# Railway Wireless Communications Channel Characterization
## 2 QoS Parameters for Communication Technology Performance Measuring  
The communication channel is the part of a system that allows sending information from the transmitter to the receiver. It can be characterized regarding different concepts such as radiofrequency or IP level. Focusing on IP level, the main IP QoS parameters that define a communication channel are the following ones:  
* RTT: duration required for a network to send a request from a starting point to a destination and back again to the starting point. \[RTT=t_{1}+t_{2}\] where \(t_{1}\) is the time required to a packet from the source to the destination, and \(t_{2}\) is the time required to a packet from the destination to the source.
* Jitter: measurement of the latency variation above and below the mean latency value. The maximum jitter is defined as the maximum latency variation above and below the mean latency value [7].
* Bandwidth: in terms of data network, bandwidth quantifies the data rate at which a network link or a network path can transfer; the amount of data a link or network path can deliver per unit of time [8]. \[BW\;(bps)=\frac{\sum\;bits}{second}\] (1)
* Packet loss: the failure of a packet to traverse the network to its destination [9; 10]; it is measured as the percentage of lost packets over the sent ones. It occurs when one or more packets transmitted over an IP network fail to arrive at their destination. Packet loss is typically caused by what is generally referred to as network congestion, but also because of distance or poor line quality. Excessive packet loss is perceived as disconnections (broken or missing communication). \[\text{Packet loss }(\%)=\frac{1-\sum\;\frac{received\;packets}{sent\;packets}}{second}*100\] (2)
* Throughput [11]: the number of payload bits successfully received per second for a reference measurement channel in a specified reference condition. \[\text{Thoughput }(bps)=\frac{payload\;bits}{second}\] (3)
* Disconnections: a time interval where there is no connection. One of the reasons could be the lack of coverage. Moreover, regarding radiofrequency scope for wireless communication, other parameters exist that allow to find out the behavior of the communication channel, such as:
* Received Signal Strength Indicator (RSSI): defined as the power of a certain signal of interest divided by the sum of the interference power (from all the other interfering signals) and the power of some background noise. Furthermore, other parameters are specific for technologies, e.g., for LTE, such as:
* Signal-to-interference-plus-noise-ratio (SINR): defined as the power of a certain signal of interest divided by the sum of the interference power (from all the other interfering signals) and the power of some background noise. \[\text{SINR}=\frac{S}{I+N}\] (4) where \(S\) is the desired Signal (W), \(I\) is the interference (W), and \(N\) is the noise (W).  
* Reference Signal Received Power (RSRP): defined as the linear average over the power contributions (in (W)) of the resource elements that carry cell-specific reference signals within the considered measurement frequency bandwidth [12].
* Reference Signal Received Quality (RSRQ): defined as the ratio \(N\times\) RSRP/(E-UTRA carrier RSSI), where \(N\) is the number of RBs of the E-UTRA carrier RSSI measurement bandwidth [12]. \[\text{RSRQ}=\frac{N\times\text{RSRP}}{\text{LTE carrier RSSI}}\] (5) where \(N\) is the number of Resource Blocks of the LTE carrier of RSSI measurement bandwidth.