Multiple Access in Mobile Networks PDF

Multiple access in mobile networks How to ensure multiple communications so that individual connections do not interfere with each other, or interfere moderately?...

Multiple access in mobile networks How to ensure multiple communications so that individual connections do not interfere with each other, or interfere moderately? 1. Packet access - the entire radio interface is allocated for single user at a time. Example of random access - ALOHA. Many resources and terminals are connected for short time. A repeated retransmission of corrupted data ensures successful communication session. 2. Parallel channel access. (a) non-overlapping or åf j ¹i j (t ) ® 0 (b) orthogonal signals f(t) are used for separate communications. Tb å ò f (t ) f j ¹i 0 i j (t )dt ® 0 0 < t < Tb Tb - symbol duration j - index of point-to-point link Tb i - useful signal index ò 0 f i (t ) f i (t )dt ¹ 0 Multiuser packet access 1. Pure Aloha access 2. Slotted Aloha access 3. Carier-sense multiple access Pure ALOHA access Slotted ALOHA access Reduces probability of collisions Time Troughput of ALOHA access schemes Parallel channel multiple access 1. Frequency Dvision Multiple Access (FDMA), when active users (i.e. after session request) are allocated different frequency carriers; 2. Time Dvision Multiple Access (TDMA), when active users using the same frequency band are allocated different time slots. A mixed access method is often used, where user groups are divided using frequency multiplexing and users in the same group are separated using TDMA. 3. Code Dvision Multiple Access (CDMA), when active users using the same frequency band are not separated over time but are assigned different codes. Again, some systems also use mixed frequency-code multiplexing.  Code FDMA TDMA Code Frequency Frequency Time Time Code CDMA Frequency Time Frequency division multiple access (FDMA) Uplink Downlink frequency band frequency band 1 2 3 4 N 1 2 3 4 N … … Guard band Frequency Bound channels Cross-channel interference c(t ) = a1 cos( 2pf 1t ) + a 2 cos( 2pf 2 t ) + a3 cos( 2pf 3 t ) Here: f1, f2, f3 - carrier frequencies, a1(t), a2(t), a3(t)- signals carrying information. At the output of nonlinear amplifier we will have the following signal ciš(t): ciš (t ) = b0 + b1c (t ) + b2 c 2 (t ) + b3 c 3 (t ) +... f 1 = 2 f1 - f 2 , f 2 = 2 f1 - f 3 f 3 = 2 f2 - f3 Time division multiple access (TDMA) 1 2 3 N User 3 3 1 2 3 N User 1 1 1 2 3 N User 2 2 TDMA frame structure Frame beginning Information Frame end bits bits Slot 1 Slot 2 Slot 3 …… Slot N Slot Sinchroniza Information Guard beginning tion bits interval bits Dynamic TDMA and FDMA access There can be dynamic TDMA or dynamic FDMA y c access (scheduled) or both (LTE case). u en eq Fr In the case of packet transmission, TDMA and FDMA are inefficient: y c en In case of TDMA, the terminal often e qu has nothing to transmit, even though Fr the slot is assigned to it In case of FDMA, it is often not c y possible to allocate a large bandwidth u en eq because then the transmitter power Fr would exceed the allowable level. Time User 1 User 2 User 3 Code division multiple access Code division multile access is closely related to spread spectrum communications In such systems, code sequence is the sign of information channel [K. D. Wong, Fundamentals of wireless communication engineering technologies. John Wiley & Sons, 2012] Code division multiple access The most widely known civilian applications are: GPS - Global Positioning System WLAN IEEE 802.11b – Wireless Local Area Networks Bluetooth- Personal Area Network- personal networks 2G CDMA Mobile Networks (e.g. IS95) 3G W-CDMA Mobile Networks (UTRA for UMTS- Terrestrial Radio Access for Universal Mobile Telecommunication System) CDMA channel capacity Gaussian channel capacity according to C.E. Shannon: C = W * log2(1+S/N)= 1.44W * ln(1+S/N) Here C - channel capacity (bps) - maximum information transmission rate in the channel, W - channel bandwidth (Hz), S/N - signal/(white Gaussian) noise ratio within bandwidth W. Let the signal bandwidth B = W and the spectral density Sf of the signal in the channel be independent of frequency. Then: C = B * 1,44 * ln(1+Sf /Nf) Here Sf /Nf - signal-to-noise spectral density ratio. If Sf > Nf (here Nf - thermal noise) in the receiver antenna. Let only one transmitter sends information to a given receiver. All other K-1 signals from the receiver are interpreted as interference. If the properties of such interference are close to Gaussian noise, we can rewrite capacity equation as follows: C = 1.44 B Sf /Nf Sf 1 C = 1,44 B = 1,44 B ( K - 1) S f K -1 C B = ( K - 1) 1 , 44 Exercise: CDMA capacity with multiple transmitters Let K = 100 transmitters work together each transmitting 9.6 kbps of data. How much spectrum does the signal need to transmit in order for transmitters could keep transmit 9.6 kbps data? Spread spectrum technologies 1. The carrier is further modulated by phase modulation in a binary sequence (code sequence) with a clock frequency much higher than the bandwidth of the information signal. Such systems are called Direct Sequence Spread Spectrum (DSSS) systems 2. The carrier frequency jumps discretely from frequency to frequency in a certain order. The frequencies and the order in which they are used are determined by a certain code sequence. Such systems are called Frequency Hopping Spread Spectrum (FHSS) systems 3. Time hopping and time-frequency hoping systems, in which the transmitter's operating time slot (usually very short) or frequency is determined by a code sequence Direct Sequence Spread Spectrum (DSSS) principles The principle of spectrum spreading: the carrier is modulated not only by the information signal, but also by the so-called code sequence of high chip rate (separating “chips” from “bits” - chips do not transmit information). The rate of the code sequence is much higher than the rate of the information signal, so the spectrum of the modulated signal is expanded. Modulation is in two stages: 1) The carrier (or code sequence) is modulated by an information signal; 2) A modulated information signal carrier modulated by a code sequence (or a harmonic carrier modulated by an already modulated information signal code sequence).  Example of DSSS system Δf fc f fc f X A(t) cos2πfct S (t ) = PN (t ) A(t ) cos2pf c t PN(t)=±1 f Transmitter fc f fc f Δf X bandpass A(t) cos2πfct filter Receiver PVN(t)=±1 f PN (t ) A(t ) cos 2pf c t × PNV (t + dt ) ¾dt¾ ¾® A(t ) cos 2pf c t ®0 DSSS system processing gain Spectrum spreading factor: k = Tb/Tc = Rc/Rb The receiver multiplier performs spectral compression only on the synchronized component of the received signal. For the asynchronous part of the received signal (noise, interference), the receiver multiplier performs a spectrum expansion. The power Pf of each asynchronous spectrum component of frequency f is located at frequencies f-Rc... f+Rc. Only the fraction of power of the unwanted signal passes through the filter: Unwanted signal fraction = Pf*Δf(signal) /Δf(chips) = Pf / k At the same time, receiver passes full power of useful signal. S/(N+I) ratio after receiver multiplier is increased k times. k is DSSS system processing gain. Conclusion: In order to disrupt the communication, the receiver needs to receive a sufficiently high interference power -- much higher than the useful signal power. DSSS system processing gain