CDMA Principles: Complete Beginner's Guide For students.

CDMA (Code Division Multiple Access) is a wireless technique in which many users share the same frequency band at the same time, separated by unique digital spreading codes. Each user's narrowband signal is multiplied by a high-rate code, spreading it over a wide bandwidth. The receiver multiplies by the same code to despread the wanted signal, while other users' signals stay spread out and look like background noise.

CDMA matters because one radio channel can serve dozens of users without rigid frequency or time slots, raising capacity and resisting interference and jamming. It powered 2G IS-95/cdma2000 networks, underlies 3G WCDMA/UMTS, and its spreading idea lives on in GPS. It is the foundation of all spread-spectrum systems.

Key Concepts

• Spreading code: a pseudo-random +1/-1 sequence (chips) unique per user. Walsh codes are perfectly orthogonal; PN sequences are nearly orthogonal.
• Chip vs bit: each data bit is multiplied by many chips. IS-95 uses 1.2288 million chips/second against bit rates near 9.6 kbps.
• Processing gain: the interference suppression from despreading, equal to chip rate / bit rate in dB.
• Orthogonality: despreading with one code ideally gives zero output for other codes, so users do not interfere.
• Near-far problem: a nearby user's strong signal can swamp a distant one, so CDMA needs fast power control equalizing received powers.
• Soft capacity: no hard user limit; each extra user only slightly raises the noise floor.

Formulas

• Processing gain: G_p = chip rate / bit rate; in dB, G_p(dB) = 10 log_10(chip rate / bit rate).
• Signal quality: E_b/N_0 after despreading = G_p x (signal power / total interference). Larger G_p tolerates more simultaneous users.
• Assumptions: synchronized codes, working power control, and interference resembling white noise; multipath reduces the real gain.

Worked Example

Processing gain of an IS-95 voice channel (chip rate 1.2288 Mcps, bit rate 9.6 kbps):

1. Spreading factor = 1,228,800 / 9,600 = 128.
2. G_p = 10 log_10(128) = 10 x 2.1072 = 21.07 dB.
3. Meaning: the despreader suppresses interference by ~21 dB, so bits recover even buried 21 dB under total interference. That margin lets many users share one 1.25 MHz carrier.

Common Misconceptions

• "CDMA is always better than GSM." They are just different access methods (code vs time division); GSM won globally on business and roaming grounds.
• "Spreading equals encryption." Spreading resists jamming and interception, but codes are public standards; real secrecy needs encryption on top.
• "Orthogonal codes mean zero interference." Only with perfect synchronization (forward link). On the reverse link, unsynchronized mobiles make PN codes merely near-orthogonal, hence power control.

Real-World Applications

• 2G/3G networks: IS-95, cdma2000 and WCDMA/UMTS air interfaces.
• GPS: each satellite's unique C/A code separates dozens of satellites on one frequency.
• Military radios: jam-resistant spread-spectrum links.
• Soft handoff: a mobile talks to two base stations on the same frequency during handover.

CDMA separates users by code rather than frequency or time, giving soft capacity and interference resistance. Its key numbers are spreading factor and processing gain; its key challenge, the near-far problem, is solved by power control. From 2G phones to GPS, its ideas are everywhere.