How does in-call handover compare across GSM, UMTS, and LTE — and why is soft handover unique to UMTS?
GSM does hard handover (break-before-make), UMTS adds soft/softer handover (make-before-break, connected to several cells at once), and LTE returns to hard handover but hides the gap with X2 data forwarding. Soft handover exists only in UMTS because its CDMA air interface lets one phone receive several cells on the same frequency simultaneously.
The three generations solve the same problem — keep a call alive while the radio link moves — with radically different radio technology, which dictates what kind of handover is even possible.
| Generation | Air interface | Handover style | Who decides | Combining point |
|---|---|---|---|---|
| GSM (2G) | FDMA/TDMA | Hard (break-before-make) | old BSS → MSC | n/a (one link only) |
| UMTS (3G) | CDMA | Soft / softer (make-before-break) | RNC (mobile-assisted) | RNC (soft) / NodeB (softer) |
| LTE (4G) | OFDMA | Hard, masked by data forwarding | source eNodeB → MME | n/a (one link, packets forwarded) |
Why only UMTS gets soft handover: in CDMA every cell transmits on the same frequency and is distinguished by a scrambling code, so a phone can lock onto two or three NodeBs at once with no retuning, and the network combines their signals. GSM's TDMA radio is tuned to one frequency/timeslot, so it physically cannot hold two links — it must drop one before grabbing the next (hard). LTE's OFDMA could in principle do something similar, but the designers chose simple hard handover and instead forward the in-flight packets from the old to the new base station, so the user never notices the break.
The constant across all three: the phone only measures and reports; the network decides (old BSS in GSM, RNC in UMTS, eNodeB/MME in LTE). And indirect routing via the home network (anchor MSC in GSM, home P-GW tunnel in LTE) keeps the moving subscriber reachable.
Tip: One-liner for the oral exam — "GSM hard, UMTS soft because of CDMA, LTE hard again but with packet forwarding; measure at the phone, decide in the network."
Go deeper:
UMTS handover — hard, soft, softer & inter-RAT (Electronics Notes) — the hard/soft/softer taxonomy that distinguishes the generations, with the rake-receiver combining that makes UMTS soft handover physical.- Soft handover (Wikipedia) — make-before-break and the rake receiver that make UMTS multi-cell connection physically possible.