CDMA Technology

CDMA Technology: The Spread Spectrum Cellular Revolution

Code Division Multiple Access (CDMA) brought spread spectrum technology — originally developed for military anti-jamming — into commercial cellular networks. Pioneered by Qualcomm and standardized as IS-95 (cdmaOne) in 1993, CDMA demonstrated that multiple users could share the same frequency simultaneously by using unique mathematical codes. This represented a fundamentally different philosophy from GSM's time-division approach and ultimately proved to offer 3-6 times the voice capacity.

The Spread Spectrum Principle

Think of it this way: in a library where everyone must whisper, you can only have a few conversations before they blur together. But if each pair spoke a different language, many more simultaneous conversations could coexist — each listener filters out everything except their own language. CDMA assigns each user a unique "language" (spreading code) so multiple conversations can occupy the same frequency band simultaneously.

Direct Sequence Spread Spectrum (DS-SS):

1. User data bit (e.g., duration 0.8 μs in IS-95)
2. Multiplied by spreading code (64 chips per bit, chip rate = 1.2288 Mcps)
3. Signal bandwidth spreads from ~10 kHz to 1.25 MHz
4. At receiver: multiply by same code → data recovered, all other users become noise

Processing gain: PG = Chip rate / Data rate = 1,228,800 / 9,600 = 128 = 21 dB

This 21 dB processing gain is the "magic" that allows many users to coexist — each interferer is suppressed by 21 dB.

IS-95 (cdmaOne) System Design

IS-95 was the first commercial CDMA cellular standard:

Parameters:

• Bandwidth: 1.25 MHz
• Chip rate: 1.2288 Mcps
• Voice codec: QCELP at 8 kbps (variable rate: 9.6, 4.8, 2.4, 1.2 kbps)
• Modulation: QPSK (downlink), O-QPSK (uplink)
• Forward link: 64 Walsh codes for channelization
• Reverse link: Long PN code (2⁴² - 1 chips) for user separation

Downlink (base station → mobile):

• Each user assigned one of 64 Walsh codes (orthogonal channels)
• All codes transmitted synchronously → perfect orthogonality
• Pilot channel (Walsh 0): Provides phase reference for coherent detection
• All traffic channels coherently demodulated using pilot

Uplink (mobile → base station):

• Each mobile uses a unique long PN code offset for identification
• Signals arrive asynchronously → codes not perfectly orthogonal
• RAKE receiver combines multipath components constructively
• Non-coherent detection (no pilot) in IS-95; coherent in CDMA2000

Power Control: The Critical Requirement

CDMA systems live or die by power control. The near-far problem means that without precise power control, a mobile phone 100 meters from the tower would overwhelm signals from a phone 2 km away:

Open-loop power control:

• Mobile estimates path loss from received pilot signal strength
• Sets initial transmit power accordingly
• Accuracy: ±10 dB (coarse — not sufficient alone)
• Updates: Continuous, but slow (responds to average path loss)

Closed-loop power control:

• Base station measures received SIR (Signal-to-Interference Ratio)
• Compares to target SIR (set by outer loop)
• Sends power control bit every 1.25 ms (800 Hz rate)
• Each command: "UP" or "DOWN" by 1 dB (IS-95) or 0.5 dB (CDMA2000)
• Accuracy: ±1 dB of target
• Tracks Rayleigh fading up to ~60 km/h vehicle speed

Outer-loop power control:

• Adjusts target SIR based on measured frame error rate (FER)
• If FER too high → increase target SIR
• If FER too low → decrease target SIR (saves interference budget)
• Update rate: ~10-100 Hz

Soft Handoff

CDMA's soft handoff is unique — the mobile connects to multiple base stations simultaneously:

Process:

1. Mobile monitors pilot signals from neighboring cells
2. When neighbor pilot exceeds threshold → add to Active Set
3. Mobile communicates with both old and new base stations simultaneously
4. Both BSs send same data; mobile combines (rake-like diversity)
5. When old pilot drops below threshold → remove from Active Set

Advantages:

• No instantaneous break in communication (compare to GSM hard handoff)
• Macro-diversity: Combining signals from multiple BSs reduces fading
• "Make before break" — new connection established before old is released
• Reduced dropped call probability during handover

Cost: Mobile and multiple BSs use resources simultaneously → overhead

CDMA2000 and Evolution

CDMA2000 evolved from IS-95:

• 1×RTT (Release 0): 153 kbps peak data, same 1.25 MHz bandwidth
• 1×EV-DO (Rev A): 3.1 Mbps downlink, data-optimized
• 1×EV-DO Rev B: Multi-carrier, up to 14.7 Mbps
• Used primarily in North America, Korea, Japan until LTE replacement

CDMA Capacity Analysis

Theoretical capacity of a single CDMA cell:

N = (W/R) × (1/(Eb/N₀)) × (1/(1+f)) × Gv × Gs

Where:

• W/R = processing gain (128 for IS-95)
• Eb/N₀ = required energy per bit ratio (~5 dB = 3.16 for voice)
• f = other-cell interference factor (~0.6)
• Gv = voice activity gain (~2.5, since people listen ~60% of the time)
• Gs = sectorization gain (~2.5 for 3-sector cell)

N ≈ 128/(3.16) × 1/(1.6) × 2.5 × 2.5 ≈ 40 users per sector

This compares to ~8 users per sector for AMPS (analog) and ~15-20 for GSM TDMA — demonstrating CDMA's significant capacity advantage.

Key Takeaways

1. CDMA allows multiple users to share the same 1.25 MHz bandwidth simultaneously using unique spreading codes, achieving 3-6× capacity over TDMA systems.
2. Processing gain (21 dB in IS-95) is the fundamental mechanism suppressing inter-user interference.
3. Precise closed-loop power control at 800 Hz is absolutely critical — without it, CDMA cannot function due to the near-far problem.
4. Soft handoff provides macro-diversity by simultaneously connecting to multiple base stations, eliminating handoff gaps.
5. Variable-rate voice coding and voice activity detection reduce average interference, directly increasing system capacity.
6. CDMA principles evolved through IS-95 → CDMA2000 → WCDMA (3G) before cellular systems transitioned to OFDMA-based 4G/5G.