How to Maintain the Orthogonality among Various Multiplexed Users
ASSIGNMENT
Discuss in details the PRN code design in DS-CDMA and
also discuss how to maintain the orthogonality among
various multiplexed users.
(so we have to discuss how can we decrease the cross-
correlation between signature codes and mention the two
types of PRN code)
Potential source: There is this book called “"Wireless Communications
Principles and Practice" – there is some interesting information which can
help you in chapter 9.
Solution
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Pseudo random code (PNR) is a fundamental component of the GPS, which is very
convoluted progression of on and off pulses (Trimble, 2016). The code is very much complicated
that it almost looks like an electrical noise. There is a pair of this code. Namely, the Coarse
Acquisition (C/A) code that functions to modulating L1 carrier. It harmonizes the L1 at a rate of
1MHz and reruns more than a thousand bits. This forms the basis for the civilian GPS use. The
other PNR code is known as Precise (P) code, which reruns on weekly basis and inflects both the
L1 and L2 carriers at a rate of 10MHz. this is specifically intended for military users and thus is
possible to be encrypted. Once encrypted, it is known as “Y” code. Due to its uniqueness than
C/A, P code is a bit difficult for receivers to acquire (Trimble, 2016).
Mobile users are supposed to share finite amounts of radio spectrum concurrently. Hence,
the sharing spectrum should be in a position to achieve a high capacity via allocation of the
available bandwidth to a couple of users. The digital address codes that are applied in the DS-
ACDMA must contain favorable features besides the low cross-section correlation. The code
must be periodic and easy to generate regularly and easy to time-shift and phase control for
synchronization. All the codes in a particular DS-ACDMA system should be efficient enough to
accommodate cross-correlation value.
Samples of the codes that meet this requirement s are the feedback shift registers. If the
proper designing of the logic, the output codes can be manipulated to be periodic and thus allow
the register to act as a free-running code oscillator. Such codes resulting in this manner are thus
referred to as shift register codes. The categories of these shift register codes that have a binary
waveform correlation are called pseudorandom noise (PRN) codes. The PRN codes are
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recognized of exhibiting the longest period associated with a particular register size and thus
known as the maximal-length shift register codes.
The autocorrelation functionality of the PRN codes is more convenient for synchronizing
two versions of a similar code because a high correlation takes place only when the codes are
aligned. Meanwhile, low correlations, which can even be zero, occur at any of the offsets.
How to Maintain the Orthogonality among Various Multiplexed Users
Communication for a handful of terminals ion both forward and reverse links can be
supported concurrently by use of a multiple access system. This because the multiple access
system is capable of transmitting links simultaneously to the two links. This is easily achievable
by multiplexing the multifarious data transmission on each of the forward and reverse links to be
orthogonal to one another in all attributes of time, frequency, and domain (Sutivong & Agrawal,
2010). This objective o0f orthonogality is not achieved in most cases because of several factors
such as receiver imperfections, conditions of the channel among others. However, multiplexing
ensures that the transmitted data for each of the terminals interferes with the transmission of the
other to a very lower extent (Sutivong & Agrawal, 2010).
Source: Untitled Document (n.d)
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From the diagram shown, the distinctive stations at the sources provoke own signals and
then each transmits its signal to the multiplexor. The multiplexor takes up the role of aggregating
the signals emanating from the different stations into one signal stream. The single signal is then
sent over with adequate bandwidth to convey the aggregated signal. The signal is disassembled
on reaching the demultiplexer into the initial single signals and then forwarded to an appropriate
station at the destination. Proper synchronization is required in order to use the orthogonal codes,
which in return break into a multipath channel. On the other hand, quasi-orthogonal codes are
responsible in causing self-interference, which is the dominant of the performance in most
CDMA systems.
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References
Goldsmith, A. (2005). Wireless communications. Cambridge university press.
https://books.google.com/books?hl=en&lr=&id=ZtFVAgAAQBAJ&oi=fnd&pg=PP1&dq=wirel
ess-communications-principles+and+practise&ots=Wp2ah3eEmw&sig=JNaV72vuF-
OJH6pNwVJ87AyhU3M
Sutivong, A., & Agrawal, A. (2010). Multiplexing for a multi-carrier cellular communication
system. Retrieved 31 March 2016, from
http://howtomaintaintheorthogonalityamongvariousmultiplexedusers
Trimble,. (2016). Trimble – GPS Tutorial – Pseudo Random Code. Trimble.com. Retrieved 31
March 2016, from http://www.trimble.com/gps_tutorial/sub_pseudo.aspx
Untitled Document. Www4.ncsu.edu. Retrieved 2 April 2016, from
http://www4.ncsu.edu/~chou/course/LectureNotes/multiplexing.htm
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