IT351 - Mobile & Wireless Computing
Tutorial_2
1. Wireless Communications have to tackle many problems at the physical layer
due to the nature of the media (spectrum)
a. Modulation techniques can help to overcome interference. Draw the
graphs for the number 001101 for ASK, FSK, and PSK
b. Why is digital modulation not enough for radio transmission? What are
the general goals of digital modulation? What are typical schemes?
2. Digitised voice is generated by a PCM codec. The sampling rate is 8000
samples/sec. Each sample consists of 8 bits. What is the data rate required for 1
voice channel? How many voice channels can be multiplexed on a 1.54 Mbps
line (ignore required guard)?
3. Motion video: NTSC; 640 pixels/line, 525 lines, 8 bits/pixel, 30 frames/sec.
What is the bandwidth required?
4. Ten signals, each requiring 4000 Hz, are multiplexed on to a single channel
using FDM. How much minimum bandwidth is required for the multiplexed
channel? Assume that the guard bands are 400 Hz wide.
5. Provide a comparison of DCA, FCA and HCA and consider a scenario where
each might be appropriate.
6. What limits the number of simultaneous users in a TDM/ FDM system
compared to a CDM system? How does an additional user influence the other
users in the both systems?
7. What are the means to mitigate narrowband interference? What is the
complexity of the different solutions
8. Frequency hopping is a technique widely used for transmission of data in
wireless systems, such as Bluetooth and Wireless LANs.
i)
Explain the technical concepts of frequency hopping.
ii)
Highlight the advantages of using frequency hopping in
wireless communications systems by comparing it to other
spread spectrum techniques such as DSSS.
9. Assume that we have several signals that are multiplexed using FDM with
guard space between them. If these signals are spreaded using DSSS OR
frequency hopping, what replaces the guard space? How can DSSS systems
benefit from multipath propagation?
10. What are the main reasons for using cellular systems? How is SDM typically
realized and combined with FDM? How does DCA influence the frequencies
available in other cells?
Homework:
1) Consider a wireless channel of Bandwidth 1Mhz. This channel is to be
shared by 100 digital mobile stations. Two multiplexing schemes are being
considered: Frequency Division Multiplexing (FDM), and Code Division
Multiplexing (CDMA) with 10 chips per bit. What would be the capacity
gain obtained by using CDM instead of FDM? Explain your answer.
2) Communication in a wired medium is different from that in wireless
medium. Justify
Answers
1-a) Amplitude Shift Keying (ASK), below, applies two different amplitudes, one
each to 0 and1.
Frequency Shift Keying (FSK) applies a different frequency, one each to 0 and 1.
Phase Shift Keying (PSK), below, applies a shift phase to each new bit. A phase
shift is a realignment of the starting position of the wave.
1-b) Worldwide regulation always uses FDM for separating different systems (TV,
WLAN, radio, satellite, …). Thus, all radio systems must modulate the digital signal
onto a carrier frequency using analogue modulation. The most prominent system is
the traditional radio: all music and voice use frequencies between, e.g., 10 Hz and 22
kHz. However, many different radio stations want to transmit at the same time.
Therefore, all the original signals (which use the same frequency range) must be
modulated onto different carrier frequencies. Other reasons and motivations for
analogue modulation are antenna and medium characteristics.
The goals for digital modulation are to achieve spectral efficiency, power efficiency
and robustness. Typical schemes are ASK, PSK, FSK.
2) Data rate required = 8000*8 = 64 Kbps
Number if voice channels = 1.54 * 106 / 64 * 103 = 24 channels
3) Required bandwidth = 640 * 525 * 8 * 30 = 80.64 Mbps
4) Required bandwidth = 10 * 4000 + 9 * 400 = 43.6 K Hz
5) Fixed Channel Assignment is the simplest case. Channels are allocated in advance
irrespective to traffic in the network. Once channels are assigned they remain
assigned until reassigned by FCA. Supply tries to meet demand in advance.
Dynamic Channel Assignment is the most complex case. Channels are assigned
according to the demand in the network. Once a channel is assigned it can be
reassigned by DCA as needed. Supply tries to meet demand as it happens.
Hybrid Channel Assignment is a compromise between the complexity (but
flexibility) of DCA and the simplicity (but inflexibility) of FCA. Some of the
channels are assigned in a fixed way with a pool of channels to be assigned using
DCA.
FCA is most useful where the predictability of the traffic is at its greatest, or where
over-provisioning is possible, for example in a Wireless LAN. DCA is most powerful
in networks that have highly unpredictable traffic patterns, or where resources are
extremely scarce, such as in city centres. HCA is a compromise situation so is suitable
for such scenarios, but mostly it is suitable where the complexity of system
management dictates that DCA is impractical to implement reliably.
6) TDM/FDM-systems have a hard upper limit of simultaneous users. The system
assigns a certain time-slot at a certain frequency to a user. If all time-slots at all
frequencies are occupied no more users can be accepted. Compared to this “hard
capacity” a CDM system has a so-called “soft-capacity” (compare filling a box with
bricks or tissues). For CDM systems the signal-to-noise-ratio typically limits the
number of simultaneous users. The system can always accept an additional user.
However, the noise level may then increase above a certain threshold where
transmission is impossible.
In TDM/FDM systems additional users, if accepted, do not influence other users as
users are separated in time and frequency (well, there is some interference; however,
this can be neglected in this context). In CDM systems each additional user decreases
transmission quality of all other users (the space for the tissues in the box gets
tighter).
7) Several mechanisms exist to mitigate narrowband interference (which might be
caused by other senders, too):
- Dynamic Frequency Selection: Senders can sense the medium for interference
and choose a frequency range with lower/no interference. HiperLAN2 and 802.11h
use this scheme. Network operators can also this scheme to dynamically assign
frequencies to cells in mobile phone systems. DFS has a relatively low complexity.
- Frequency hopping: Slow frequency hopping (several symbols per frequency)
may avoid frequencies with interference most of the time with a certain probability.
This scheme may be used in GSM. Furthermore, wireless systems can use this
principle for multiplexing as it is done in Bluetooth systems (still slow hopping as
Bluetooth sends many symbols, indeed a whole packet, on the same frequency).
Fast hopping schemes transmit a symbol over several frequencies, thus creating
a spread spectrum. FH systems have medium complexity. Main topic is
synchronisation of the devices.
- Direct sequence spread spectrum: Data is XORed with a chipping sequence
resulting in a spread signal. This is done in all CDMA systems, but also in WLANs
using, e.g., Barker sequences for spreading (e.g., 802.11b). The signal is spread
over a large spectrum and, thus, narrowband interference only destroys a small
fraction of the signal. This scheme is very powerful, but requires more powerful
receivers to extract the original signal from the mixture of spread signals.
8) Frequency hopping is a technique widely used with wireless transmission. It is
based on the fact that a transmitting device changes the frequency of the carrier signal
regularly, thus hopping from frequency to frequency.
Frequency hopping is used to overcome some of the inherent problems associated
with wireless transmission:
• It helps prevent multipath interference, as when the reflected signals arrive
at the receiver, this may already have changed to a new frequency and will
not detect the late signal;
• It helps to provide security of the transmission, as someone eavesdropping
or trying to jam the signal would need know not only the transmission
bands, but the hoping sequence used;
• It can act as a form of CDMA, as devices hopping according to different
sequences are unlikely to interfere with each other. This is used in the
Bluetooth system.
• Comparing to DSSS, frequency hopping is easier to implement and use
only a small portion of the spectrum at any time. However, DSSS is more
robust and provides better secutity
8) Guard spaces are now the orthogonality of the chipping sequences or hopping
patterns. The higher the orthogonality , the lower the correlation of spread signals or
the lower the collision probability of frequency hopping systems. DSSS system
typically use rake receivers that recombine signals travelling along different paths.
Recombination results in a stronger signal compared to the strongest signal only.
10) The main reason is the support of more users. Cellular systems reuse spectrum
according to certain patterns. Each cell can support a maximum number of users.
Using more cells thus results in a higher number of users per km². Additionally, using
cells may support user localisation and location based services. Smaller cells also
allow for less transmission power (thus less radiation!), longer runtime for mobile
systems, less delay between sender and receiver. Well, the downside is the
tremendous amount of money needed to set-up an infrastructure with many cells.
Typically, each cell holds a certain number of frequency bands. Neighbouring cells
are not allowed to use the same frequencies. According to certain patterns (7 cluster
etc.) cellular systems reuse frequencies. If the system dynamically allocates
frequencies depending on the current load, it can react upon sudden increase in
traffic by borrowing capacity from other cells. However, the “borrowed” frequency
must then be blocked in neighbouring cells.