Wireless Notes

TDMA Time Division Multiple Access

Learn TDMA with time slot allocation, frame structure, GSM implementation, synchronization, guard time, advantages disadvantages, and comparison with FDMA CDMA OFDMA for engineering students.

In TDMA, a single frequency channel is divided in time – multiple users share the same frequency but in different time slots. Each user is periodically assigned a dedicated time slot.

🎯 What is TDMA?

In TDMA, time is divided into smaller slots. Each user transmits in their assigned time slot, then waits for their next slot. This allows a single frequency to be shared among multiple users.

┌─────────────────────────────────────────────────────────────────────┐ │ TDMA – TIME DIVISION │ │ │ │ Frequency ↑ │ │ │ │ │ │ ┌──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┐ │ │ Channel │ │U1│U2│U3│U1│U2│U3│U1│U2│U3│U1│U2│U3│ │ │ │ └──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┘ │ │ │ │ │ └──────────────────────────────────────────▶ Time │ │ │← Frame →│← Frame →│← Frame →│ │ │ │ │ Same frequency shared by 3 users taking TURNS │ │ U1 transmits in slot 1, U2 in slot 2, U3 in slot 3 │ │ Each user gets periodic access (every frame) │ └─────────────────────────────────────────────────────────────────────┘

⚙️ Working Principle

Steps:

Frequency channel divided into repeating frames
Each frame divided into N time slots
Each user assigned one (or more) time slot(s) per frame
User transmits burst data only during assigned slot
User is silent during other slots (saves power!)
Process repeats every frame

Key Characteristics:

Shared frequency – multiple users on same channel
Burst transmission – transmit only during slot
Synchronization required – all users must be time-aligned
Digital only – needs buffering for burst mode
More efficient than FDMA for voice (silence detection)

│ User 1's view	│
│ User 2's view	│
│ User 3's view	│
│ Users take turns	share the channel fairly │
│ During "wait" time	phone can do other tasks, save battery │

📐 Frame Structure

│ Each Time Slot Contains	│
│ Guard bits	Prevent overlap between adjacent slots │
│ Training sequence	For channel estimation/sync │
│ Data	Actual user information │

GSM Frame Parameters:

Parameter	Value
Frame duration	4.615 ms
Slots per frame	8
Slot duration	0.577 ms (577 μs)
Bits per slot	156.25 bits
Bit rate (per slot)	270.833 kbps (total)
User data rate	~13 kbps (after coding)
Channel BW	200 kHz

⏱️ Synchronization

Strict synchronization is essential in TDMA – if users transmit outside their slot, other users' data will be corrupted.

Timing Issues:

Propagation delay – Signal travel time varies with distance
Clock drift – User devices ke clocks slightly different
Timing Advance – Base station tells mobile to transmit early to compensate for propagation delay

Without Timing Advance

User (far) ──┤TX├─── arrives LATE → overlaps next slot! ❌

With Timing Advance

User (far) ┤TX├───── arrives ON TIME ✅

(transmits early by propagation delay amount)

🛡️ Guard Time

Guard time is a small empty gap between adjacent slots that prevents overlap.

│ Slot N │ Guard │ Slot N+1 │ │ data │(empty)│ data │ Guard time accounts for: • Propagation delay uncertainty • Timing advance errors • Transmitter ramp-up/down time GSM guard time: ~30 μs (≈8.25 bit periods)

Trade-off: Longer guard = safer but wastes capacity. Shorter guard = more data but risk of collision.

📱 TDMA in GSM

GSM (Global System for Mobile Communications) uses a TDMA + FDMA combination:

GSM Structure:

FDMA	200 kHz channels across the band
TDMA	Each 200 kHz channel divided into 8 time slots
Total	FDMA × TDMA = many more users!
• Band	890-915 MHz (uplink), 935-960 MHz (downlink)
• 25 MHz bandwidth	125 frequency channels (FDMA)
• Each channel	8 time slots (TDMA)
• Total traffic capacity	125 × 8 = 1000 users per cell

GSM Features:

Frequency Hopping – Changes frequency each frame (anti-interference)
DTX (Discontinuous TX) – Don't transmit during silence (save power)
Timing Advance – Up to 233 μs (35 km cell radius)
Power Control – Adjusts TX power based on need

✅ Advantages & Disadvantages

Advantages:

Advantage	Hindi
More efficient than FDMA	More users in the same spectrum
No frequency guard bands between users	No frequency waste
Supports data services (packet)	Can also send data
Battery saving (TX only in slot)	Battery bachti hai
Flexible bandwidth (assign more slots)	More slots = more speed
Digital – encryption possible	Security easy
Frequency hopping possible	Interference se bachav

Disadvantages:

Disadvantage	Hindi
Synchronization required	Time sync is essential
Guard time wastes capacity	No data during guard time
Multipath limits cell size	Delay spread > guard = problem
Complex handset (burst mode)	Phone ka design complex
Limited to moderate data rates	Not suitable for very high speeds
Pulsed interference (to other devices)	Pulse pattern can cause interference

🌐 Applications

System	TDMA Details
GSM (2G)	8 slots/frame, 200 kHz channel
IS-136 (D-AMPS)	3 slots/frame, 30 kHz channel
DECT (cordless)	24 slots/frame, 1.728 MHz
PDC (Japan 2G)	3 slots/frame, 25 kHz channel
Satellite (some)	TDMA uplink sharing
IEEE 802.16 (WiMAX)	TDD-TDMA component

📝 Summary

Concept	Key Point
Principle	Divide time into slots, users take turns
Users share	Same frequency, different time slots
Frame	Repeating group of N time slots
Sync	Strictly required (timing advance)
Guard time	Empty gap between slots (prevents overlap)
Efficiency	Better than FDMA (shared channel)
Used in	GSM (2G), IS-136, DECT
Replaced by	CDMA (3G), OFDMA (4G/5G)

❓ FAQ

Q: What if a GSM user needs more speed? A: Multiple time slots can be assigned. In GPRS/EDGE, a user could get up to 8 slots (multi-slot operation) for faster data.

Q: How is cell size limited in TDMA? A: Signal propagation delay must not exceed the guard time. In GSM, the max timing advance is 233 μs = ~35 km cell radius. A user farther than this will overlap into the next slot.

Q: Is TDMA used in 4G/5G? A: Not pure TDMA, but in TDD (Time Division Duplex) mode, uplink and downlink are divided in time. Plus, the scheduler allocates resources in the time-domain (TDMA-like behavior).

Continue the lesson path

Study next in this module

4 connected lessons

01Multiple Access TechniquesSDMA Space Division Multiple AccessLearn SDMA with spatial separation, smart antennas, beamforming, MU-MIMO, massive MIMO 5G, antenna arrays, and how SDMA multiplies capacity without extra spectrum for engineering students.02Multiple Access TechniquesOFDMA Orthogonal Frequency Division Multiple AccessLearn OFDMA with OFDM basics, subcarrier allocation, resource blocks, cyclic prefix, SC-FDMA uplink, 4G LTE 5G NR implementation, scheduling, and advantages over CDMA for engineering students.03Multiple Access TechniquesFDMA Frequency Division Multiple AccessLearn FDMA with working principle, frequency channel allocation, guard bands, advantages disadvantages, AMPS 1G system, and comparison with TDMA CDMA OFDMA explained for engineering students.04Multiple Access TechniquesCDMA Code Division Multiple AccessLearn CDMA with spread spectrum, Walsh codes, processing gain, near-far problem, soft handoff, WCDMA CDMA2000 3G implementation, advantages disadvantages for engineering students.

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