--- title: "CDMA merits/demerits; GSM frame efficiency" subject: "Wireless Communication" chapter: 7 key: "semester_7/wireless_communication:multiple-access-cdma-frame-efficiency" source: https://www.bctengineer.com/q/semester_7/wireless_communication/multiple-access-cdma-frame-efficiency site: BCT Engineering --- # CDMA merits/demerits; GSM frame efficiency Wireless Communication · Chapter 7: Multiple Access in Wireless Communication - Asked in: 2070 Bhadra, 2+6+4 marks - Video: [CDMA ll Code Division Multiple Access Explained with Solved Example in Hindi](https://www.youtube.com/watch?v=n9-VQvIFvFc) — 5 Minutes Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/multiple-access-cdma-frame-efficiency - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7 ## Question Define multiple access. What are the merits and demerits of Code Division Multiple Access? If a normal GSM time slot consists of 6 trailing bits, 8.25 guard bits, 26 training bits, and 2 traffic bursts of 58 bits of data, find the frame efficiency. ## Short answer Multiple access allows many users to share limited radio spectrum. CDMA merits include universal reuse, soft capacity, and soft handoff, but suffers from near-far problem and self-jamming. GSM frame efficiency: overhead per slot is 40.25 bits out of 156.25 bits, so useful data fraction is 116/156.25 = 74.24%. ## Answer #### Multiple access Multiple access is the technique by which **many users share a common, limited radio spectrum at the same time** without causing serious interference to each other. The shared resource is divided in frequency (FDMA), time (TDMA), code (CDMA) or space (SDMA). #### Merits of CDMA 1. **Universal frequency reuse** (N = 1): all cells use the same carrier; no frequency planning. 2. **Soft capacity**: more users can be added with a gradual drop in quality. 3. **Soft handoff**: the mobile connects to two base stations at once; fewer dropped calls. 4. **Multipath advantage**: RAKE receiver combines multipath components (diversity). 5. **Voice activity and sectorisation gains** directly increase capacity. 6. **Privacy and interference rejection** due to PN spreading and processing gain. 7. Wide bandwidth reduces the effect of frequency-selective fading. #### Demerits of CDMA 1. **Near-far problem** – needs fast and accurate power control. 2. **Self-jamming** – codes not perfectly orthogonal; interference from other users limits capacity. 3. **Complex receivers** – code acquisition, tracking and RAKE processing. 4. **Strict timing** – IS-95 base stations need GPS synchronisation. 5. **Cell breathing** – coverage shrinks as load increases. #### GSM frame efficiency Given: tail (trailing) bits = 6, guard bits = 8.25, training bits = 26, data = 2 × 58 = 116 bits. ```text Bits per slot = 6 + 8.25 + 26 + 2×58 = 156.25 bits Bits per frame b_T = 8 slots × 156.25 = 1250 bits Overhead/slot = 6 + 8.25 + 26 = 40.25 bits Overhead/frame b_OH = 8 × 40.25 = 322 bits η_f = (1 − b_OH / b_T) × 100% = (1 − 322 / 1250) × 100% = (1 − 0.2576) × 100% = 74.24 % ``` (Equivalently, useful data per slot = 116/156.25 = 0.7424.) **Answer: Frame efficiency η = 74.24 %** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is soft handoff in CDMA? A. The mobile connects to two base stations at once, so fewer calls drop B. The call is dropped then reconnected C. The mobile changes frequency D. The call is moved by time slot Answer: A. Soft handoff keeps connections to both base stations during transfer. (Key point: Soft handoff) 2. What is cell breathing? A. Coverage grows with load B. Coverage shrinks as load increases C. Hop rate changes D. Frequency reuse changes Answer: B. More interference from more users reduces effective coverage radius. (Key point: Cell breathing) 3. In the GSM slot example, which bits are overhead? A. The 116 data bits B. Data and training bits C. Tail 6 + guard 8.25 + training 26 = 40.25 bits D. Only the guard bits Answer: C. Overhead per slot = 6 + 8.25 + 26 = 40.25; data is 2 x 58 = 116. (Key point: Overhead bits) 4. What is the frame efficiency of the GSM slot (8 slots/frame)? A. 25.76 % B. 80 % C. 65 % D. 74.24 % Answer: D. Frame has 1250 bits with 322 overhead: (1 - 322/1250) x 100 = 74.24 % (= 116/156.25). 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