are just different ways to view or measure the same signal change In AM. The amplitude of a high-frequency carrier signal is varied in proportion to the instantaneous amplitude of the modulating message signal.
Frequency Modulation (FM) is the most popular analog modulation technique used in mobile communications systems. In FM, the amplitude of the modulating carrier is kept constant while its frequency is varied by the modulating message signal. Amplitude and phase can be modulated simultaneously and separately, but this is difficult to generate and especially hard to detect. Instead, in practical systems the signal is separated into another set of independent components: I (In phase) and Q (Quadrature). These components are orthogonal and do not interfere with each other.
MSK (minimum-shift keying) | CT2
Minimum Shift Keying (MSK) is derived from OQPSK by replacing the rectangular
Pulse in amplitude with a half-cycle sinusoidal pulse. The MSK signal is defined as:
S(t) = d(t) cos (°t/2T) cos 2°ft + d(t) sin (°t/2T) sin 2°ft The MSK modulation makes the phase change linear and limited to ±€ (°/2) over a bit interval T. This enables MSK to provide a significant improvement over QPSK. Because of the effect of the linear phase change, the power spectral density has low side lobes that help to control adjacent-channel interference. However the main lobe becomes wider than the quadrature shift keying.
GMSK (Gaussian MSK)
In MSK we replace the rectangular pulse with a sinusoidal pulse. A Gaussian-shaped impulse response filter generates a signal with low side lobes and narrower main lobe than the rectangular pulse. Since the filter theoretically has output before input, it can only be approximated by a delayed and shaped impulse response that has a Gaussian – like shape. This modulation is called Gaussian Minimum Shift Keying (GMSK).
The relationship between the premodulation filter bandwidth, B and the bit period, T
defines the bandwidth of the system. GSM designers used a BT = 0.3 with a channel
Data rate of 270.8 kbs. This compromises between a bit error rate and an out-of-band
Interference since the narrow filter increases the inter symbol interference and reduces
the signal power.
There are two methods to generate GMSK. One is Frequency shift keyed modulation and
the other is Quadrature phase shift keyed modulation. The GMSK VCO-modulator architecture as shown in the first is simple but is not however, suitable for coherent demodulation due to component tolerance problems. This method requires that the frequency deviation factor of the VCO exactly equals 0.5, but the modulation index of conventional VCO based transmitter’s drifts over time and temperature. The implementation in the second employs a quadrature baseband process followed by a quadrature modulator. With this implementation, the modulation index can be maintained at exactly 0.5. This method is also cheaper to implement. Both methods lead to the same GMSK modulated signal
QPSK | NADC (CDMA) –
Phase Shift Keying
One of the simplest forms of digital modulation is binary or Bi-Phase Shift Keying (BPSK). One application where this is used is for deep space telemetry. The phase of a constant amplitude carrier signal moves between zero and 180 degrees. The symbol rate is one bit per symbol.
A more common type of phase modulation is Quadrature Phase Shift Keying (QPSK). It is used most commonly in applications including CDMA (Code Division Multiple Access) cellular service, wireless local loop, Iridium (a voice/data satellite system) and DVB-S (Digital Video Broadcasting – Satellite). Quadrature means that the signal shifts between phase states which are separated by 90 degrees. These points are chosen as they can be easily implemented using an I/Q modulator. Only two I values and two Q values are needed and this gives two bits per symbol.
Quadrature Amplitude Modulation
Another member of the digital modulation family is Quadrature Amplitude Modulation (QAM). QAM is used in applications including microwave digital radio, DVB-C (Digital Video Broadcasting-Cable), and modems. In 16-state Quadrature Amplitude Modulation
(16QAM), there are four I values and four Q values. This results in a total of 16 possible states for the signal. It can transition from any state to any other state at every symbol time.
DQPSK | NADC (TDMA), PDC, PHP
The second variation is differential modulation as used in differential QPSK (DQPSK) and differential 16QAM (D16QAM). Differential means that the information is not carried by the absolute state; it is carried by the transition between states. In some cases there are also restrictions on allowable transitions. This occurs in π/4 DQPSK where the carrier Trajectory does not go through the origin. A DQPSK transmission system can transition from any symbol position to any other symbol position. The π/4 DQPSK modulation format is widely used in many applications including
-NADC- IS-54 (North American digital cellular)
-PDC (Pacific Digital Cellular)
If the two bit streams I and Q are offset by a 1/2 bit interval, then the amplitude
Fluctuations are minimised since the phase never changes by 180o. This modulation
Scheme, Offset Quadrature Phase shift Keying (OQPSK) is obtained from QPSK by
Delaying the odd bit stream by half a bit interval with respect to the even bit stream. Thus the range of phase transitions is 0o and 90o (the possibility of a phase shift of
180o is eliminated) and occurs twice as often, but with half the intensity of the QPSK.
M-ary PSK (some wireless LANs)
Multi-level modulation techniques permit high data rates within fixed
Bandwidth constraints. A convenient set of signals for M-ary PSK is
M-ary PSK requires more complex equipment than BPSK signalling. Carrier
Recovery is also more complicated. The requirement that the carrier be
Recovered can be mitigated by using a comparison between the phases of two
Successive symbols. This leads to M-ary differential PSK, and is in principle
Similar to DPSK
Bit rate is the frequency of a system bit stream. Take, for example, a radio with an 8 bit sampler, sampling at 10 kHz for voice. The bit rate, the basic bit stream rate in the radio, would be eight bits multiplied by 10K samples per second or 80 Kbits per second. (Figure 10 is an example of a state diagram of a Quadrature Phase Shift Keying (QPSK) signal. The states can be mapped to zeros and ones. The symbol rate is the bit rate divided by the number of bits that can be transmitted with each symbol. If one bit is transmitted per symbol, as with
BPSK, then the symbol rate would be the same as the bit rate of 80 Kbits per second. If two bits are transmitted per symbol, as in QPSK, then the symbol rate would be half of the bit rate or 40 Kbits per second. Symbol rate is sometimes called baud rate. If more bits can be sent with each symbol, then the same amount of data can be sent in a narrower spectrum. This is why modulation formats that are more complex and use a higher number of states can send the same information over a narrower piece of the RF spectrum.
CONSTRAINS FACED by modulations
Attenuation: Attenuation has two types, the deterministic type
Open air signal propagation attenuation
The stochastic attenuation fading form
This parameter has two types as well. Delay can either be deterministic or stochastic (jitter). Jitter can cause serious problems in transmitting data of such applications through wireless networks, which services are sensible to stochastic changing of delay (typically including voice and video traffic).
Multi-path propagation: A typical phenomenon in urban environment that a receiver can receive a signal through several paths. Different signal paths have in most cases different propagation characteristics resulting an Inter symbol Interference (ISI) in the receiver.
Fading is one of the most important problems during signal transmission. Fading can depend on several parameters, in case of wireless transmission frequency selective and time selective Fading are of importance.
In case of fast moving devices (of high mobility) frequency offset can occur, possibly causing signal detection problems. Near-far effect: If more than one user is active in a wireless system and the interference or disturbing source is closer to the receiver Disturbance sources with natural origin. In most cases this can typically originate from the atmospheric sources.
Disturbance sources with artificial origin:
They can typically include two sources. One of them originates from other WLANs operating in the same geographical area and the other originates from other users of the ISM band (from medical and scientific devices, from wireless phones, wireless stereo speakers and from other similar devices and photocopiers, microwave ovens). Another problem is posed by radio amateurs possibly generating disturbances in the ISM band.
The present paper investigates the modulation procedures applied in wireless systems. Each and every modulation techniques has its own credits and limitation depending upon the purpose, applications and devices including the physical environment, that each of the investigated modulation methods can meet requirements posed by general data transfer tasks (Data over WLAN, Voice over WLAN). – Modulation procedures are adequate ensuring good protection against interference and impacts of other wireless methods operating on the given geographical region and being able to achieve high data transmission rates. Also in the modernise world the wireless technology has increased the power and the strength of the device. Each modulation method will reviews show that the end-to-end system losses, examines RF spectrum bandwidth, and discusses the spectrum improvement factor resulting from baseband filtering.
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