CDMA Signal Spreading - The VERY basics of how it's done

Summary

This video explains the process of spreading user data using Walsh codes in a Code Division Multiple Access (CDMA) system. It demonstrates how each user's data is spread using a unique code, creating a waveform that represents their signal. The signals from multiple users are then combined into a composite waveform. The recovery of each user’s original data is accomplished by correlating the composite signal with the user's unique spreading code, allowing for interference-free communication. This method highlights how CDMA efficiently enables multiple users to share the same frequency spectrum without interference.

Takeaways

• 😀 A user wants to send data by spreading it using a unique Walsh code for each user.
• 😀 Each user's data consists of two bits, and the Walsh code used for spreading is specific to each user.
• 😀 The XOR (exclusive OR) operation is used to apply the spreading code to the user's data, transforming it into a spread signal.
• 😀 A spread signal consists of multiple chips (binary values) representing the original data, expanded through the spreading code.
• 😀 The data for each user is spread individually using their unique Walsh code, resulting in a spread signal with multiple chips.
• 😀 Each spread signal is represented as a voltage waveform, where 0 is represented as +1 volt and 1 is represented as -1 volt.
• 😀 After spreading the signals, they are combined into a composite waveform, where the signals from all users are added together.
• 😀 The combined composite waveform contains overlapping signals from all users, each representing different data.
• 😀 To recover the original data, the user's unique Walsh code is applied to the composite signal through multiplication (XOR).
• 😀 By averaging the results of applying the Walsh code to the composite signal, the original data for each user is recovered successfully.

Q & A

What is the purpose of using a Walsh code in the script?

The purpose of using a Walsh code in the script is to spread the user's data over a wider bandwidth, allowing multiple users to share the same frequency band without causing interference. This technique is often used in systems like CDMA.

What does the XOR operation do in the context of spreading data?

In the context of spreading data, the XOR operation is used to combine each bit of the user's data with the corresponding bit from the Walsh code. If the bits are the same, the result is 0; if they are different, the result is 1. This spreads the data by modulating it with the spreading code.

How are the signals of different users combined in the system?

The signals of different users are combined by adding their spread signals together. Each user's spread signal is a sequence of bits, and the sum of these bits at each time point forms a composite signal, which represents the combined data from all users.

What is the final goal of spreading and combining the users' signals?

The final goal is to allow each user to transmit their data without interference, even though they are using the same frequency. By using unique Walsh codes, each user’s data is spread and combined into a composite signal, which can later be despread to recover the original data.

How is the composite signal used to recover the original data?

The composite signal is recovered by applying the same Walsh code used during spreading to the composite signal. This process, known as despreading, extracts the original data for each user by multiplying the composite signal with the user’s unique Walsh code and averaging the results.

What role do voltage levels (+1 and -1 volts) play in the script?

The voltage levels of +1 and -1 volts are used to represent binary data. In the script, a 0 bit is represented by +1 volt, and a 1 bit is represented by -1 volt. These voltages are used to encode the data in the spreading process and to create waveforms for signal transmission.

What happens when the signals from all users are added together?

When the signals from all users are added together, the result is a composite waveform that combines the data from all users. The individual voltages from each user’s signal are summed at each time point, creating a new waveform that contains the superposition of all users’ data.

What does the script mean when it says the resulting signal is 'spread'?

When the script says the resulting signal is 'spread', it refers to the process of using a Walsh code to modulate the user's data. This spreading increases the bandwidth of the signal, which helps to reduce interference and allows multiple users to transmit their data over the same frequency band simultaneously.

What is the significance of averaging the results in the despreading process?

Averaging the results in the despreading process is crucial because it helps to eliminate noise and recover the original data more accurately. By averaging the values obtained after multiplying the composite signal with the Walsh code, the system ensures that the correct data is retrieved, even if there is some interference or noise.

Can the process of spreading and despreading work with more than three users?

Yes, the process can work with more than three users, provided each user has a unique Walsh code. The system can handle as many users as there are unique Walsh codes available. The key is that each user's signal must be spread using a different code to avoid interference when combining the signals.