TDMA, FDMA, and CDMA TDMA, FDMA, and CDMA Telecomunicazioni Undergraduate course in Electrical Engineering University of Rome La Sapienza Rome, Italy 2007-2008
T ime ime D D ivision ivision M M ultiple ultiple A A ccess (TDMA) ccess (TDMA) T Each user is allowed to transmit only within specified time intervals (Time Slots). Different users transmit in differents Time Slots. When users transmit, they occupy the whole frequency bandwidth (separation among users is performed in the time domain). 2
TDMA : Frame Frame Structure Structure TDMA : TDMA requires a centralized control node, whose primary function is to transmit a periodic reference reference burst burst that defines a frame and forces a measure of synchronization of all the users. The frame so-defined is divided into time slots, and each user is assigned a Time Slot in which to transmit its information. Frame T F T S Reference Burst Time Slot 3
TDMA : Frame Structure TDMA : Frame Structure User 3 User 2 User 1 4
TDMA : guard guard times times TDMA : Since there are significant delays between users, each user receives the reference burst with a different phase, and its traffic burst is transmitted with a correspondingly different phase within the time slot. There is therefore a need for guard guard times times to take account of this uncertainty. Each Time Slot is therefore longer than the period needed for the actual traffic burst, thereby avoiding the overlap of traffic burst even in the presence of these propagation delays. without guard time with guard time misalignment misalignment 5
TDMA : preamble preamble TDMA : Since each traffic burst is transmitted independently with an uncertain phase relaive to the reference burst, there is the need for a preamble preamble at the beginning of each traffic burst. The preamble allows the receiver to acquire on top of the coarse synchronization provided by the reference burst a fine estimate of timing and carrier phase. preamble information 6
TDMA: reference reference transmitter transmitter scheme scheme TDMA: SLOW IN FAST OUT Pulse TDMA S TX S Mod Shaper coder Digital BUFFER signal f P Code Carrier generator generator 7
TDMA: a case a case study study TDMA: User s (j) (t) j ( ) ( ( ) ( ) ) � = � � j j s t a t kT 1 k k 0.8 0.6 Digital signal of user j 0.4 Sequence of equally spaced 0.2 0 binary antipodal symbols -0.2 -0.4 a k (j) : k -th binary antipodal -0.6 symbol generated by user j -0.8 T : time period between symbols -1 0 1 2 3 4 5 6 7 8 9 10 x 10 -3 8
TDMA: a case study TDMA: a case study Compressed signal The symbols of the original signal SLOW IN ( ) ( ) are organized in groups of N bps ( ) ( ) FAST OUT s j t s j t symbols. Each group is transmitted C in a single Time Slot of duration T S . Time Slots are organized in frames BUFFER of duration T F . 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 0 1 2 3 4 5 6 7 8 9 10 x 10 -3 9
TDMA: a case study TDMA: a case study ( ) ( ( ) ( ) ) � = � � j j s t a t kT k k N bps ( ) ( ) ( ) � � ( ) = � � � j j s t a t kT mT + C k mN C F m bps = k 1 T C : time interval between symbols after compression 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 0 1 2 3 4 5 6 7 8 9 10 x 10 -3 10
TDMA: a case study study TDMA: a case from the ( ) ( ) ( ) ( ) s j t TDMA s j t buffer TDMA C coder TDMA Coded Signal The position in time of each group is modified according to the TDMA code, Code which is assigned to the user. generator In other words, the TDMA code indicates which slot inside each frame must be occupied by the user. 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 0 1 2 3 4 5 6 7 8 9 10 x 10 -3 11
TDMA: a case study study TDMA: a case N bps ( ) ( ) ( ) � � ( ) = � � � j j s t a t kT mT + C k mN C F m bps = k 1 N ( ) bps ( ) � � ( ) ( ) ( ) = � � � � j j j s t a t kT c T mT + TDMA k mN C m S F m bps = k 1 c m (j) : TDMA code assigned to user j for the m -th frame 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 0 1 2 3 4 5 6 7 8 9 10 x 10 -3 12
TDMA: a case study TDMA: a case study e h r e t d ( ) Transmitted signal m ( ) o s j c t Pulse o r A S (j) TX (t) at Radio Mod f M TDMA Shaper D Frequencies T All users adopt the same carrier frequency f p for Carrier modulating the base- f P band signal generator ( ) ( ) s j t Base-band bb signal 1 0.8 100 100 0.6 0.4 50 50 0.2 0 0 0 -0.2 -0.4 -50 -50 -0.6 -0.8 -100 -100 -1 0 1 2 3 4 5 6 7 8 9 10 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 x 10 -3 13
TDMA: a case study study TDMA: a case N ( ) bps ( ) � � ( ) ( ) ( ) j = j � � � j � s t a t kT c T mT + TDMA k mN C m S F m bps = k 1 For the sake of simplifying the notation, let us consider the simple case of BPSK (in phase carrier modulation) ( ) ( ) ( ) ( ) ( ) ( ) ( ) = � � + � j j j s t 2 P s t g ( t ) sin 2 f t TX TX TDMA 0 P P TX : transmitted power g 0 (t) : energy-normalized impulse response of the f P : carrier frequency ϕ (j) : istantaneous phase Pulse Shaper. It has unitary energy. 14
TDMA: a case study study TDMA: a case ( ) ( ) ( ) ( ) s j t s j t TX RX 15 100 10 ] V [ 50 e 5 d u t i 0 0 l p m A -5 -50 -10 -100 -15 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 Time [s] Received signal after BEWARE! propagation over a At risk for multi user two-paths channel interference! 15
TDMA: a case study study TDMA: a case Received waveform Front-end filtering Front-end filtering Demodulation Demodulation .004 0.006 0.008 0.01 0.012 0.014 0.016 Time [s] Sampling Sampling Threshold detection Threshold detection Received binary antipodal signal 1 2 3 4 5 6 7 8 9 10 x 10 16
F requency requency D D ivision ivision M M ultiple ultiple A A ccess (FDMA) ccess (FDMA) F Each user transmits with no limitations in time, but using only a portion of the whole available frequency bandwidth. Different users are separated in the frequency domain. 17
FDMA vs. TDMA FDMA vs. TDMA Frequency division is very simple: all transmitters sharing the medium have output power spectra in non-overlapping bands. Many of the problems experienced in TDMA due to different propagation delays are eliminated in FDMA. The major disadvantage of FDMA is the relatively expensive and complicated bandpass filters required. TDMA is realized primarily with much cheaper logic functions. Another disadvantage of FDMA is the rather strict linearity requirement of the medium. 18
FDMA: reference reference scheme scheme FDMA: Pulse S S TX Mod Shaper Digital signal Code Carrier generator generator 19
FDMA: a case study study FDMA: a case Digital binary signal Base-band signal FDMA-coded signal ( ) ( ) ( ) ( ) ( ) ( ) s j t s j s j t t bb FDMA Generated bit stream for each user Signal after Pulse Shaping Signal after FDMA coding 60 60 1 40 40 0.5 20 20 0 0 0 -20 -20 -0.5 -40 -40 -1 -60 -60 0 5 10 0 0.005 0.01 0.015 0 0.005 0.01 0.015 x 10 -3 20
FDMA: a case study study FDMA: a case ( ) ( Digital binary signal ( ) ( ) ) � = � � j j s t a t kT k k Base-band signal ( ) ( ) ( ) ( ) ( ) = � j j s t s t g t bb 0 FDMA-coded signal ( ( ) ) ( ) ( ) ( ) ( ) ( ) ( ) = � + � + � j j j j s t 2 P s ( t ) sin 2 f c t f t FDMA TX bb P Δ f : frequency spacing between adjacent users S TX (j) (t) c (j) : FDMA code assigned to user j 21
FDMA: a case study study FDMA: a case 8 6 ] Transmitted V 4 [ e d 2 u t i 0 signal at RF l p m -2 A -4 Propagation Propagation -6 -8 0 0.005 0.01 0.015 0.02 ] Time [s] Demodulation Demodulation V Received Signal after Demodulation (Decoding) [ 40 Received (Decoding Decoding) ) e ( d 20 u t base-band 0 i l -20 p waveform m Transmitted -40 A Received -60 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 Sampling Sampling x 10 -3 Samples of the received waveform Samples at 4 2 the receiver 0 -2 output -4 -6 Threshold Threshold -4 -2 0 2 4 6 8 10 12 14 16 detection detection Received 1.5 1 binary 0.5 stream 0 -0.5 -4 -2 0 2 4 6 8 10 12 14 16 22
TDMA + FDMA TDMA + FDMA FDMA TDMA + FDMA 23
TDMA + FDMA in GSM900 standard TDMA + FDMA in GSM900 standard 24
C ode ode D D ivision ivision M M ultiple ultiple A A ccess (CDMA) ccess (CDMA) C 25
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