Simulation of Code Division Multiple Access protocol using Walsh codes.
Code Division Multiple Access, CDMA is an innovative use of direct sequence spread spectrum technology used to provide a multiple access scheme for mobile telecommunications and other wireless systems. CDMA used the property of DSSS that unless the transmitter and receiver used the same spreading code for both ends of the process, the signal could not be decoded and in this way it was able to provide a means of enabling a variety of different users to use the same channel to access a base station without mutual interference.
CDMA has a number of distinguishing features that are key to spread spectrum transmission technologies:
- Use of wide bandwidth: CDMA, like other spread spectrum technologies uses a wider bandwidth than would otherwise be needed for the transmission of the data. This results in a number of advantages including an increased immunity to interference or jamming (AKA jamming margin), and multiple user access.
- Spreading codes used: In order to achieve the increased bandwidth, the data is spread by use of a code which is independent of the data.
- Level of security: In order to receive the data, the receiver must have a knowledge of the spreading code, without this it is not possible to decipher the transmitted data, and this gives a measure of security.
- Multiple access: The use of the spreading codes which are independent for each user along with synchronous reception allow multiple users to access the same channel simultaneously.
- Graceful Degradation: CDMA systems minimize the impact of interference from a new user by distributing the interference power (inversely proportional to the length of spreading code) equally among all existing users. This ensures no single user/group of users are affected in particular.
Correlation is a method of measurement of how precisely a given signal matches with a desired code. In CDMA technology, each user is assigned with a different code, the code which is being assigned or chosen by the user is very important to modulate the signal because it is related to the performance of the CDMA system.
One will get best performance when there will be clear separation between the signal of desired users and signals of the other users. This separation is made by correlating the desired signal code which was locally generated and other received signals. If the signal matches with the code of the user, then the correlation function will be high and the system can extract that signal. If the user's desired code has nothing in common with the signal, the correlation should be as close to zero as possible (thus eliminating the signal); also known as cross correlation.
It can be shown that orthogonal codes have a cross-correlation equal to zero; in other words, they do not interfere with each other. Orthogonal codes are mainly used in synchronous CDMA.
Walsh Codes are most commonly used in the orthogonal codes of CDMA applications. These codes correspond to lines of a special square matrix called the Hadamard matrix. For a set of Walsh codes of length N, it consists of n lines to form a square matrix of n×n Walsh code. Walsh matrices are just Hadamard matrices and can be created using the given formula, given an initial matrix. This is also why Walsh codes are also called Walsh-Hadamard codes
The code in this repository makes use of python's inbuilt socket function to simulate simple CDMA-based communication. The program takes in 2 main arguments via CLI.
- Number of users
- Message to be sent by each user.
The code contains the following 3 files
Contains code for encoding the messages with respective Walsh codes and generating the final signal (matrix here) to be sent in the channel
Contains code for decoding the signal (matrix) from channel and decoding it with the same Walsh codes to obtain the respectives messages
Contains methods to generate Walsh matrices required (depending on no. of users) and to convert text string to binary array and vice versa.
The socket arguments are specified as follows:
- address:
localhost - port:
3300
These can be changed as per requirements.
Additional comments providing a rough overview of the codeflow is also given
- Clone this repository and move into the cloned directory
- Run
python receiver.py <NO_OF_USERS>first - Run
python transmitter.py <NO_OF_USERS>and enter the message to be sent for each user
- Set logging level to
DEBUG. (Default isINFO) - The comments are written to be more useful if used in conjecture with Better Comments Extension
- There are also some methods in
utils.pyto enable sending images instead of text usingbase64encoded strings. However, it is not used in main code as there might be some buffer issues in socket for large strings.
With logging level INFO
With logging level DEBUG
- This code only simulates the use of Walsh codes in CDMA systems and thus ignores the other aspects in a complete CDMA system like modulation, demodulation, noise, etc. For a complete simulation of CDMA, see these:
- This system is used mainly in voice communications, however dealing with strings is much easier than sound in python.
[1] Walsh Codes, PN Sequences and their role in CDMA Technology
[2] CDMA basics and Walsh codes
[3] Code division multiple access, Wikipedia
[4] CDMA Techniques
[1] T. Rappaport, Wireless Communications: Principles and Practice, 2nd ed., (Upper Saddle River, N.J.: Prentice Hall, 2002).
[2] A. Molisch, Wireless Communications, (Chichester, UK: IEEE press - John Wiley, 2005).
This software is licensed under the terms of the MIT License.