--- title: "SDMA and hybrid SS techniques against near-far" subject: "Wireless Communication" chapter: 7 key: "semester_7/wireless_communication:sdma-hybrid-near-far" source: https://www.bctengineer.com/q/semester_7/wireless_communication/sdma-hybrid-near-far site: BCT Engineering --- # SDMA and hybrid SS techniques against near-far Wireless Communication · Chapter 7: Multiple Access in Wireless Communication - Asked in: 2075 Bhadra, 2+6 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/sdma-hybrid-near-far - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7 ## Question What is space division multiple access? Explain any two hybrid spread spectrum multiple access techniques which minimize the effect of near-far effect. ## Short answer SDMA separates users by position using smart antennas that form narrow beams, reusing the same frequency and time in different directions. Hybrid DS/FHMA spreads data with a PN code then hops the center frequency, so a near user rarely shares a hop with a far user. TCDMA assigns each cell a code and each user a time slot, so only one user per cell transmits at a time, avoiding near-far. ## Answer #### Space division multiple access Space Division Multiple Access (SDMA) separates users by their **position in space**. The base station uses directional or **smart (adaptive) antennas** that form narrow beams pointing at each user, so the same frequency and time slot can be reused for users in different directions within the same cell. ```text beam 1 -> user A [BS]==== beam 2 -> user B (same f, same t) beam 3 -> user C ``` - Advantages: higher capacity, lower interference, less transmit power, larger range. - Ideal SDMA would give an infinitely narrow beam to each user; practical systems use sectoring or adaptive arrays. #### Hybrid spread spectrum techniques that reduce the near-far effect **1. Hybrid DS/FHMA (direct sequence + frequency hopping)** - The data is first spread with a PN code (DS). The centre frequency of the DS signal then hops pseudo-randomly over the wide band. - At any time a user occupies only a small part of the band. A strong nearby user is very unlikely to be on the same sub-band at the same moment, so it does not swamp the weak user. Hence the near-far problem is largely avoided. ```text data->(x)PN code->(x)->[BPF]-> Tx ^ [Freq synthesizer]<-[hop PN gen] ``` - Disadvantage: complex hardware; poor support for soft handoff because the base station must follow many hopping patterns. **2. Time Division CDMA (TCDMA)** - Each cell is given a different spreading code (codes separate cells). - Inside a cell, users are separated in **time**: each user gets a time slot, so only one user transmits at a time in a cell. - Because there is never a near and a far user of the same cell received together, the near-far effect is avoided. On handoff, the user's code changes to that 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| ``` - Disadvantage: needs slot synchronisation; per-cell capacity limited by number of slots. Other hybrids (FCDMA, TDFH) also exist, but DS/FHMA and TCDMA are the two usually cited for near-far protection. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does SDMA separate users? A. By their time slots B. By their spreading codes C. By their position in space using narrow beams from smart antennas D. By their carrier frequencies Answer: C. SDMA forms beams toward each user so the same frequency and time slot can be reused in different directions. (Key point: SDMA principle) 2. Which is an advantage of SDMA? A. Higher capacity and lower interference with less transmit power B. Needs no antennas C. Requires precise slot synchronisation D. Uses wider guard bands Answer: A. Narrow beams reduce interference and power and raise capacity and range. (Key point: SDMA advantages) 3. Why does hybrid DS/FHMA reduce the near-far effect? A. The PN code is made orthogonal B. A strong near user is rarely on the same hop frequency at the same time as the weak user C. Only one user transmits per cell D. Power is fixed at a minimum Answer: B. Since the DS signal's centre frequency hops, users occupy only a small part of the band at a time and seldom collide. (Key point: DS/FHMA avoids near-far) 4. In TCDMA, how are users inside a cell separated and how are cells separated? A. Users by codes, cells by time slots B. Users by frequency, cells by beams C. Users by time slots, cells by different spreading codes D. Both by the same code Answer: C. Each cell has its own code and users take turns in time slots, so only one user per cell transmits at a time. (Key point: TCDMA operation) ## Related in Wireless Communication - [Near-far effect and a hybrid SS technique](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Anear-far-hybrid-technique) — Wireless Communication, chapter 7 - [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 - [FHMA principle and hybrid techniques for near-far](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afhma-hybrid-near-far) — 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 - [Hybrid multiple access techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ahybrid-multiple-access) — Wireless Communication, chapter 7 - [Spread spectrum multiple access variants](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Assma-variants-applications) — Wireless Communication, chapter 7