Ec 2401 wireless communication unit 4

1. Unit - 4
SIGNAL PROCESSING IN WIRELESS
SYSTEMS
Prepared by
JAI GANESH S
Asst.Professor - ECE

2. Syllabus
• Diversity
– Principles
– Macro Diversity
– Micro Diversity
– Signal Combining Techniques
– Transmit diversity
• Equalizer
– Linear feed back equalizers
– Decision feed back equalizers
• Coding techniques
– Channel Coding Techniques (Review)
– Speech Coding Techniques (Review)

3. Basic - Understanding
Dispersive
Channels

4. Diversity - Understanding
Dispersive
Channels

5. Diversity - Principle
• The principle of diversity is to ensure that the same
information reaches the receiver on a statistically
independent channels.

6. Diversity
• This is a technique implemented to improve the
channel quality and performance.
• This is achieved by placing the Tx signals in a
statistically independent channels.
• This reduces the over all fading of the received signals
at the receiver side.
• This technique mainly addresses the fading problems.

7. Correlation Coefficient
• Diversity is most efficient when the different
transmission channels carry independently faded copies
of the same signals.
• The correlation coefficient characterises the correlation
between signals on different diversity branches .
• Correlation can be done on different parameters such as
complex part of the signal, phase relations etc,
• Most important one is correlation between the signal
envelopes.

8. Lack of Correlation - Understanding
Dispersive
Channels

9. • The correlation coefficient is given by:
• Where x and y are the envelope of the signals.
• The signals are said to be “effectively” decorrelated if , ρ is
between a certain threshold value (typically 0.5 or 0.7)

10. Types of Diversity
• How to obtain the diversity in the signals ?
• Diversity are classified based on the fading methods.
–Small scale fading
• Microscopic diversity techniques are used.
–Large scale fading
• Macroscopic diversity techniques are used.

11. Micro diversity Techniques.
• There are 5 methods available.
– Spatial diversity
• Antenna elements separated by space
– Temporal diversity
• Transmission of signals at different time
– Frequency diversity
• Transmission of signals at different frequencies
– Angular diversity
• Multiple antennas with different antenna patterns
– Polarization diversity
• Multiple antenna with different polarizations

12. 1. Spatial diversity
• Signals are Transmitted / Received by more than one antenna and
the best signal is selected for processing.
• Oldest and simplest form of diversity
• Large correlation between the signals are undesirable, as it decreases
the effectiveness of diversity.
• Important factor is to establish the relation between the antenna
spacing (Different for Tr side and Different for the Rx side)
Transmitter Side
Receiver side 

13. Envelope Correlation Coefficient as a function of
antenna separation

14. 2.Temporal Diversity
• As the wireless propagation channels are time
variant, signals that are received at different times
are uncorrelated.
• Temporal diversity can be realized in 3 different
ways.
– Repetition coding
– Automatic repeat request
– Combination of interleaving and coding

15. • Repetition coding:
– The signal is repeated several times, where the repeat intervals
are sufficient for decorrelation of signals.
– This method is highly bandwidth inefficient
• Automatic Repeat ReQuest – (ARQ):
– Rx sends a message to transmitter to indicate weather it
received the data with sufficient quality.
– If not then transmission is repeated again.
– Efficiency is better than repetition coding.
– Retransmission occurs only at certain cases.
• Combination of interleaving and coding:
– The signals are interleaved and coded. Instead of sending the
actual data those code words are sent.
– The transmitted codeword can be reconstructed .

16. 3. Frequency Diversity
• Frequency diversity is implemented by transmitting
information on more than one carrier frequency.
• It does not mean that same messages are transmitted in
two different frequencies.
– When this is done then the signals undergoes different fading
levels.
• Instead, information is spread over a large bandwidth, so
the small parts of the information are conveyed by
different frequency components.
• Then the receiver averages over the different frequencies
to recover the original information.

17. Frequency Spreading

18. • The spreading is achieved by different methods:
– Compressing the information in time.(TDMA)
– Code division multiple access.(CDMA)
– Orthogonal frequency division multiplexing(OFDM)
– Frequency hopping in conjunction with coding.
Demerits:
- It requires large band width.
- More number of receivers are required.
- High cost.

19. 4. Angular Diversity
• It enhances the decorrelation of signals at closely
spaced antennas.
• Different antenna patterns can be achieved very
easily.
• This effect is due to Mutual Coupling.

20. Mutual Coupling
• Place 2 identical
antennas close to each
other
• Here antenna B acts as a
reflector for antenna A.
• Antenna A acts as a
reflector for Antenna B.
• Hence the pattern of
both the antennas are
skewed as shown in
figure.
• Of Course Antennas with
different radiation
pattern can also be used

21. • Mutual Coupling can also be increased by locating the
antenna at different parts of the casing.
• The various possibilities are shown in the figure.

22. 5. Polarization Diversity
• Signals are differentiated with the horizontally and vertically
polarizations.
• Reflection and the diffraction depends upon the polarizations.
• The fading levels at each polarization are independent. Thus the
diversity is achieved.

23. Macro Diversity
• Macro Diversity:
– To reduce the large scale fading these macro diversity is used.
– Large scale fading are generally caused by shadowing.
– If there is an hill between the BS and the MS then simply
increasing the Tx antenna and Rx antenna doesn’t make any
difference.
– Thus a intermediate BS (BS2) is placed in between the BS and MS
so that the hill does not lie in between the BS and BS2.
–Receive – Amplify – Retransmit.
Merits:
– Distance between BS and MS can be increased.
Demerits:
– It requires large bandwidth.
– frequency repeaters causes delay dispersion.

24. Diversity Achieved. How to Resolve?
Selection
Diversity
Combining
Diversity

25. Signal Combining Techniques
– Selection diversity
 Best signal is selected, and the rest are discarded.
• Selection
• Switched
• Feed back
– Combining diversity
All signals are combined together and then it is decoded.
• Maximal ratio combining
• Equal gain diversity

26. Selection Diversity
• There are 2 selection criteria:
– RSSI – Received Signal Strength Indication
– BER – Bit Error Rate
• The receiver selects the signal with the largest
instantaneous power

27. RSSI – Driven Selection Diversity

28. BER – Driven Selection Diversity
• Working…
– We first transmit the Training Sequence. (ie) Know signal
/ sequence.
– The Rx then demodulates the signal from each antenna
and compares / correlates it with the transmit signal.
– The antenna which received the smallest BER is chosen as
the best and that signal is processed further.

29. BER – Driven Selection Diversity

30. Merits and Demerits of RSSI and BER
• RSSI
– Merits:
• Only one RF Chain is used
• Process is done on only one signal
• Easy to implement
– Demerits:
• Waste of signal energy by discarding (n-1) received signals
• Not an optimum method
• BER
– Demerits:
• More number of Rx are used.
• Implementation is complex
• Training sequence is to be repeated again and again.
• Tradeoff between the duration of training and BER should be maintained.

31. Switched Selection Diversity
• The main drawback of the selection diversity is its
criteria.
• RSSSI and BER has to be monitored continuously
on all branches.
• Leads to complex designs and heavy hardware
requirements.
These drawbacks are eliminated by switched selection
diversity.

32. • In this method the selection criteria is monitored only in
the active branches.
• If it falls below a certain threshold value, then the
receiver switches to a different antenna.
• Case 1: All branches have equal power then the selection
of active branch is RANDOM.
• Case 2: All branches are below threshold level, then the
receiver just switches back and forth until an active line
is detected.
The performance of the switched diversity is worse than
the selection diversity. Hence it is not considered widely.

33. Feed Back Diversity
• Also called scanning diversity.
• This is a combination of selection and switched
diversity.
• All the available channels are scanned first in a fixed
sequence until one is found above the threshold level.
• The signal is received from that antenna until it falls
off the threshold value and scanning process is
initiated again.

34. 2. Combining Diversity
• It exploits all available copies of signals. Each signal
copy is multiplied by a (complex) weight and then
added up.
• Weight = phase correction + weight of amplitude
• Phase correction is done to make the signals coherent.
• Amplitude weighting has 2 methods:
– Maximal ratio combining (MRC).
– Equal gain combining (EGC).

35. Maximum Ratio Combining
• This method weighs all signal copies by their amplitude.
• They also does the phase correction for different antennas.
 Merits:
– Output are acceptable even when all the received signals are faded highly.

36. Equal Gain Combining
• This method weighs all the signals with equal amplitude and
performs phase correction to give equal gain diversity.

37. END
OF
DIVERSITY

38. EQUALISERS

39. Introduction
• Inter symbol interference is the major problem in
wireless communication which leads to the BIT
ERRORS at the receiver.
• Equalization is a technique used to reduce the inter
symbol interference.
• This device equalizes the dispersive effect of the
channel. (dispersion due to fading)
• Equalizers are mostly used at the receiver side.

40. Classification of equalizers.

41. • Linear equalizers:
– If the output is not used in the feed back path to adapt
the equalizer is called linear equalizer.
• Non linear equalizers:
– If the output is fed back to change the subsequent outputs
of the equalizer is called as non linear equalizers.

42. LINEAR EQUALIZERS
• They are simple and resembles the filter structures.
• The product of the transfer function of the channel
and equalizer must satisfy certain criteria.
• The criteria can be,
– Either, Achieving a completely flat transfer function of
the channel – filter concatenation.
– Or, Minimizing the mean square error at the filter output.
• The basic structure of the linear equalizer is shown in
the figure.

43. • Ci  Transmit Sequence sent over the channel.
• Ui  Sequence available at the Equalizer input.
• Now we have to convert the Ci to C^
i .
• The aim of this conversion is to produce ZERO Deviation.
OR
• To produce minimum mean square error.

44. Types of Linear Equalizers
• There are 2 types of linear equalizers, they are:
– Zero Forcing Equalizer (ZF)
– Minimum Mean Square Error Equalizer (MMSE)

45. Zero Forcing Equalizers Vs MMSE Equalizers

46. Merits and demerits
• Merits
– Simple and easy to implement
– It has faster convergence
– Unique structure
– When channel becomes more time dispersive, the length of the
equalizer can be increased.
• Demerits
– Structure is complicated than compared to a linear equalizer.
– Not suitable for severe distortion channels.

47. 2. MMSE Equalizers
• In mmse the ultimate aim is to reduce the BER but
not the ISI.
• This can be achieved by minimizing the mean square
error between the signals.
• For minimizing the error the coefficients are found
first.

48. ALGORITHMS FOR MMSE
• Least Mean Square - LMS
• Recursive Least Square - RLS

49. LMS Algorithm

50. RLS Algorithms
• No assumptions are made in general
• Each signal is received individually and then they are
analyzed for the type of dispersion.
• This is more advantageous than the LMS alg.

51. NON LINEAR EQUALISERS