PN Sequence generation

1. SPREAD SPECTRUM
MODULATION
UNIT II
PN sequences, A Notion of spread spectrum, Direct –
Sequence spread coherent BPSK, signal – space
dimensionality and processing gain, probability of error,
frequency – hop spread spectrum, synchronization of
spread spectrum signals : Acquisition and tracking

2. Why Spread Spectrum?
• Occupies more bandwidth than conventional
schemes
• Spread is before tx with a code and
despreaded at rx.
• Secure Communication
• Rejects interference (intentional or
unintentional)
• Multipath Rejection

3. Types
• Direct Sequence
• Frequency Hop
Basis….PN Sequence

4. PN SEQUENCES
•Feedback shift register is said to be linear when…Feedback logic
contains…Modulo 2 adders…
•Zero state is not permitted
•Period of PN Sequence cannot exceed
•When the period is exactly equal to It is called Maximal
Length Sequence…or m-sequence
12 m
12 m

5. Maximal Length Sequences
(m sequences)Initial State is Assumed to be ….100 (left to right)
Succession of States …
Repeats itself after

6. Properties of Maximal length
Sequences

7. Properties
• Property 1:
– Has 2n-1 ones and 2n-1-1 zeros
• Property 2:
– For a window of length n slid along output for N (=2n-1) shifts, each n-
tuple appears once, except for the all zeros sequence
• Property 3:
– Sequence contains one run of ones, length n
– One run of zeros, length n-1
– One run of ones and one run of zeros, length n-2
– Two runs of ones and two runs of zeros, length n-3
– 2n-3 runs of ones and 2n-3 runs of zeros, length 1

9. Comparison

10. Choosing a Maximal length Sequences

17. Notion of Spread Spectrum

20. DS-SS with coherent BPSK

21. Model for analysis

22. Waveforms

23. Analysis

30. Jamming Margin

31. Eavesdropping
• Listening ones private message without any
right.

32. Near Far Problem
• GSM is less sensitive to the near far effect because it uses Time and Frequency
division duplexing: every MS has its own channel and time slot assigned to it by
the BS to transmit/Receive during the uplink/downlink.
In CDMA more users share the same bandwidth at the same time: every user
encode its transmission with a unique spreading code chosen between those
belonging to an orthogonal set, so the BS is capable to separate users from the
interference resulting from the sum of all MS transmitting at that time by
multiplying the received signal by the code assigned to the user of interest.
This is possible if the TX power of all MS is controlled in a way that at BS end each
user is received "more or less with the same power" and the resulting multi-user
interference is kept below a threshold that is managed by the processing gain.
But if for some reason a user (Near user) is transmitting with an uncontrolled
power its stronger signal will mask the weakest signals (Far users) and BS is no
more capable to demodulate them correctly (some times in literature you could
find references to pirate terminal to describe the behaviour of MS operating with
uncontrolled power loop).
For this reason in CDMA cellular systems power control loop of MS is more
accurate than in GSM systems and is implemented in cooperation with BS using a
dedicated channel.

33. Processing Gain
• In a SS system, the process gain (or
'processing gain') is the ratio of the spread (or
RF) bandwidth to the unspread (or baseband)
bandwidth. It is usually expressed in decibels
(dB).
• For example, if a 1 kHz signal is spread to
100 kHz, the process gain expressed as a
numerical ratio would be 100,000/1,000 =
100. Or in decibels, 10log10(100) = 20 dB.

34. Processing Gain
• The bandwidth expansion ratio, or processing
gain Pg=Wc/Wx, Wc=Bandwidth of PN
Sequence, Wx=Bandwidth of the baseband
signal.
• A measure of system immunity to
interference.
• The larger the Pg, the better the system
immunity to interference.

35. Jamming Margin
• A measure of system’s ability to operate in the
presence of interference
• 10log(J/Sr)=10log(Pg)-10log(Eb/Nj)