Digitalization and Multiplexing Techniques in Telecommunications
1. Digitalization of Telephone Channel, Quantization Distortion (Noise)
Telephone channel, unidirectional path from 300 to 3400 BW:3100Hz. Its digitalization with PCM(pulse code mod), if adds TDM(time div mult), it increments the efficiency utilization of the transmission paths. PCM converts analogue samples to digital forms (increments noise immunity transm). 3 ops(A/D converter) to analogue signal before mult & transm: 1-Sampling(signal discrete in time, Fsampling=1/Ts, sampling theorem Fmax = Fs/2, for the phone fmax 3400, Fs = 8000 Hz(with some reserve), 2-quantization (signal discrete in time & amplitude) 3-coding(to binary). Emisor multiplexes digital signal, transports, demu in the receptor and D/A conv(or PCM decoder) restores converted signal. Digitalization generates distorsions in orig signal- quantization distortion (key quality parameter in digital transm of analogue signals) or quantiz noise (frequency spectrum of residual signal is distributed equally in the whole band of transmitted frequencies).
2. Nonlinear Quantization- Compression Characteristics A and Mu Law
For transm of signals with lower level in sufficient quality, non-uniform division of quantization levels is used. 2 types of compression: Mu-law. PCM 24-digital signal of 1order(DS1), USA and Japan. A-law. PCM 30/32-digital signal of 1order(E1), Europe. Quantization and coding from 12 to 8 bits. Transmission rate of sampling freq & length of a code word after compression, vp=N·fs = 8·8K = 64kbit/s.
4. Utilization of CRC-4 in Connection with PCM Signals
CRC-4 protection method is based on cyclic coding scheme according to ITU-T,G.706, gradually transmission of 4 bits control group. Each block of information elements(k-bits) is supplemented with control elements(r-bits), final group of n = (k + r) elements create the protected transmitted signal. This method goes in TS0. Information pattern A(D). Protection patt– R(D) remainder 2^r*A(D)/G(D). r ceros tras A y div por G (1/1 0 0/1 1 1/0 1).Generating patt G(D). Protected patt F(D)
5. Methods Used for Multiplexing Signals
Telecomm systems support many functions – one is mult signals efficiently before their transm over the path and adaptation of aggregated signals for trans over specific media. There’re multiple utilizations of transm paths: –FDM (frec division multiplex) Each signal is modulated to a certain frequency (Carrier); this way the different signals are bundled and sent at the same time independently of each other. It also inserts some protection bands in every signal to avoid overlapping. To demultiplex them we have to filter them properly. A major advantage is the independence between signals. Method used in CATV or ADSL/VDSL. –TDM (time) Data of different channels is sent in certain timeslots (Δt) over a single line. 2 options– STD (sync time division), each input channel always gets a timeslot. ATD(asynchronous) the time-slots are assigned only when it’s necessary, time-slots carry extra info about the channel also called (Statistical TDM). This method allows trans of analogue samples or processed samples by an impulse modulation (PAM, PPM, PWM). Used in cable and wireless apps, e.g.PCM 30/32. –WDM (wavelength) uses the same principal as FDM, but is used in optical signals or paths. 2 options available– DWDM (Dense WDM) – spacing carriers under 1 nm. CWDM (Coarse WDM) – spacing over 10 nm –CDM (code) makes use of certain properties of a suitably constructed code. Used in modern wireless systs, e.g. 3G and Wi-Fi.
6. Plesiochronous Digital Hierarchy PDH-Characteristics, Stuffing Utilization
-Multiplexed signals don’t have defined fixed time position — asynchronous multiplexing. -Structure of higher-level signal contains a reserve used for compensation of lower level signal transmission rate deviation. -PDH interlaces several multiplexed signals freely, bit by bit, to a frame of the higher-level signal, but there isnt any definite relation between the frames of the lower and of the higher-level signal. -Possibility to restore lower-level signals through sequential demultiplexing from higher-level signal, may cause signal degradation if its repeated. In PDH signal we can found some reserves used for transm of signals with slightly higher rate, method called stuffing. 3 types: positive: transmission rate of higher-level signal is higher than the multiple of lower-level signals. Negative(Idem lower). Assymetric(combination of previous).