As per my knowledge (please correct me if I'm wrong), a mobile hotspot is an intermediate device between mobile base station (BS) and mobile user (MU). The link from BS to hotspot is cellular link and the link from hotspot to MU is Wi-fi.
1. During switching between cellular and Wi-fi spectra, what modifications are adapted in terms of channel width, modulation, coding schemes and so on?
2. What are the effect of these modifications on throughput (may be in terms of processing delay)?
3. Is any concept of "spectrum sensing" adapted there or only "carrier sensing"?
Cellular and WiFi both use a variety of standards, depending on the version you are using. In general, a single RF channel of both WiFi and cellular 4G is 20 MHz wide, although in the case of cellular, that 20 MHz can be created out of multiple narrower slices. Also, the newer 4G and the newer variants of WiFi can aggregate multiple 20 MHz channels, to achieve RF channel widths of 40, 80, 160, or more MHz. They need these wide channels to provide the b/s capacity they advertize. You should search for the standards that apply to cellular and to WiFi (IEEE 802.11), to see all the details of the different physical layer specs, for each version.
As to spectrum sensing, WiFi does that, attempting to find an open channel, when the access point is powered up.
The answers to your questions are involved, and vary with version numbers, so it's not a simple request. For instance, in terms of modulation, cellular 3G uses wideband CDMA, and IEEE 802.11b uses a similar direct sequence spread spectrum scheme. While cellular 4G uses LTE (which in turn uses OFDM). The newer and faster variants of 802.11 (g, n, and ac) use OFDM. And the RF frequencies are different too, where cellular uses a variety of bands, from 700 MHz on up to bands above 1 GHz, while WiFi uses either the 2.4 GHz band or the 5 GHz band. So, best to search out these details online.
Also, the physical layer translation going on, when you use mobile hotspots, is really no different from what any home modem/router has to do, for your home broadband service. In essence, you are buffering each received frame, as switches do, and then transmitting them to the other side using the appropriate RF standard for that end. Wireless or wired makes no difference in this regard. You're usually doing a translation in both directions, buffering frames, because you're almost always going to have mismatched speeds at the two sides of the modem/router/mobile hotspot.
1. Interfacing Cellular networks to Hotspots has become an important technical challenge to save spectrum. It is generally done with the help of Generic Access Network. The modulation schemes in cellular and Wi-Fi are different. The BW is larger in Wi-Fi.Channel coding schemes are also different. References like GSM manual and 811.15.2 will help to get details.
2. The throughput in Wi-Fi is higher as the BW is more but it is neutralized by the requirement of more data for coding.
3. Here, only carrier sensing is required (FHSS).
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