A mobile phone signal is the signal strength received by a mobile phone from
a cellular network. Depending on various factors, such as proximity to a tower,
any obstructions such as buildings or trees, etc., this signal strength will
vary. Most mobile devices use a set of bars of increasing height to display the
approximate strength of this received signal to the mobile phone user.
Traditionally five bars are used; see five by five.
Generally, a strong mobile phone signal is more likely in an urban area, though
these areas can also have some "dead zones" where no reception can be
obtained. Cellular signals are designed to be resistant to multipath reception,
which is most likely to be caused by the blocking of a direct signal path by
large buildings such as high-rise towers. By contrast, many rural or
sparsely inhabited areas lack any signal or have very weak fringe reception; many
mobile phone providers are attempting to set up towers in those areas most likely
to be occupied by users, such as along major highways. Even some national parks
and other popular tourist destinations away from urban areas now have cell
phone reception, though location of radio towers within these areas is
normally prohibited or strictly regulated, and is often difficult to
arrange. In areas where signal reception would
normally be strong, other factors can have an effect on reception, or may
cause complete failure. From inside a building with thick walls or of mostly
metal construction, signal attenuation may prevent a mobile phone from being
used. Underground areas, such as tunnels and subway stations, will lack reception
unless they are wired for cell signals. There may also be gaps where the service
contours of the individual base stations of the mobile provider do not completely
overlap. In addition, the weather may affect the
strength of a signal, due to the changes in radio propagation caused by clouds,
precipitation, and temperature inversions. This phenomenon, which is
also common in other VHF radio bands including FM broadcasting, may also
cause other anomalies, such as a person in San Diego "roaming" on a Mexican
tower from just over the border in Tijuana, or someone in Detroit "roaming"
on a Canadian tower located within sight across the Detroit River in Windsor,
Ontario. These events may cause the user to be billed for "international" usage
despite being in their own country, though mobile phone companies can
program their billing systems to re-rate these as domestic usage when it occurs
on a foreign cell site that is known to frequently cause such issues for their
customers. The volume of network traffic can also
cause calls to be blocked or dropped due to a disaster or other mass call event
which overloads the number of available radio channels in an area, or the number
of telephone circuits connecting to and from the general public switched
telephone network. Dead zones
Areas where mobile phones cannot transmit to a nearby mobile site, base
station, or repeater are known as dead zones. In these areas, the mobile phone
is said to be in a state of outage. Dead zones are usually areas where mobile
phone service is not available because the signal between the handset and
mobile site antennas is blocked or severely reduced, usually by hilly
terrain, dense foliage, or physical distance.
A number of factors can create dead zones, which may exist even in locations
in which a wireless carrier offers coverage, due to limitations in cellular
network architecture, limited network density, interference with other mobile
sites, and topography. Since cell phones rely on radio waves, which travel though
the air and are easily attenuated, mobile phones may be unreliable at
times. Like other radio transmissions, mobile phone calls can be interrupted by
large buildings, terrain, trees, or other objects between the phone and the
nearest base s viders work continually to improve and upgrade their networks in
order to minimize dropped calls, access failures, and dead zones. For mobile
virtual network operators, the network quality depends entirely on the host
network for the particular handset in question. Some MVNOs use more than one
host, which may even have different technologies.
Dead zones can be filled-in with microcells, while picocells can handle
even smaller areas without causing interference to the larger network.
Personal microcells, such as those for a home, are called femtocells, and
generally have the range of a cordless phone, but may not be usable for an MVNO
phone. A similar system can be set up to perform inmate call capture, which
prevents cellphones smuggled into a prison from being used. These still
complete calls to or from pre-authorized users such as prison staff, while not
violating laws against jamming. These systems must be carefully designed so as
to avoid capturing calls from outside the prison, which would in effect create
a dead zone for any passersby outside. In the event of a disaster causing
temporary dead zones, a cell on wheels may be brought in until the local
telecom infrastructure can be restored. These portable units are also used where
large gatherings are expected, in order to handle the extra load.
Dropped calls A dropped call is the common term for a
wireless mobile phone call that is terminated unexpectedly as a result of
technical reasons, including presence in a dead zone. In technical circles, it is
called an abnormal release. One reason for a dropped call is when
the mobile phone moves out of range of a cellular network. An active call cannot
usually be maintained across a different company's network, resulting in the
termination of the call once a signal cannot be maintained between the phone
and the original network. Another common reason is when a phone is
taken into an area where wireless communication is unavailable,
interrupted, interfered with, or jammed. From the network's perspective, this is
the same as the mobile moving out of the coverage area.
Occasionally, calls are dropped upon handoff between cells within the same
provider's network. This may be due to an imbalance of traffic between the two
cell sites' areas of coverage. If the new cell site is at capacity, it cannot
accept the additional traffic of the call trying to "hand in." It may also be
due to the network configuration not being set up properly, such that one
cell site is not "aware" of the cell to which the phone is trying to handoff. If
the phone cannot find an alternative cell to which to move that can take over
the call, the call is lost. Co-channel and adjacent-channel
interference can also be responsible for dropped calls in a wireless network.
Neighbouring cells with the same frequencies interfere with each other,
deteriorating the quality of service and producing dropped calls. Transmission
problems are also a common cause of dropped calls. Another problem may be a
faulty transceiver inside the base station.
Calls can also be dropped if a mobile phone at the other end of the call loses
battery power and stops transmitting abruptly.
Sunspots and solar flares are rarely blamed for causing interference leading
to dropped calls, as it would take a major geomagnetic storm to cause such a
disruption. Experiencing too many dropped calls is
one of the most common customer complaints received by wireless service
providers. They have attempted to address the complaint in various ways,
including expansion of their home network coverage, increased cell
capacity, and offering refunds for individual dropped calls.
Various signal booster systems are manufactured to reduce problems due to
dropped calls and dead zones. Many options, such as wireless units and
antennas, are intended to aid in strengthening weak signals.
ASU Arbitrary Strength Unit is an integer
value proportional to the received signal strength measured by the mobile
phone. It is possible to calculate the real
signal strength measured in dBm by a formula. However, there are different
formulas for 2G and 3G networks. In GSM networks, ASU maps to RSSI.
dBm = 2 × ASU - 113, ASU in the range of 0..31 and 99.
In UMTS networks, ASU maps to RSCP level.
dBm = ASU - 116, ASU in the range of -5..91 and 255.
In LTE networks, ASU maps to RSRP. The valid range of ASU is from 0 to 97. For
the range 1 to 96, ASU maps to (ASU - 141) ≤ dBm