Deck 22: Personal Communication Systems

Question 1

What is sleep mode for a cell phone, and what is the advantage of being able to use a sleep mode

Answer 1

Sleep mode is the mode when cell phone is in idle mode and waiting for either voice call or teleservice message.
In TDMA (time-division multiple access) digital control channel, there is a time slot dedicated to paging channel which helps cell phone in switching to sleep mode. To understand the sleep mode for a cell phone, monitor the indicators in the time slots used for paging and which in turn inform the phone about the broadcast information changes. Thus, as long as broadcast information remains same, the phone only have to wake up and read its paging slot and perform channel measurements. This provides an efficient sleep mode and also provides the fast response of broadcast updates. This in turn provides extend battery life.

Question 2

Why are soft handoffs not possible with TDMA or GSM

Answer 2

Handoff refers to the process of transferring an active call from one channel in a cell to another channel. Soft handoff is a technology in which two or more base stations are connected simultaneously to a mobile during a call. Soft handoff technology is used by code division multiple access (CDMA) systems. In CDMA systems all the cells uses all the frequencies due to which a mobile receives signals from one or more base station. So in that case if a mobile moves out of cell, since it receives signal from one or more base station it will just use soft handoff instead of suddenly changing frequency (handoff).
Soft Handoff is not possible with TDMA and GSM networks as all the cells in the TDMA and GSM systems do not use all the frequencies. So, mobile used in these systems do not receive more than one signal simultaneously during a call like in CDMA network. Since, the mobile do not receive more than one signal, soft handoff is not possible with TDMA and GSM networks. TDMA and GSM system uses handoff technique. Handoff is a technique of transferring continuing call from one cell site to another cell site without disconnecting it.

Question 3

(a) Calculate the length of a quarter-wave monopole antenna for the center of the cellular radio band (860 MHz) and the center of the PCS band (1910 MHz).
(b) Assuming equal transmitter power applied to both of the antennas in the previous item (a), would there be any difference in the power density generated at a receiving antenna Assume free-space propagation. Explain.
(c) If these antennas are used as receiving antennas, which of the two would produce a stronger signal at the receiver input, and by how many decibels Assume equal power density at the antenna. Explain.

Answer 3

a)
A quarter wave monopole antenna has antenna length (l) equal to
of it's wavelength ( ).
(i)
The center of cellular radio band has a frequency
.
Calculate the wavelength
.
The length of antenna is,
Thus, the length of the antenna is
.
(ii)
The center of a PCS band has a frequency
.
Calculate the wavelength
.
The length of antenna is,
Thus, the length of the antenna is
.
b)
The receiver antenna flux density or power density is,
Here,
is the Power Transmitted
is the Transmitter Gain
R is the Distance between transmitter and receiver
Thus for the same transmitted power , power density at the receiver antenna is same for both the cellular network and PCS (provided the gain of transmitters is also same).
However the actual power received varies as a square of wavelength. According to the following formula
Received Power
Here,
is the Effective area of antenna
F is the Power Density
Therefore, the formula for radiated power gets modified as follows
Here, D is the Diameter of the antenna
c)
If at the receiver flux density is same then the receiver power is directly proportional to the antenna aperture.
Received Power
And Effective area of antenna is,
.
Assume receiver power and wavelength for cellular frequency range antenna are
and
respectively.
Similarly, assume power and wavelength of PCS frequency antenna be
and
respectively.
…… (1)
Since
and
that is
, thus the antenna with cellular frequency produces a stronger signal at the receiver.
Convert equation (1) to decibels.
Thus, the power received by a cellular antenna is greater than the power received by the PCS antenna by
.

Question 4

Why won't a North American TDMA phone work in Europe

Question 5

What is the use of the Walsh codes for each of the forward and reverse links of the CDMA system

Question 6

Compare the spectral efficiency of the TDMA and GSM PCS and analog AMPS in terms of kHz used per voice channel.Proceed as follows:
(a) Take a simplistic approach to get a rough estimate. Find the width of each RF channel and the number of voice channels per RF channel. Ignore control channels. Calculate spectrum use in kHz per channel.
(b) Refine your estimate by considering the proportion of the overall bandwidth used by control channels in each system and therefore unavailable for voice. Assume a 12-cell repeating pattern with one control channel for each cell in TDMA and GSM. Also assume the use of one of the larger PCS frequency blocks. Check Chapter 21 to refresh your memory about AMPS control channels.
(c) Compare CDMA with the others. First find the bandwidth per voice channel for one cell, then take into account the gain in capacity that results from the ability to use all channels in all cells to arrive at an equivalent bandwidth per voice channel.

Question 7

The systems described in this chapter are sometimes called second-generation PCS. What was the first generation What is expected from the third generation

Question 8

Compare the number of TDMA channels on one RF channel for the TDMA and GSM systems.

Question 9

What is the use of the long code in each of the forward and reverse links in the CDMA system

Question 10

(a) Suppose that the SMSCH in a TDMA system occupies five slots in each superframe. What is the total bit rate available for short messages
(b) Suppose that five users in the cell or sector have messages addressed to them at the same time. What is the bit rate available for each user

Question 11

What frequency range is used for North American PCS How were frequencies assigned to service providers

Question 12

Compare the RF channel bandwidths for the TDMA and GSM systems. Which uses bandwidth more efficiently

Question 13

How does the CDMA system take advantage of pauses in speech to increase system capacity Explain how it is done in both forward and reverse channels.

Question 14

(a) A CDMA mobile measures the signal strength from the base as 85 dBm. What should the mobile transmitter power be set to as a first approximation (b) Once the connection is made with the previous power level, 85 dBm), the base station needs the mobile to change its power to + 5 dBm. How long will it take to make this change

Question 15

How is interference between base and mobile transmissions prevented in PCS

Question 16

Why do GSM and TDMA sometimes have to use traffic channels for control information

Question 17

Why does QPSK require linear amplifiers while GMSK does not Which PCS use each of these modulation schemes

Question 18

(a) Calculate the time for one complete repetition of the Walsh code on the forward channel.
(b) Repeat (a) for the long code. (Now do you see why it's called the long code )

Question 19

Give two other terms for the RF channel from base to mobile.

Question 20

How does GSM handle handoff requests from base to mobile

Question 21

What advantage does offset QPSK have over normal QPSK Where is offset QPSK used in PCS

Question 22

A rake receiver in a mobile receives a direct signal from a base 1 km away and a reflected signal from a building 0.5 km behind the mobile. It also receives a signal from another base station 3 krn away. See Figure 22.20 for the situation. Calculate the amount of time delay each "finger" of the receiver needs to apply.

Question 23

Give two other terms for the RF channel from mobile to base.

Question 24

What does a GSM mobile do, during a call, in those time slots when it is neither transmitting or receiving voice information

Question 25

Why is control of the mobile transmitter power more important in CDMA than in the other PCS

Question 26

As we saw earlier in this book, ordinary digital wireline telephony uses a bit rate of 64 kb/s for each voice channel. Find the ratio by which the data has been compressed in each of the digital systems discussed. Assume full-rate vocoders for TDMA and GSM.

Question 27

How is it possible for a digital system to use less bandwidth than a traditional analog FM system

Question 28

How does frequency hopping improve the reliability of a communication system in the presence of multipath interference

Question 29

Explain the difference between open-loop and closed-loop power control.

Question 30

Compare TDMA, GSM, and COMA by.finding for each (take note that there are two different modulation schemes in use):
(a) the RF channel rate in bits per second per hertz of bandwidth
(b) the RF channel rate in symbols per second per hertz of bandwidth

Question 31

What other advantages do digital systems have over analog

Question 32

Which of the three North American PCS can use frequency hopping to improve its performance

Question 33

What is a rake receiver, and what benefits does it provide

Question 34

(a) What is the difference in propagation time for signals from two mobiles, one 150m from the base, and the other 2 km away (b) How many bit periods in the RF channel correspond to this difference, for each of the three systems discussed in this chapter

Question 35

Do current PCS implementations have any disadvantages compared with analog 800-MHz cellular radio

Question 36

Describe the GSM SIM and explain how it makes traveling to other countries easier for GSM users than for those with other types of PCS phones.

Question 37

Explain the difference in the way PCS handle short messages and longer data transmissions.

Question 38

Suppose a wireless system operates at 200 kb/s using BPSK.What data rate would be possible in the same bandwidth using:
(a) QPSK
(b) 8-DPSK

Question 39

How does IS-136 PCS TDMA differ from the TDMA used at 800 MHz

Question 40

How does GSM prevent people from using the system without authorization

Question 41

What is the difference between circuit-switched and packet-switched data transmissions

Question 42

What are the meaning and function of the SCF bits in the TDMA digital control channel

Question 43

Why won't a North American GSM phone work in Europe

Question 44

Which of the personal communication systems has the highest speed for circuit-switched data

Question 45

What happens if two mobiles transmit simultaneously on the reverse control channel in the TDMA system

Question 46

How would a North American GSM user obtain service on a trip to Europe

Question 47

Why is it difficult to measure signal strength in a CDMA PCS

Question 48

What is meant by a logical channel, and how is the control channel divided into logical channels in the TDMA system

Question 49

Explain the difference between frequency-hopping and di-rect-sequence spread-spectrum systems. Which of these is used with the CDMAone personal communication system

Question 50

What are the data rates envisioned by the IMT-2000 specifications, and how do they compare with current systems

Question 51

What is the meaning of a superframe in TDMA What information must be transmitted completely in one superframe

Question 52

How does CDMA improve the frequency reuse situation compared with TDMA and GSM

Question 53

The second generation of PCS uses three incompatible standards in North America. Does the third generation improve on this situation

Question 54

What is a hyperframe in TDMA, and what is its function

Question 55

Explain what is meant by a soft handoff. What advantages does it have

Question 56

Explain time-division duplexing. How does it compare with traditional duplexing methods