--- title: "Multiple access, TCDMA and TDFH" subject: "Wireless Communication" chapter: 7 key: "semester_7/wireless_communication:tcdma-tdfh" source: https://www.bctengineer.com/q/semester_7/wireless_communication/tcdma-tdfh site: BCT Engineering --- # Multiple access, TCDMA and TDFH Wireless Communication · Chapter 7: Multiple Access in Wireless Communication - Asked in: 2071 Bhadra, 7 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/tcdma-tdfh - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7 ## Question Explain the terms multiple access, Time Division CDMA (TCDMA) and Time Division Frequency Hopping as related to wireless communication system. ## Short answer Multiple access allows many users to share scarce spectrum by dividing it in frequency, time, code, or space. TCDMA assigns each cell a different spreading code and uses time slots within a cell so only one user transmits at a time, avoiding near-far effect but needing time sync. TDFH combines TDMA with slow frequency hopping, where the subscriber changes carrier frequency at each new TDMA frame, gaining frequency diversity and averaging interference, as used in GSM. ## Answer #### Multiple access Multiple access is the method that allows **many mobile users to share a limited radio spectrum simultaneously**. The common resource is divided in frequency (FDMA), time (TDMA), code (CDMA) or space (SDMA). Wireless systems often combine these in **hybrid** forms to get the advantages of each, for example GSM uses FDMA + TDMA, and hybrid spread spectrum schemes combine CDMA with TDMA or frequency hopping. #### Time Division CDMA (TCDMA) TCDMA (also called TDMA/CDMA) is a hybrid technique that combines code division between cells with time division inside a cell. - A **different spreading code is assigned to each cell**. - Within a cell, each user is given a **time slot**, so only **one CDMA user transmits at a time** in each cell. - When the user hands off to another cell, its spreading code is changed to the code of the new cell. ```text Cell 1, code C1: |U1|U2|U3|U1|U2|U3| ... Cell 2, code C2: |U4|U5|U6|U4|U5|U6| ... time -------------------------> ``` - Advantage: **avoids the near-far effect**, because a near and a far user of the same cell are never received at the same time. Code separation still lets neighbouring cells use the same frequency. - Disadvantage: needs time synchronisation; capacity within a cell is limited by the number of slots. #### Time Division Frequency Hopping (TDFH) TDFH combines TDMA with slow frequency hopping. - The subscriber **hops to a new carrier frequency at the start of each new TDMA frame**, following a hopping sequence set by the network. - Within one frame (burst) the frequency is constant; it changes from frame to frame. ```text freq ^ f3 | [U1] f2 |[U1] [U1] f1 | ... +---frame1--frame2--frame3--> time ``` - Advantages: frequency diversity – a user does not stay in a deep fade or on a heavily interfered channel for long; co-channel interference is averaged over many users, which helps in heavily loaded cells. - Disadvantage: needs a central hopping plan and frame synchronisation between base station and mobile. - Example: **GSM** uses TDFH (slow frequency hopping, about 217 hops/s, one hop per TDMA frame of 4.615 ms). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In TCDMA, what is assigned to each cell and each user? A. A frequency to each cell and a code to each user B. A different spreading code to each cell and a time slot to each user in the cell C. A beam to each cell and a code to each user D. A time slot to each cell and a frequency to each user Answer: B. TCDMA combines code division between cells with time division inside a cell. (Key point: TCDMA structure) 2. What happens to the code when a TCDMA user hands off? A. It stays the same B. It is removed C. It is changed to the new cell's code D. It becomes a Walsh code Answer: C. Codes are per cell, so the user switches to the new cell's code. (Key point: TCDMA handoff) 3. Why does TCDMA avoid the near-far effect? A. Near and far users in one cell are never received at the same time B. Users hop frequencies C. Power is fixed D. Codes are orthogonal Answer: A. Only one CDMA user per cell transmits at a time. (Key point: TCDMA benefit) 4. When does a TDFH subscriber change frequency? A. Every chip B. At the start of each new TDMA frame C. Only at handoff D. Every symbol Answer: B. Frequency is constant within a frame and hops from frame to frame. (Key point: TDFH operation) 5. What is a key advantage of TDFH in GSM? A. No synchronisation needed B. Removes near-far C. Frequency diversity and averaging of co-channel interference D. Increases guard times Answer: C. A user does not stay in a deep fade or heavily interfered channel for long. (Key point: TDFH advantage) ## Related in Wireless Communication - [CDMA vs TDMA; FHSS multiple access](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acdma-advantages-fhss-types) — Wireless Communication, chapter 7 - [Spread spectrum multiple access; FDMA vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-ma-fdma-cdma) — Wireless Communication, chapter 7 - [Multiple access; FDMA vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amultiple-access-fdma-vs-cdma) — Wireless Communication, chapter 7 - [Frequency hopped multiple access (FHMA)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afhma-operation) — Wireless Communication, chapter 7, asked 5× - [Multiple access and near-far effect mitigation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amultiple-access-near-far) — Wireless Communication, chapter 7 - [Advantages and disadvantages of FDMA, TDMA, CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afdma-tdma-cdma-pros-cons) — Wireless Communication, chapter 7 ## Related in other subjects - [Spread spectrum frequency hopping in Bluetooth](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Afreq-hopping-bt) — Instrumentation II, chapter 5, asked 2× - [Multiplexing vs multiple access](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Amux-vs-multiple-access) — Data Communication, chapter 6 - [Multiple access protocols](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Amultiple-access-protocols) — Computer Networks and Security, chapter 3, asked 2× - [Multiple access in IEEE 802.5](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aieee-802-5-access) — Computer Networks and Security, chapter 3