CDMA
Understanding CDMA
Spread Spectrum and Code Orthogonality
Unlike FDMA, which divides the spectrum by frequency, and TDMA, which divides it by time, Code Division Multiple Access (CDMA) allows all users to share the same frequency band simultaneously. This is achieved using spread-spectrum technology. The data signal is multiplied by a unique, pseudo-random spreading code (such as a Walsh code or PN sequence) that runs at a much higher rate, known as the chip rate.
This multiplication spreads the narrow-bandwidth data signal across the entire channel bandwidth. At the receiver, the composite signal is multiplied by the same spreading code. Because the code is orthogonal to the codes assigned to other users, this despreading operation collapses only the desired signal back to its original narrow bandwidth, while all other users' signals remain spread and appear as low-level background noise.
Processing Gain and the Power Control Imperative
The performance of a CDMA system relies on its processing gain, which is the ratio of the spread-spectrum chip rate to the data rate. A high processing gain allows the receiver to extract the desired signal even when the total interference power is significantly stronger than the signal itself. However, CDMA systems are interference-limited, meaning that each active user adds to the noise floor, gradually reducing the capacity of the cell.
The primary challenge in CDMA is the near-far problem: if a user close to the base station transmits at the same power as a user at the cell edge, the near user's signal will arrive much stronger and overwhelm the far user's signal. To prevent this, CDMA networks implement closed-loop power control at a high update rate (typically 800 Hz in IS-95). The base station measures the received signal strength of each handset and continuously commands it to adjust its transmit power, keeping all received signals at the same power level.
Key Mathematical Relations
Technical Specifications Comparison
| Multiple Access Scheme | User Separation Method | Frequency Allocation | Time Allocation | Capacity Limit | Key Challenge |
|---|---|---|---|---|---|
| FDMA | Frequency bands | Dedicated narrow bands | Continuous | Frequency slot count | Guard band spectral overhead |
| TDMA | Time slots | Shared wide band | Periodic bursts | Time slot count | Strict timing synchronization |
| CDMA | Orthogonal codes | Shared wide band | Continuous | Interference-limited | Near-far problem (requires power control) |
| OFDMA | Orthogonal subcarriers | Shared subcarrier sets | Continuous/Burst | Subcarrier count | High PAPR (requires linear PAs) |
Frequently Asked Questions
What is the difference between Walsh codes and PN sequences?
Walsh codes are perfectly orthogonal codes used on the downlink (base station to mobile) to separate users within the same cell sector. Pseudo-Noise (PN) sequences are pseudo-random codes used on the uplink (mobile to base station) to distinguish different mobile devices, as perfect synchronization is not possible in the uplink.
How does CDMA handle multipath interference?
CDMA uses a RAKE receiver, which contains multiple correlators (fingers) tuned to different time-delayed replicas of the incoming signal. The RAKE receiver despreads each multipath component independently and combines them coherently, turning multipath propagation into a diversity gain instead of an interferer.
Why did LTE and 5G networks transition from CDMA to OFDMA?
CDMA is interference-limited, meaning its efficiency drops as data rates increase because the processing gain decreases. OFDMA separates users in both time and frequency, avoiding inter-user interference. It also supports wider bandwidths (up to 100+ MHz) and MIMO spatial multiplexing much more efficiently than CDMA.