Wireless Notes

CDMA Code Division Multiple Access

Learn CDMA with spread spectrum, Walsh codes, processing gain, near-far problem, soft handoff, WCDMA CDMA2000 3G implementation, advantages disadvantages for engineering students.

In CDMA, all users transmit on the SAME frequency at the SAME time! How are they separated then? Each user is assigned a unique code. The receiver only responds to its assigned code and rejects all others.

🎯 What is CDMA?

In CDMA, neither frequency nor time is divided – CODE is divided! All users use the full bandwidth and full time, but each signal is "spread" using a unique code.

┌─────────────────────────────────────────────────────────────────────┐ │ CDMA – CODE DIVISION │ │ │ │ Frequency ↑ │ │ │ ┌─────────────────────────────────────────────┐ │ │ │ │ User 1 (Code A) ░░░░░░░░░░░░░░░░░░░░░░░ │ │ │ │ │ User 2 (Code B) ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓ │ │ │ Entire │ │ User 3 (Code C) ████████████████████████ │ │ │ Bandwidth │ │ User 4 (Code D) ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ │ │ │ │ │ ALL OVERLAP in frequency AND time! │ │ │ │ └─────────────────────────────────────────────┘ │ │ └──────────────────────────────────────────────▶ Time │ │ │ │ ALL users use ENTIRE bandwidth at the SAME time │ │ Separated by unique CODES (orthogonal/pseudo-random) │ └─────────────────────────────────────────────────────────────────────┘

Analogy: Imagine 10 pairs talking in a room – all in the SAME language. FDMA = each pair in a separate room. TDMA = one pair talks at a time. CDMA = all in the same room at the same time, but each pair speaks a unique language that only they understand!

📡 Spread Spectrum Concept

CDMA uses spread spectrum technique – a narrowband signal is "spread" across a wide bandwidth using a code.

Original signal:	████	(Narrowband, high power)
(bandwidth = B)	████
	████
░░░░░░░░░░░░░░░░░░░░ ──▶	████	(Original recovered!)

Direct Sequence Spread Spectrum (DSSS):

Data bit (1): ┌──────────────────────┐ │ 1 │ └──────────────────────┘ Code (chip sequence): ┌┐┌┐ ┌┐ ┌┐┌┐ ┌┐┌┐ │││││ ││ │││││ ││││ └┘└┘└──┘└──┘└┘└──┘└┘└┘ +1-1+1-1+1 -1+1 -1+1+1-1+1-1+1 (example) Spread signal = Data × Code (chip rate >> data rate)

⚙️ How CDMA Works

Transmitter (Spreading):

User data (Rb) ──×──▶ Spread signal (Rc = Rb × N) │ Unique code (chip rate Rc)

Receiver (Despreading):

Received signal ──×──▶ Integrate ──▶ Decision ──▶ Data (all users) │ & Dump Same code (synchronized)

Example with 4 Users:

User 1	[+1,+1,+1,+1]
User 2	[-1,+1,-1,+1] (data × code)
User 3	[+1,+1,-1,-1]
User 4	[-1,+1,+1,-1]
Sum	[0, +4, 0, 0] ← This is what's in the air
To extract User 1	Multiply by Code 1 and sum:
[0,+4,0,0] × [+1,+1,+1,+1] = 0+4+0+0 = +4	positive → bit = +1 ✅
To extract User 2	Multiply by Code 2 and sum:
[0,+4,0,0] × [+1,-1,+1,-1] = 0-4+0+0 = -4	negative → bit = -1 ✅

🔑 Codes in CDMA

Code Type	Property	Use
Walsh Codes	Perfectly orthogonal	Downlink (same cell)
PN Sequences	Pseudo-random, near-orthogonal	Uplink, cell separation
Gold Codes	Good cross-correlation	Multiple users
Kasami Codes	Best cross-correlation	Large user sets

Orthogonality:

Code A	[+1,+1,-1,-1]
Code B	[+1,-1,+1,-1]
Orthogonal codes	zero interference between users (ideal)

📊 Processing Gain

Processing gain is the strength of CDMA – it indicates how much SNR improvement is obtained from spreading.

│ In dB	Gp(dB) = 10×log₁₀(W/R) │
│ Example (WCDMA)	│
│ This means	can tolerate 25 dB of interference! │

⚠️ Near-Far Problem

This is the biggest challenge of CDMA! If one user is very close to the tower (strong signal) and another is far away (weak signal), the strong signal "drowns" the weak signal. Power control is essential to solve this.

│ │←── 100m ──────────────────│──────────────── 2 km ──	│ │
│ (received	-60 dBm) │ (received: -100 dBm) │
│ Problem	User A's signal is 40 dB stronger! │
│	User B cannot be decoded! ❌ │
│ Solution	POWER CONTROL │
│ Base station tells User A	"Reduce your power!" │
│ Goal	All users arrive at base station with EQUAL power │
│ After power control	│
│ User A (reduced)	-80 dBm │ User B (max): -80 dBm │

Solution: Fast power control (800 times/second in WCDMA!) adjusts every user's TX power so all arrive at equal level at base station.

🤝 Soft Handoff

A unique feature of CDMA – a phone can be simultaneously connected to two or more base stations (soft handoff). Since codes are different, there is no interference.

FDMA/TDMA	Hard handoff (break-before-make)
CDMA	Soft handoff (make-before-break)
Result	No call drop during handoff, better quality at cell edge

📱 CDMA in 3G

WCDMA (3GPP – UMTS):

Parameter	Value
Chip rate	3.84 Mcps
Channel BW	5 MHz
Duplex	FDD
Spreading factors	4-512 (variable)
Voice data rate	12.2 kbps
Max data (HSPA)	42 Mbps (DL)
Power control	1500 Hz

CDMA2000 (3GPP2):

Parameter	Value
Chip rate	1.2288 Mcps
Channel BW	1.25 MHz
Duplex	FDD
Voice rate	8/13 kbps
Max data (EV-DO)	3.1 Mbps (DL)
Regions	Americas, Asia

✅ Advantages & Disadvantages

Advantages:

Advantage	Hindi
High capacity (many users)	Supports many users
Soft handoff (no call drop)	Seamless handover
No frequency planning needed	Frequency reuse = 1
Inherent security (coded signal)	Signal coded = secure
Resistant to narrowband interference	Interference spread out
Graceful degradation	Users badhne pe slowly degrade
Multipath exploitation (RAKE)	Multipath se faayda

Disadvantages:

Disadvantage	Hindi
Near-far problem	Power control essential
Complex power control	800-1500 Hz power updates
Self-interference (all users)	Users apas mein interfere
Complex receiver	RAKE receiver complex
Capacity is soft (interference-limited)	No hard limit, noise limited
Code synchronization needed	Codes sync karne padtein

📝 Summary

Concept	Key Point
Principle	All users share same freq+time, separated by codes
Spreading	Narrowband signal spread to wideband using code
Processing gain	Gp = Chip rate / Data rate
Near-far	Solved by fast power control
Soft handoff	Connect to multiple BSs simultaneously
Codes	Walsh (orthogonal), PN (pseudo-random)
3G systems	WCDMA (5 MHz), CDMA2000 (1.25 MHz)
Key advantage	High capacity, soft capacity limit, security
Replaced by	OFDMA (4G/5G) for higher data rates

❓ FAQ

Q: How many users can CDMA support? A: It is a soft limit – each new user increases interference. Approximately: Users ≈ Gp / (Eb/N₀ required) × voice activity. WCDMA cell: ~60-80 voice users per carrier.

Q: 4G/5G ne CDMA kyun chhoda? A: At high data rates, CDMA's self-interference problem increases. OFDMA uses orthogonal subcarriers that are interference-free – much better for broadband data.

Q: How is frequency reuse of 1 possible in CDMA? A: Every cell uses the same frequency but different scrambling codes. Interference from adjacent cells is handled by processing gain. No frequency planning needed!

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for CDMA Code Division Multiple Access.

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