Chapter 7 Multiple Division Techniques 1
Outline Frequency Division Multiple Access (FDMA) Division Multiple Access (TDMA) Code Division Multiple Access (CDMA) Comparison of FDMA, TDMA, and CDMA Walsh Codes Near-far Problem Types of Interferences Analog and Digital Signals Basic Modulation Techniques Amplitude Modulation (AM) Frequency Modulation (FM) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) Quadrature Phase Shift Keying (QPSK) Quadrature Amplitude Modulation (QAM) 2
Frequency Division Multiple Access (FDMA) Frequency User n User 2 User 1 Single channel per carrier All first generation systems use FDMA 3
Division Multiple Access (TDMA) Frequency User 1 User 2 User n Multiple channels per carrier Most of second generation systems use TDMA 4
Code Division Multiple Access (CDMA) Frequency User n... User 2 User 1 Code Allowing several transmitters to send information simultaneously over a single communication channel (multiple access.) Users share bandwidth by using code sequences that are orthogonal to each other (Each user is associated with a different code) Some second generation systems use CDMA & CDMA Most of third generation systems use 5
Types of Channels Control channel Forward (Downlink) control channel Reverse (Uplink) control channel Traffic channel Forward traffic (traffic or information) channel Reverse traffic (traffic or information) channel 6
Types of Channels (Cont d) Reverse channel (Uplink) Control channels f f f 1 f 2 f n f 1 f 2 f n MS Traffic channels Forward channels (Downlink) BS 7
FDMA MS #1 MS #2 f 1 f 2 f 1 f 2 f n MS #n Reverse channels (Uplink) f n Forward channels (Downlink) BS 8
FDMA: Channel Structure Guard Band W g Sub Band W c 1 2 3 4 N Frequency Total Bandwidth W=NW c f 1 f 2 f n Reverse channels Protecting bandwidth f 1 f 2 f n Forward channels Frequency 9
TDMA Frequency f Slot Frequency f MS #1 MS #2 #1 #2 #1 #2 t t t t #2 #1 #2 #1 MS #n #n #n t t #n #n Frame Frame Frame Frame BS Reverse channels (Uplink) Forward channels (Downlink) 10
TDMA: Channel Structure f Frame Frame Frame #1 #2 #n #1 #2 #n #1 #2 #n t (a). Forward channel f Frame Frame Frame #1 #2 #n #1 #2 #n #1 #2 #n t (b). Reverse channel 11
TDMA: Frame Structure (Cont d) Frequency f = f Frame Frame #1 #2 #n #1 #2 #n #1 #2 #n #1 #2 #n Forward channel Reverse channel Forward channel Reverse channel Channels in Simplex Mode 12
TDMA: Frame Structure (Cont d) Frequency Frame Frame Frame #1 #2 #n #1 #2 #n #1 #2 #n Guard time Head Data 13
Code Division Multiple Access (CDMA) MS #1 Frequency f C 1 Frequency f C 1 MS #2 C 2 C 2 MS #n C n C n Reverse channels (Uplink) Forward channels (Downlink) BS Note: Ci x Cj = 0, i.e., Ci and Cj are orthogonal codes (Walsh Codes ) Ci x Cj = 0, i.e., Ci and Cj are orthogonal codes (Orthogonal codes have zero cross-correlation) 14
Orthogonal Codes Example. Let b = (1, -1, -1, 1) c = (-1, -1, 1, 1 c.b = 1 (-1) + (-1) (-1) +(-1) x 1+ 1 x 1 = 0 then b and c are orthogonal. Definition. A finite subset M of F is said to be an orthogonal code if any two vectors in M, called codewords, are orthogonal. Zero correlation is obtained if the product of two signals, summed over a period of time, is zero 15
Comparisons of FDMA, TDMA, and CDMA (Example) Operation FDMA TDMA CDMA Allocated Bandwidth 12.5 MHz 12.5 MHz 12.5 MHz Frequency reuse 7 7 1 Required channel BW 0.03 MHz 0.03 MHz 1.25 MHz No. of RF channels 12.5/0.03=416 12.5/0.03=416 12.5/1.25=10 Channels/cell 416/7=59 416/7=59 12.5/1.25=10 Control channels/cell 2 2 2 Usable channels/cell 57 57 8 Calls per RF channel 1 4 * 40 ** Voice channels/cell 57x1=57 57x4=228 8x40=320 Sectors/cell 3 3 3 Voice calls/sector 57/3=19 228/3=76 320 Capacity vs FDMA 1 4 16.8 Delay??? * Depends on the number of slots ** Depends on the number of codes 16
Comparisons of FDMA, TDMA, and CDMA 17
Direct Sequence Spread Spectrum for CDMA A spread spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power. Transmitter Spreading Receiver Despread Digital signal s(t) Spreading signal m(t) Digital signal s(t) Power Code c(t) Power Code c(t) Power Frequency Frequency Frequency 18
Direct Sequence Spread Spectrum for CDMA In Direct-Sequence CDMA, the user signal is multiplied by a pseudo-noise code sequence of high bandwidth. This code sequence is also called the chip sequence. The resulting coded signal is transmitted over the radio channel. 19
Concept of Frequency Hopping Spread Spectrum Transmitter Spreading Receiver Despread Digital signal Spreading signal Digital signal Hopping Pattern Hopping Pattern Power Power Power Frequency Frequency Frequency 20
An Example of Frequency Hopping Pattern Frequency 21
Walsh Codes (Orthogonal Codes) Based on Hadamard Matrix Orthogonal codes are easily generated by starting with a seed of 0, repeating the 0 horizontally and vertically, and then complementing the 1 diagonally. This process is to be continued with the newly generated block until the desired codes with the proper length are generated. Sequences created in this way are referred as Walsh code. 22
Walsh Codes 1. The code length is the size of the matrix. 2. Each row is one Walsh code of the size N, for example W0=00, W1=01 taken from 2x 2 matrix. 3. A base station can communicate on up to 64 channels. It has one pilot signal, one synch channel and 8 paging channels. The remaining are used for traffic (54). 4. The communications between the mobile and the base station takes place using specific channels. The forward channel (from base station to mobile) is made up of the following channels: Pilot channel (always uses Walsh code W0), establish the contact with the MS. Paging channel(s) (use Walsh codes W1-W7), name of the network, ring tone, message Sync channel (always uses Walsh code W32), time of the day, GPS Traffic channels (use Walsh codes W8-W31 and W33- W63) The reverse channel (from mobile to base station) is made up of the following channels: Access channel Traffic channel 23
Near-far Problem MS 2 BS MS 1 Received signal strength Distance 0 Distance MS 2 d 2 d BS 1 MS 1 24
Near-far Problem The receiver will receive more power from the nearer transmitter. This makes the farther transmitter voice more difficult to understand. Since one transmission's signal is the other's noise the signal-to-noise ratio (SNR) for the farther transmitter is much lower. The closer transmitters use less power so that the SNR for all transmitters at the receiver is roughly the same. In CDMA systems, this is commonly solved by dynamic output power adjustment of the transmitters. 25
Types of Interference in CDMA Interference baseband signals Baseband signal Frequency Spreading signal Frequency Despread signal Interference signals Frequency Interference in spread spectrum system in CDMA 26
Adjacent Channel Interference in CDMA Channel 1 Channel 2 Power f 1 f 2 Frequency 27
Power Control in CDMA Controlling transmitted power P r P t = 1 4πdf c α P t = Transmitted power P r = Received power in free space d = Distance between receiver and transmitter f = Frequency of transmission c = Speed of light α = Attenuation constant (2 to 4) 28
Modulation Why need modulation? Small antenna size Antenna size is inversely proportional to frequency e.g., 3 khz 50 km antenna 3 GHz 5 cm antenna Limits noise and interference, e.g., FM (Frequency Modulation) Multiplexing techniques, e.g., FDM, TDM, CDMA 29
Analog and Digital Signals Analog Signal (Continuous signal) Amplitude S(t) 0 Digital Signal (Discrete signal) Amplitude + 1 0 1 1 0 1 0 _ Bit 30
Hearing, Speech, and Voice-band Channels Human hearing Human speech 100 Voice-grade Telephone channel.. 10,000 Frequency (Hz) Pass band Guard band Guard band Frequency cutoff point 0 200 3,500 4,000 Frequency (Hz) 31
Amplitude Modulation (AM) Message signal x(t) Carrier signal AM signal s(t) Amplitude of carrier signal is varied as the message signal to be transmitted. Frequency of carrier signal is kept constant. 32
Frequency Modulation (FM) Message signal x(t) Carrier signal FM signal s(t) FM integrates message signal with carrier signal by varying the instantaneous frequency. Amplitude of carrier signal is kept constant. 33
Frequency Shift Keying (FSK) 1/0 represented by two different frequencies slightly offset from carrier frequency Carrier signal 1 for message signal 1 Carrier signal 2 for message signal 0 1 0 1 1 0 1 Message signal x(t) FSK signal s(t) 34
Phase Shift Keying (PSK) Use alternative sine wave phase to encode bits Carrier signal sin( 2πf c t) Carrier signal sin( 2π f c t + π ) Message signal x(t) PSK signal s(t) 1 0 1 1 0 1 35
QPSK Signal Constellation Q Q 0,1 1 0 I 1,1 0,0 I 1,0 (a) BPSK (b) QPSK 36
All Possible State Transitions in π/4 QPSK 37
Quadrature Amplitude Modulation (QAM) Combination of AM and PSK Two carriers out of phase by 90 deg are amplitude modulated Q 1000 1100 0100 0000 1001 1101 0101 0001 I 1011 1111 0111 0011 1010 1110 0110 0010 Rectangular constellation of 16QAM 38