Almost every mobile phone user has come accross a situation in which during the conversation the call gets suddenly dropped. After re-dialing the same number again the connection is usually back to normal and the conversation can be continued. Possible causes of this phenomenon will be described below.
Poor radio propagation
The cellular network consists of transmitters arranged in an appropriate configuration. The range of a single base station is limited by its maximum luminosity. Details are presented in the article: “Fading, shadowing, and other effects. Part I”. Usually, the power of the transmitters is selected in such a way that between adjacent stations there is ensured a sufficient signal level that does not cause dropping or breaking the connection. But this is not always the case.
The user during a voice call can be in a place with bad radio propagation. For example, when you drive a car, ride a bike or walk by foot you can be out of range of the base station by being in a “dead zone” of a given operator. The article “Dead zone in 3G coverage” was devoted to such places found in Warsaw by users of RFBenchmark mobile application.
An example of a moving mobile within a few stations is shown in the illustration below:
The reasons for the formation of “dead zones” in network coverage may be various. It could be the case that the operator did not cover the range of certain areas. For example forests, fields, places with very low population density or any other place which is not visited or seldom visited by mobile users.
Another reason may be the terrain. If the terrain is quite irregular, for example there are mountains, passes, slopes and changes in altitude, then the signal from the base station will not travel to the spots which are difficult to reach. If the mobile is used in such a spot, the call will be dropped.
In the cities, the formation of “dead zones” may be caused by covering the path in which the signal travels; this is usually done by buildings and attenuation introduced by their structures. In the case of rapid urbanization and the erection of new buildings it may turn out that the network which was well designed has now “dead zones.” In addition, the lack of coverage is most common in underground garages and basements, due to high attenuation of the signal by the structure of the object.
Unsuccessful handover (handoff from one base station to another)
When moving while on a phone, the connection is being transferred between consecutive base stations whenever the user is in their reach. This process is called a handover and is of course performed automatically. When the signal from one base station increases and and from the second station decreases, at some point there is a handover performed.
It may happen that the signal level may be high, however, due to the lack of resources the handover will not be performed to an overloaded channel. Dropped calls will be caused by poor radio conditions if there is no other channel, to which the connection could be transferred to, and the mobile will be out of range of the current base station.
An example of inability to perform a handover of the mobile connection to an adjacent base station:
Providing mobile network services requires proper signal-to-noise ratio. Signal level without any reference to the noise level does not inform us about its quality at a given moment of time but only helps to maintain an appropriate relationship to interference. In the case of heavy traffic on one channel or interference from adjacent channels it can happen that despite increasing transmission power to the maximum, the minimum signal-to-noise ratio of the signal being received (for the given service) will not be retained. In this case the call will be dropped.
In the case of 3G technology, where the radio signal is scattered in the spectrum, a value for Ec/I0 is used, which is the ratio of energy per chip of the scattered signal to noise ratio. In the receiver, after extraction of scattered signal from the noise by the use of spreading sequence, Eb/N0 is used instead of Ec/I0, which is the ratio of energy per bit to noise spectral density level. Interferences from other users and from the neighboring base stations, from the point of view of the receiver have noise effect.
The value of Ec/I0 is negative (in decibels) as it relates to the spread spectrum where both interference and the desired signal are reduced to the same scale. In contrast, Eb/N0 is positive (in decibels) because the desired signal level after the extraction with the use of spreading.
The process of spreading the spectrum to the narrowband signal is presented below:
|Transmitted narrowband signal before spectrum spreading.||The transmitted signal after spectrum spreading in the radio channel along with other signals (eg. adjacent base stations)||The received signal after reapplying the same spreading sequence is “extracted” from the noise|
A dropped call may also be caused by a low battery in the phone. Broken connections can also occur in case of damage to the phone, in particular damage to the antenna and the transmitter module.
The same applies to transmitters. Dropped calls can result from malfunction of base stations or the whole network.
Summing up any mobile phone user can experience the problem of a dropped call. The reasons for that may be various, starting with those associated with deficiencies in the radio coverage, through those associated with short-term lack of resources caused by other users or the introduction of excessive interference, ending with the problems with the network or user’s mobile. Statistically speaking, in the current mobile networks with good radio conditions, dropped call should be a relatively rare phenomenon.