Mobile Internet Technology Communication & Distributed C!O!M! S!Y!S! Presence Exercise SS 2012 24.05.2012 Name: Matr.No.: First Name: Study: Information: (Please read carefully!) • The presence exercise consists of 4 sections on 8 pages, plus 2 additional pages for notes. • On request, we supply more additional sheets of paper. Clearly attribute answers on additional sheets to questions! • Please state your name and student number on top of EVERY sheet you hand in. • Time limit: 60 minutes. Total points: 50. • You need 25 points to get the admission to the exam. • Be brief in your answers, but do not answer in single unexplained words. • Questions may be answered in either English or German. • Mobile phones and any aids are not permitted. • You have to write in blue or black color. I hereby declare that the provided answers are solely results of my own work. Signature Summary: Task 1 2 3 4 Σ Points achievable 12 13 12 13 50 Points achieved Name: Matr.No.: Solution 1 (Modulation and Bandwidth) (4+2+2+2+2) = 12 Points There are several alternatives to manipulate a carrier wave in order to transmit digital information over a radio interface. a) A combination of amplitude shift keying and phase shift keying shall be considered for a carrier wave with a constant frequency f . The following conventions are used: • The bits are coded according to the following table: Symbol Amplitude Phase Shift 000 1 0 001 2 0 010 1 pi2 (90 ◦) 011 2 pi2 100 1 pi 101 2 pi 110 1 3pi2 111 2 3pi2 • Each symbol is transmitted for 12 · 1f seconds. The following diagram shows the carrier wave s(t) = sin(2pift) for 1f seconds. Complete the diagram by the signal that represents the bit sequence 000110111001. 0 2 1 0 -1 -2 A t b) Comparing PSK and ASK for doing digital modulation, which one is preferable for data transmission? Give reasons. PSK, because it is more robust (attenuation and noise affect the amplitude of a signal). Mobile Internet Technology SS 2012 (Reference Solution) 24.05.2012 Page 1 Name: Matr.No.: c) You are asked to set up a communication system which can operate on a bandwidth of 5 MHz, and you want to use 64-QAM for modulation. What is the data rate you can achieve with this system, applying the theorem of Nyquist? Nyquist: S = 2 ·B · ld(n), thus S is 10 million symbols per second, multiplied with the number of bits per symbol. Assuming 64-QAM (n = 64) you achieve 6 bits per symbol, thus: 60 MBit/s. d) In reality, modulation is done in two steps: first a digital modulation. followed by an analog modulation step. What is the purpose of each of these steps? Simply spoken: digital modulation converts digital bits into an analog representation which can be transmitted on a restricted bandwidth. Analog modulation means bringing this information onto a cer- tain carrier frequency / channel. e) What is dynamic rate shifting and why is it used in wireless transmission systems? Choose modulation depending on signal quality. Thus, adapt the modulation (and thus the data rate) depending on the signal quality so that you still can communication even in case of much noise (even if with lower data rate); it always is offered the highest possible data rate at a certain error rate. Mobile Internet Technology SS 2012 (Reference Solution) 24.05.2012 Page 2 Name: Matr.No.: Solution 2 (Signal Propagation) (6+2+2+3) = 13 Points a) Which factors are influencing the received signal power in a wireless transmission in a free space setup? Name 3 factors and for each of them shortly state how they are influencing the received signal power. Several possible solutions - e.g. • Distance — decreases receive power proportional to square of distance between sender and re- ceiver • Antenna gain of sender — increase signal strength by focussing sending power with special hard- ware • Antenna gain of receiver — increase signal strength by combining receive powers with special hardware • Frequency — the higher the frequency, the faster the signal strength goes down with distance • Sending power — the more power the sender feeds into a signal, the more power is present on receiver side Not valid are answers like shading, reflection, diffraction etc. In free space are no obstacles, thus such effects cannot occur. b) Name and shortly describe a possibility to mitigate the effects of narrowband interference. Spreading - spread a signal over a larger bandwidth to distribute the sending power. If one small part is disturbed, the signal comes through on the rest of the bandwidth. c) What is multipath propagation and how is it caused? A transmitter radiates omnidirectional (or more or less directed). Several waves are produced, traveling in different directions. By e.g. reflection, not only the LOS wave is received but also reflected signals – we receive the same transmission several times, with phase shifts. Mobile Internet Technology SS 2012 (Reference Solution) 24.05.2012 Page 3 Name: Matr.No.: d) Shortly describe what Inter-Symbol-Interference (ISI) is. Additionally, give one possibility to protect against ISI. Different parts of the signal reach the receiver maybe at the same time, maybe at different times (due to multipath propagation, see above). ISI is interference between two neighbored symbols of a data stream due to high delay on one (or several) path. We may receive symbol a over a long path that late, that over a shorter path we already receive symbol b. So our own symbols interfere. One solution e.g. would be lowering the symbol rate and/or integrate guard times – as done by OFDM. Mobile Internet Technology SS 2012 (Reference Solution) 24.05.2012 Page 4 Name: Matr.No.: Solution 3 (Multiplexing) (4+2+4+2) = 12 Points a) Code Division Multiple Access (CDMA) can be used to coordinate the access of multiple stations to a specific frequency band. The following orthogonal chip sequences are used by stations A, B, and C to encode a single bit. Station A: (-1 -1 +1 +1 -1 -1 +1 +1) Station B: (-1 -1 -1 -1 -1 -1 -1 -1) Station C: (-1 +1 -1 +1 -1 +1 -1 +1) The stations would like to transmit the following bits: Station A: 1 Station B: nothing Station C: 0 Fill in the following table with the chip sequence transmitted by each station and with the resulting superposition: Station A: +1 +1 -1 -1 +1 +1 -1 -1 Station B: 0 0 0 0 0 0 0 0 Station C: -1 +1 -1 +1 -1 +1 -1 +1 Superposition: 0 +2 -2 0 0 +2 -2 0 . b) Additionally to the three stations A, B, and C, a forth station D should now be considered. A well selec- ted chip sequence has to be assigned to this station to ensure that the whole system remains operational. Complete the following chip sequence for station D so that it can be used together with the sequences of A, B, and C. Station D: (+1 +1 +1 +1 -1 -1 -1 -1 ) Mobile Internet Technology SS 2012 (Reference Solution) 24.05.2012 Page 5 Name: Matr.No.: c) You are asked to set up a communication system using CDMA with a chipping rate of fc = 1 MChips/s. You are using PSK for modulation. You have to choose chipping sequences for two transmissions which should achieve data rates r1 = 500 kBit/s and r2 = 250 kBit/s. Which chipping sequences do you choose? Why? To achieve 500 kBit/s with PSK, we need 500.000 symbols per second. This is half the chipping rate given in our system, thus we can encode one bit with a repetition of two chips, e.g. (1, 1). For the second data stream, we need half the data rate, i.e. twice the number of chips – here we could choose (1, -1, 1, -1). The sequences have to be orthogonal, otherwise they cannot be reconstructed correctly on receiver side. d) When using multiplexing, guard spaces are needed in between the channels formed by the multiplexing. How are guard spaces realized between users in CDMA? We need ’different enough’ codes, i.e. there are guard spaces in our code space. (Remeber: only ortho- gonal or quasi-orthogonal codes can be used, the rest of possible chipping sequences is not allowed. Mobile Internet Technology SS 2012 (Reference Solution) 24.05.2012 Page 6 Name: Matr.No.: Solution 4 (Medium Access) (3+2+3+2+3) = 13 Points a) What is the main physical reason for the failure of many MAC schemas known from wired networks when applying them to wireless networks? What is done in wired networks to avoid this effect? Not all stations are in range of all others in our network because the signal is attenuated via propagation and after some distance cannot be detected any more (resulting in the hidden station problem). So we can fail in determining if the channel is idle and start to send. In wired networks the effect is avoided by restricting the range of the network by just defining a maximum cable length – so we ensure that the signal strength is even high enough for detection at the far end of the network. b) Assume all stations can hear all other stations. One station wants to transmit and senses the carrier idle. Why can a collision still occur after the start of a transmission? Same as in wired networks: we have a propagation delay, so other stations also can sense the carrier idle because our transmission has simply not reached them yet. c) Slotted ALOHA was an early approach for medium access in wireless networks. On which reason slots are used? How are collisions detected and handled? Slots: to reduce the effect of collisions: two messages either collide completely, or not at all. An overlap of messages as with simple ALOHA is not possible, thus a higher throughput can be achieved. Each transmission has to be ACKed. A missing ACK means data loss, probably by a collision. Just retransmit after a random time. Mobile Internet Technology SS 2012 (Reference Solution) 24.05.2012 Page 7 Name: Matr.No.: d) Multiple Access with Collision Avoidance (MACA) is designed to deal with the hidden and exposed station problems. Why can MACA still fail concerning the hidden station problem? We can have asymmetric channels, new users moving in after the RTS/CTS handshake, ... e) In a MAC schema named MACA-By-Invitation (MACA-BI) only the base station initiates a communica- tion by sending a Ready to Receive (RTR) message to one sender at a time, reducing the overhead in control messages compared to MACA. What fundamental knowledge does the base station require to schedule transmissions? For which traffic patterns MACA-BI is best suited? We need to know which station has data to transmit at which point in time. Best suited for stations with regular traffic patterns (e.g. voice traffic: generation of new packets in fixed time intervals so that you need something like TDMA), so that the base station has reliable information about the demands of all stations. Mobile Internet Technology SS 2012 (Reference Solution) 24.05.2012 Page 8
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