--- title: "Hand off and its strategy in GSM" subject: "Wireless Communication" chapter: 2 key: "semester_7/wireless_communication:handoff-strategy-gsm" source: https://www.bctengineer.com/q/semester_7/wireless_communication/handoff-strategy-gsm site: BCT Engineering --- # Hand off and its strategy in GSM Wireless Communication · Chapter 2: Cellular Mobile Communication Concepts - Asked in: 2070 Magh, 8 marks - Video: [GSM Handoff Procedure - 2G Technologies - Mobile Communication System](https://www.youtube.com/watch?v=6sxzWZgf2-4) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/handoff-strategy-gsm - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-2 ## Question What is hand off? Explain its strategy used in GSM. ## Short answer Handoff transfers an ongoing call from one cell to another without interruption. GSM uses Mobile Assisted Handoff: the mobile measures serving and neighbour signal levels, reports the six strongest to the BSC, which decides when to switch to a new channel. ## Answer **Handoff (handover)** is the process of transferring an ongoing call from one channel or base station to another without interrupting the call, as the mobile moves from one cell into another or when signal quality falls. #### Basic handoff idea - Each BS watches signal level. A handoff threshold is set a little above the minimum usable level: ```text Δ = Pr,handoff − Pr,minimum usable ``` - If Δ is too large, there are unnecessary handoffs; if too small, the call may drop before handoff completes. - The system checks that the drop in signal is not just momentary fading, so it averages the signal over time (dwell time). ```text Signal | BS1 signal BS2 signal |\ / | \______ handoff ______/ | \ threshold / |----\--------------/----- minimum usable level +------------------------------ distance BS1 A B BS2 (handoff made between A and B) ``` #### Handoff strategy in GSM: mobile assisted handoff (MAHO) GSM uses **Mobile Assisted Handoff** and **hard handoff** (break-before-make). 1. The BSC sends the MS a list of neighbour cells (BCCH allocation list). 2. In idle TDMA time slots, the MS measures the received level (RXLEV) of up to 32 neighbours, and RXLEV and RXQUAL (bit error quality) of the serving cell. 3. The MS reports the six strongest neighbours plus serving cell values to the BTS on the SACCH about every 480 ms. 4. The BTS adds its own uplink measurements and timing advance (distance). 5. The **BSC** decides when to hand off, based on level, quality, distance or traffic load, and selects a target cell. 6. The target channel is reserved, the MS is commanded to switch, it sends access bursts on the new channel, and the old channel is released. Because the MS does the measuring, handoff decisions are faster (about 1–2 s) than in network-controlled handoff of first-generation systems. #### Types of GSM handoff | Type | Between | Controlled by | |---|---|---| | Intra-cell | Two channels in the same cell | BSC | | Intra-BSC | Two cells under same BSC | BSC | | Inter-BSC (intra-MSC) | Cells of different BSCs, same MSC | MSC | | Inter-MSC | Cells under different MSCs | Both MSCs | #### Prioritizing handoffs - **Guard channels:** some channels are reserved only for handoff requests. - **Queuing of handoff requests:** a request is queued while the mobile is still in the overlap area. - Umbrella cells are used for fast-moving users to reduce the number of handoffs. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is handoff? A. Starting a new call in a cell B. Transferring an ongoing call to another channel or base station without interrupting it C. Ending a call when signal is weak D. Raising mobile transmit power Answer: B. Handoff keeps the call alive as the mobile moves or signal quality falls. (Key point: Handoff definition) 2. What happens if the handoff margin delta is too small? A. Interference stops B. Capacity doubles C. Too many unnecessary handoffs D. The call may drop before handoff completes Answer: D. A small delta leaves little time to complete the handoff, risking a dropped call; a large delta causes needless handoffs. (Key point: Threshold delta) 3. In GSM mobile assisted handoff, who measures neighbour cell signal levels? A. The mobile station in idle time slots B. The MSC C. The BSC D. The HLR Answer: A. The MS measures RXLEV of neighbours in idle TDMA slots and reports on SACCH. (Key point: MAHO) 4. Which entity decides when to hand off and selects the target cell in GSM? A. The BSC B. The target BTS C. The SIM card D. The mobile station Answer: A. The BSC uses level, quality, distance and load reports to decide. (Key point: Decision maker) 5. What are guard channels? A. Channels used for billing B. Unused spectrum between cells C. Channels with extra power D. Channels reserved only for handoff requests Answer: D. Guard channels give handoff requests priority over new calls. (Key point: Prioritizing handoffs) ## Related in Wireless Communication - [GSM traffic and control channels](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-traffic-control-channels) — Wireless Communication, chapter 8, asked 4× - [GSM frame structure, traffic and control channels](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-frame-traffic-control-channels) — Wireless Communication, chapter 8, asked 4× - [GSM frame structure and channels during a call](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-frame-call-channels) — Wireless Communication, chapter 8 ## Related in other subjects - [SCR from complementary BJTs; turning off SCR](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Acomplementary-bjt-scr-turn-off) — Advanced Electronics, chapter 7