Course: Digital Communications, Advanced Course (ETTN01

Digital communications - week 3 1

Study period Ht-2 2010.

Course: Digital Communications, Advanced Course (ETTN01)

Aim

The aim of this course is to give very good knowledge of advanced digital

communication methods.

The course gives a broad and deep understanding such that many of the communication

methods used today, as well as many future methods, can be understood.


Digital Communications, Advanced Course (ETTN01)

Examples of previous project works :

• Mobile telephony (GSM, 3G, 4G)

• Digital Radio & TV

• Modem (e.g., ADSL)

• WLAN (Wireless Local Area Network)

• Future communication methods/systems (4G, 5G,…)

• MIMO systems (multiple antenna systems)

• UWB (Ultra Wide Band, “Impulse Radio”)

• GPS (Global Positioning System)

• Bluetooth

• Home electronics (CD, DVD, remote controls, etc. )



How good can we do concerning bit rate & bit

error probability?

C=W*log2(1+SNR), Claude Shannon 1948

Practical consequences?


Remember!

Thinking in the frequency domain is necessary to be able to

understand communication methods and systems.

Problem 2.26a (M-ary PSK)!!

Problem 2.26b (M-ary QAM)!!

Problem 2.30 (Peak value of R(f); Radiation aspects (EMC),

standards)!!



A practical implementation is therefore:

General bandpass:



Example 3.1:







In 3G mobile telephony systems all users use the same

time-interval and the same frequency-interval simultaneously!

CDMA is used instead of TDMA and FDMA.

Each user has a unique code.


AM: The information is in the envelope.



A VCO is also useful to generate FSK and GSM signals!


FM

• Assume that the input signal a(t) gives the

FM-signal v(t).

• Assume now that the input signal is

doubled, i.e. =2a(t).

What is now the FM-signal?


Interpretation in the frequency domain:


Common challenge in both Wireless and Wireline applications!

Remember the training bits in GSM!


This will cause overlapping signals unless Tb is increased to 3 s!




Sent:

Received:

How do we handle this situation?


All frequencies are equally disturbed.


Gaussian probability

distribution:

What is the probability that the output noise is above a critical level A (“bit-error”)?


Very useful

tables!


Q(x) versus x


Sent:

Received:

How can we find the sent information bits?

Pb=?



(The same definition as in (2.12))


Sent message (k bits): Decided

message:


How is your decision strategy?

The disturbed image on the black-board: Is it a house or a boat?


Optimal decision strategy!

How is your decision strategy?

The disturbed image on the black-board: Is it a house or a boat?

Since the decided message must be 0,1,2,3,…,(M-2) or (M-1)

we test all cases in equation (4.13).



Furthermore, we can quite easy implement the optimal receiver!


Probability of “false alarm”: “0” is sent but the receiver decides “1”.

Probability of a “miss” : “1” is sent but the receiver decides “0”.


This receiver is optimal if the signal alternatives are equally likely.

The received signal is compared with all noise-free signal alternatives.

That is why the channel must be known to the receiver!


“Correlation receiver”:



Documents

Course: Digital Communications, Advanced Course (ETTN01