Answer:  Indoor Propagation Model:-The mobile to mobile radio communication has to be analysed with respect to the indoor propagation as we give importance to the outdoor propagation setup. The indoor propagation is also influenced by same type of mechanisms as that of the outdoor propagation. The mechanisms are:-

(i) Reflection

(ii) Diffraction and

(iii) Scattering.

It is also important that whether the indoor building is in open or closed status because the level of radio signal will vary greatly depending on this status. An antenna mounting setup also dominates the signal propagation. That is, if the antenna is mounted on ceiling then the mobile signal strength will be stronger than mounting antenna on desks within a partitioned rooms. There are many indoor model available. They are :

a. Partition losses:- Same floor in a building.

b. Partition losses:- Between floors in a building.

c. Ericsson multiple breakpoint model.

d. Log distance pathloss model.

e. Attenuation factor model.

This indoor propagation becomes important with the advent of personal communication systems$ (PCS)$ and the communication between mobile to mobile was focussed much more on considering indoor propagation aspects. If the system is not properly designed there might be greater gam loss occurence in signal propagation. The channels regarding indoor propagations is also subdivided into two important groups known as line of sight $(LOS) $and obstructed signal $(OBS)$. Some of the indoor propagation models are below:-

a) Partition loss (Same floor in a building)- Consider a multistoried building where two types of partitions may be used.

i) Hard partition

ii) Soft partition

If the partition are formed within the building as a part of building and if reconfiguring the partitions is not possible such a type is known as hard partition. On the other hand if the partitions are reconfigurable and does span to ceiling then such a type is called as soft partition. These partitions might be made up of wood as movable office partitions such that the space may be designed and easily reconfigured at any time. 

b). In Partition loss (Considering between the floors in a building)- The partition losses between the floors of a building depends on the entire plan of construction of the building. The external projections, number of windows available, presence of tinting etc. influences losses in radio wave propagation. There may be greater attenuation in the radio energy. Hence the plan involved in building construction has to be done very carefully to avoid or to reduce partition losses between floors. The metal and other materials used for construction also imparts certain amount of partition losses in radio propagation. 

 c). Ericsson (Multiple breakpoint model) : It is a radio system model which was developed due to multiple floors in an office building this model considers an upper and the lower bounds on the path loss $ (PL)$ of the radio signal. There is path loss variations observed for different frequency$f$ ranges. This model assumes that an attenuation of $30 \ dB$ for a distance of $1$meter and a frequency of $\text{900 MHz}$. Considering unity gain antennas, there is a deterministic limit on the range of the path loss $(PL)$ at certain distances by this Ericsson model.  **d). Log-distance pathloss model**:This model stresses that the indoor pathloss measurements must statisfy distance power low so that the pathloss $ PL$ in decibels is given as,  $\mathrm{PL(dB) = PL(D_o) + 10 \ n \ log \bigg(\dfrac{D}{D_o}\bigg)+X_{\sigma} }$ where  $\mathrm{PL= Pathloss}$ $\mathrm{n= Pathloss\ exponent }$ $\mathrm{X_{\sigma}=\text{ Normal random variable wire standard deviation denoted as }\sigma}$ **e). Attenuation factor model:** In this model an inbuilding site specific propagation is considered. It is a better method which can be used to minimize the standard deviation between the predicted measured pathloss value upto $4$ decibels. The attenuation factor model is given as, $\mathrm{PL\ (D)dB = PL(D_o)\ dB + 10 \ n \ log \bigg(\dfrac{D}{D_o}\bigg)+FAF \ dB+\Sigma \ PAF \ dB\ .............. }$ where $\mathrm{PL= Pathloss \ in \ decibels}$ $\text{FAF= Floor attenuation factor }$ $\text{PAF -Partition attenuation factor for a particular constructions }$ considering a single ray between transmitter$\mathrm{T}$and receiver $\mathrm{R}$.  $\mathrm{n_{sf}\rightarrow \text{Pathloss exponent in same floor}}$ The method of drawing a single ray between transmitter and receiver is known as primary ray tracing. Selective values of $FAF$ and better estimate of $n$ and cumulative pathlosses will lead to an accurate attenuation factor model. In the above equation $FAF$ is simply replaced with a pathloss exponent value, and it can be rewritten as, $\mathrm{PL\ (D)dB = PL(D_o) + 10 \ n_{mf} \ log \bigg(\dfrac{D}{D_o}\bigg)+\Sigma \ PAF \ dB\ }$ where $\mathrm{n_{mf}}=\text{ Pathloss exponent value for multiple floors }$ The in-building pathloss follows the free space loss factors that increases with range values and considering this in a multifloor building, the equation for attenuation factor model is again written as, $\mathrm{PL\ (D)dB = PL(D_o) \ dB+ 20 \ log \bigg(\dfrac{D}{D_o}\bigg)+ \alpha \ d+\ FAF \ dB+\Sigma \ PAF \ dB\ }$ $\mathrm{\alpha \rightarrow \text{Attenuation constant value for the channel. (unit is dB/m). }}$