Slide 1:Tutorial 1 Signal Classification Modulation Multiple Access Communications Duplexing technique Switching method Mobile Generation Cellular Concepts The Handoff problem GSM 900 band GSM: Global System for mobile communications 2009


Slide 2:Signals Signals May be Baseband: The baseband signal is a signal that is not modulated, this signal is usually a low frequency signal Passband: The Passband signal is a signal that is applied to a modulation technique, this frequency usually has a high frequency Another Classification Analog Signal: The Analog signal is a signal that can take any value at any time instance (CT) or at discrete time instants (DT) Digital Signal: The Digital signal is a signal that takes discrete values at discrete time instants Signals


Slide 3:Modulation Modulation: Applying a signal to a mixer (an electronic device) that is multiplied by a carrier signal (a sinusoidal), thus allowing a frequency shift to a higher frequency Modulation is divided to: Analog Modulation technique These are AM (Amplitude modulation), FM (Frequency modulation), and PM (Phase modulation) Digital Modulation technique They are Shift Keying modulation. Digital modulation needs double bandwidth of analog modulation and may encounter quantization errors Modulation Signal Carrier mixer Modulated Signal


Slide 4:Multiple access technique Types of communications: Channel Communications: Only one user per resource, this means that no access by more than user for a resource Multiple access Communications: They are methods to apply multiple access for users on the available resources Types of multiple access: 1 – FDMA: Frequency division multiple access, this means that the frequency resource is divided among users, each user has a certain band of frequencies of the available band Multiple access Band width divided among users f t The band available for the users all the time


Slide 5:Multiple access technique 2 – TDMA: Time division multiple access, this means that the time resource is divided among users, each user has all frequencies at the available period of time Multiple access All frequencies are available for users at certain periods of time t f f t The GSM uses a hybrid system mixing both FDMA and TDMA. A certain frequency band is available for users at certain periods of time, the unit is a time slot Time Slot f t


Slide 6:Multiple access technique 3 – CDMA: Code division multiple access, Same time and frequency, all band available at all the time, but each user has a code to receive the signal. The Signal is then multiplied by that code to be decoded and encoded. Multiple access Data (Low rate of change is low frequency) Code (High rate of change is High frequency) The Rate is the time dedicated for a single bit The Process of Multiplying the data by a code leads to the Spreading, which enlarges the band width


Slide 7:Multiple access technique Spreading: To hide the signal in order to not be detected by frequency synthesizers by portioning the signal among more frequencies (wider band width corresponding to high time domain rate). Spreading characterizes CDMA. Multiple access Code Coding (Spreading) Code decoding (despreading) Transmitter Receiver


Slide 8:Duplexing technique Duplexing technique The Duplexing technique is the way of dividing the resources between the sender and receiver. FDD TDD Frequency Division Duplex Time Division Duplex The frequency of sending is not like the frequency of receiving. Uplink frequency is not equal to the downlink frequency. The Uplink frequency is the same as the downlink frequency but sending and receiving is at different time instances. The wireless technologies need to operate at the pass-band and not in the base-band, because the pass-band is of higher frequencies and so the lengths of the antennas become small (low profile antenna), as the length of antennas is directly proportional to the wave length


Slide 9:Switching technique Switching technique The Switching technique decides the method of pricing (by time or by volume) Circuit Switching Packet Switching Allocating a whole path between the source and destination, Like phone calls, where the pricing is by time of usage Packets are ordered and sent in different non-fixed paths, pricing is by number of packets used GSM uses Circuit Switching