CDMA Code Division Multiple Access

Different users use different spreading codes

Real-World Applications

3G (CDMA2000/WCDMA), GPS satellite signals. All users talk at the same time on the same frequency, but each uses a different "language" (spreading code). Though noisy, you can filter out the person you want to hear by language characteristics.

Experiment Steps

Orthogonal Codes: Walsh codes ensure user orthogonality
Soft Capacity: Number of users can exceed spread factor

Simulation Experiment

Time Domain (X: Time, Y: Amplitude)

Spectrum Analysis (X: Frequency, Y: Amplitude)

CDMA Orthogonal Despreading Demo

CDMA Spreading & Despreading Demo

All users' signals are superimposed for transmission; the receiver extracts specific users via "orthogonality".

Receiver Target:

Principle:

1. Code Division Multiplexing (CDM): Users use unique Walsh codes (length 16) to spread 1-bit data into 16 chips.

2. Mixing: All spread signals are directly summed on the channel (Channel Sum). The waveform looks like noise.

3. Despreading: Receiver multiplies the combined signal by the target user's Walsh code and integrates.

4. Orthogonality Magic: Due to Walsh code orthogonality, Target Code × Other User Code integrates to 0. Only Target Code × Target Code integrates to positive (16) or negative (-16), recovering the original data.

Parameters(Real-time)

Num Users4

Spread Factor16

SNR10 dB

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