Wireless Communication. Assoc. Prof. Hakan DOĞAN Department of Electrical and Electronics Engineering - PDF

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Wireless Communication Assoc. Prof. Hakan DOĞAN Department of Electrical and Electronics Engineering Digital Communication System Source of Information Source Encoder Channel Encoder Digital Modulator

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Wireless Communication Assoc. Prof. Hakan DOĞAN Department of Electrical and Electronics Engineering Digital Communication System Source of Information Source Encoder Channel Encoder Digital Modulator Modulator RF-Stage Channel Information Sink Source Decoder Channel Decoder Digital Demodulator Demodulator RF-Stage Sampling - the conversion of a signal from continuous to discrete in time Quantization - the conversion of the signal samples from continuous to discrete in amplitude Sampling Quantization Coding Source Coding In computer science and information theory, data compression or source coding is the process of encoding information using fewer bits (or other information-bearing units) than an uncoded representation would use, through use of specific encoding schemes. Channel Coding Channel coding introduces redundancy into the data flow in order to allow the detection or even the correction of bit errors introduced during the transmission. Automatic Repeat request (ARQ), also known as Automatic Repeat Query, is an error-control method for data transmission that uses acknowledgements (messages sent by the receiver indicating that it has correctly received a data frame or packet) and timeouts (specified periods of time allowed to elapse before an acknowledgment is to be received) to achieve reliable data transmission over an unreliable service. If the sender does not receive an acknowledgment before the timeout, it usually retransmits the frame/packet until the sender receives an acknowledgment or exceeds a predefined number of re-transmissions. Forward error correction (FEC) is a system of error control for data transmission, whereby the sender adds (carefully selected) redundant data to its messages, also known as an error-correcting code. This allows the receiver to detect and correct errors (within some bound) without the need to ask the sender for additional data. Quadrature Notation where x(t) and y(t) are real-valued baseband signals called the in-phase and quadrature components of s(t) Complex Envelope Notation s t = Re x t + jy t e +j2πf ct = Re[s l (t)e +j2πf ct ] where is the complex envelope of s(t). Magnitude and Phase FSK- Frequency shift keying: Information is transmitted by shifting between two frequencies to represent zeroes and ones. ASK- Amplitude shift keying: The amplitude of a carrier is shifted between two states to represent zeroes and ones. PSK Phase shift keying: The phase of a carrier is varied between two states to represent zeroes and ones. Binary Phase Shift Keying (BPSK) is a type of phase modulation using 2 distinct carrier phases to signal ones and zeros. BPSK is the simplest form of PSK. It uses two phases which are separated by 180 and so can also be termed 2-PSK. This modulation is the most robust of all the PSKs since it takes serious distortion to make the demodulator reach an incorrect decision. It is, however, only able to modulate at 1bit/symbol 1 S t A. sin(2 f t) 0 t T 2 c c s S t A. sin(2 f t+ ) 0 t T c c s A. sin(2 f t+ ) c c T 2 2 Ac Ts 1 1 () Es S t dt Es Es E av Es Es Please note that «cos» could also be employed. 2Eav s1( t) Ac sin(2 fct) sin(2 fct) Ts 2Eav s2( t) Ac sin(2 fct) sin(2 fct) Ts 2Eav sn( t) sin(2 fct ( n 1) ), n 1,2 Ts 2 ( t) sin(2 fct) Ts Basis Function s ( ) ( ) 1 t Eav t s ( ) ( ) 2 t E t av Receiver Design based on transmitted signals Receiver Design is simplified by the usage of basis functions. Receiver Design based on basis functions (Reduced Complexity) T s t 0 () Desicion 2 ( t) sin(2 fct) Ts Amplitude and phase can be modulated simultaneously and separately to convey more information than either method alone, but is difficult to do An easier way is to separate the original signal into a set of independent components or channels: I (In-phase) and Q (Quadrature) The I and Q components are considered orthogonal or in quadrature because they are separated by 90 degrees The I and Q components are summed in a modulator circuit 2Eav sn( t) cos(2 fct (2n 1) ), n 1,2,3,4 Ts 4 π 4 3π 4 5π 4 7π ( t) cos(2 fct) Ts 2 2 ( t) sin(2 fct) Ts It is seen that higher-order modulations exhibit higher error-rates; in exchange however they deliver a higher raw data-rate. As it can be seen, 16QAM requires only arond 19dB while 16PSK requires around 23dB of E s /N 0 at a symbol error rate of QAM -- better noise performance 16 QAM has a greater distance between adjacent points in the I - Q plane and this improves its noise immunity. As a result it can achieve the same data rate at a lower signal level. This means that the demodulator must detect both phase and amplitude. Also the fact that the amplitude varies means that a linear amplifier is required to amplify the signal. PSK has a constant envelope, which reduces the complexity of your transmitter's circuitry (especially in terms of amplifier design). The vast majority of satellite systems employ a method of phase modulation, known as PSK
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