Mobile phone signal

A display of bars on a mobile phone screen

A mobile phone signal (or reception) is the strength (measured in dBm) of the connection to the mobile phone with its network. Depending on various factors, such as proximity to a tower, obstructions such as buildings or trees, etc, the signal may vary. Most mobile devices use a set of bars of varying heights to display the strength of the signal where the device is located. Traditionally five bars are used; see five by five.

Generally, a stronger mobile phone signal is easier to obtain in an urban area, though urban areas do have some "dead zones" where a reception cannot be obtained. On the contrary, many rural or minimally inhabited areas lack a signal or have a very weak reception, but many mobile phone providers are attempting to set up towers in parts of these 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 receptions.

In an area where the signal would normally be strong, certain other factors may have an effect on the reception, thereby making it either stronger or weaker, or may cause complete interference. For example, a building with thick walls may prevent a mobile phone from being used. Many underground areas, such as tunnels and subway stations, lack a reception. Additionally, the weather and volume of network traffic may impact the strength.

Dead zones

Areas where cell phones cannot transmit to a nearby cell site, base station, or repeater are known as dead zones. In these areas, the cell phone is said to be in a state of outage. Dead zones are usually areas where cell phone service is not available because the signal between the handset and the cell site antenna is blocked, usually by hilly terrain, excessive foliage, physical distance, or tall buildings.

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 (the locations of antennas), limited network density, interference with other cell sites, and topography. Since cell phones rely on radio waves, and radio waves travel though the air and are easily attenuated, cell phones may be unreliable at times. Like other radio transmissions, cell phone calls can be interrupted by large buildings, terrain, trees, or other objects between the phone and the nearest base station antennas.

Many wireless service providers work continually to improve and upgrade their networks in order to minimize dropped calls, access failures, and dead zones (which they call "coverage holes" or "no-service areas").

Dropped calls

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.

One reason for a dropped call is when the mobile phone moves out of range of a wireless network. An active call cannot usually be maintained across a different company's network (as calls cannot be re-routed over the traditional phone network while in progress), 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. Neighbour 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 (XCVR) 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.

Sun spots and solar flares are rarely blamed for causing interference leading to dropped calls.

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

ASU or Active Set Update 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 (and thereby power in Watts) by a formula. However, there are different formulas for 2G and 3G networks.

In GSM networks, ASU is equal to the RSSI (received signal strength indicator, see TS 27.007).

 dBm = 2*ASU - 113,  ASU in the range of 0..31 and 99

In UMTS networks, ASU is equal the RSCP level (received signal control power, see TS 25.125)

 dBm = ASU - 116,     ASU in the range of -5..91

Although the mobile standards define an ASU message ("Active Set Update") used in handover procedures, on Android phones the acronym ASU has nothing to do with Active Set Update. It has not been declared precisely by Google developers, but it is believed to mean "Arbitrary Scale Unit" or "Android Signal Unit". (see NeighboringCellInfo)

See also

• Cell phone tower
• Cellular network
• Subscriber Identity Module
• Cellular repeater
• Signal strength
• Cel-Fi
• Missed call (intentionally dropped calls)