1. In a typical cellular network there are 3 major elements in the cellular infrastructure [note, in some cellular standards these terms are a little bit different, and sometimes two of the three are combined into one. Just answer for the typical cellular terminology – no mention needed of which cellular systems use them]: the basestation (BS), the basestation controller (BSC), and the mobile switching center (MSC). Sometimes the BSC and BS are combined into a Base Transceiver Station or equivalent terminology. Answer the following 2 questions (i and ii), without using graphics, use just words.
i)Briefly describe the key functions of the MSC, and WHAT does it connect to?
ii)Briefly describe the key functions of the BS, and HOW does it connect to the mobiles?
b) Briefly describe the process/technique for decoding an FEC convolutional code, and explain what is meant by saying that the coding gain of a code is 13 dB.
c) Imagine a cellular system which uses TDMA (and which is not an existing or past system but it is a possible TDMA system architecture) using a 200 KHz bandwidth carrier, and carrying 8 physical channels in that carrier. i) What defines A CHANNEL, and ii) how much bandwidth does each channel occupy AND why?
d) For a QPSK signal received with an SNR (signal to noise ratio) of 13 dB, using 1 MHz bandwidth and carrying 2 Mbps, calculate i) it’s Eb/No, and ii) the BER that is achieved (assume no error correction is being used, so it’s simply the modulation BER performance. Also assume perfect filtering of the QPSK signal).
2. Modulation is a function implemented in wireless communications systems
a. Define what is meant by power efficiency and spectral efficiency for a modulation scheme? What is the actual entity measured to specify the two (its physical or mathematical definition), with the units used for the spectral efficiency?
b. Order the following modulation types in terms of spectral efficiency (best to worse) and explain why (be specific by showing a number for each representing the spectral efficiency, and how you got it. The correct explanation is what is being asked for here – a simple ordering of best to worse without a correct explanation will get you no points): QPSK, 8-PSK, 64-QAM, and BPSK. You can assume that r=0, ie, perfect filtering.
c. Describe the OFDM concept (what is it comprised of and how the signal is generated), explain what is technically important about orthogonality in the OFDM concept, and what makes OFDM better able to work well for higher bandwidths?
3. Answer parts a and b
a) Select which two statements ARE FALSE. No need to explain here.
(1) For antennas used at lower frequencies to have the same gain as those used at higher frequencies, the lower frequency antennas generally have to be bigger.
(2) Antennas can have omnidirectional radiation patterns or they can have directional beams in their patterns.
(3) In a cellular architecture a forward link is the wireless link from the cell phone to the basestation, and the reverse link is the other way around.
(4) ARQ is an error control technique that uses detections of bit errors, and requests retransmissions of frames with erroneous bits.
(5) Free space loss is wireless propagation loss in a link where there are free O2 (ie, oxygen) molecules absorbing energy from the propagating signal, and thus further attenuating the signal.
WHICH TWO ARE FALSE?
b) Select which three (3) statements ARE TRUE. No need to explain here.
(1) 3G cellular systems use various forms of spread spectrum modulation and multiple access based on the spread spectrum codes.
(2) FEC can correct many erroneously received bits in wireless
(3) At cellular frequencies the main source of RF noise in the receiver of a cell phone is a combination of atmospheric noise and other people nearby talking on their desk phones.
(4) The Shannon’s capacity formula says that under certain conditions (such as when the noise is white Gaussian) the maximum capacity of a channel is proportional to that channel’s bandwidth.
(5) Mechanisms provided by cellular architectures to handle user or cell phone mobility requires having the basestations know when a cell phone is inside its coverage area. This is done by the basestations using GPS to obtain the location of each cell phone.
WHICH THREE ARE TRUE?
4. On propagation, multipath and other wireless effects and techniques
a) Propagation in a wireless channel is sometimes modeled using the propagation index n. Which of the following three statements is/are TRUE, and which is/are FALSE. NO need to explain here, it is a TRUE or FALSE question for each.
i) The propagation index n varies as function of the physical environment for the path, and the frequency. The index n is a function of the matter that make up the path, the objects, gases or liquids in the path, or pure vacuum, and it is then typically higher if there is more solid material (more objects in between), such as buildings, hills or parts of them, vegetation and trees, and other natural and manmade objects.
ii) The propagation index depends on the transmit power.
iii) The signal will travel further for a propagation index of 4 than for a propagation index of 3, for the same frequency, transmit power levels, and antenna gains.
WHICH IS TRUE AND WHICH IS FALSE IN EACH PART OF a)?
b)
i) describe fast fading in multipath,
and ii) describe in detail (ie, not just mention, say what it is and how/why it works) ONE technique to counter it. [Mentioning a number of techniques without explaining how/why one works will get you no points]
5. a) Consider a direct sequence spread spectrum (DSSS) system attempting to transfer data. The DSSS system has a chipping rate of 2 Mcps (2 million chips per sec). A data source provides a 20 Kbps data rate (20 thousand bits/sec) input into that DSSS system, which spreads the signal and transmits it.
i) Calculate the processing gain being used. Show each of your steps;
and ii) If you needed to increase the processing gain for the system, while supporting the same source, how would you do it?
b) Describe/explain i) what are spread spectrum orthogonal codes, ii) why they are useful, and iii) why sometimes those signals with orthogonal codes transmitted do not stay orthogonal?
