Communication and Mobile Devices
Communication and Mobile Devices
This domain tests cellular generations, SIM and IMEI identity, RF front ends, antennas, charging and battery circuits, displays and firmware service procedures.
Core rules
- GSM, UMTS and LTE use different air interfaces and bands. A handset must support the operator's band and be network-unlocked.
- The SIM identifies the subscriber; the IMEI identifies the handset. Never alter an IMEI; cloning is illegal.
- The RF chain includes antenna, switch/filter, power amplifier, transceiver and baseband. A damaged antenna or coax can cause no network or weak signal.
- Charging path: charger, cable, connector, protection, charging IC, battery. Measure current draw with a USB meter under load.
- Lithium-ion cells need correct charge voltage and protection; swollen or leaking cells must be replaced, not recharged.
- Flashing firmware requires the correct model and region build; a wrong image can brick the device.
Worked example
A phone does not charge. A USB power meter shows 5 V but only 0.05 A. Try a known charger and cable, inspect and clean the port, then check battery voltage. If the battery is deeply discharged, a preconditioning current may be needed. If current pulses and the phone warms near the charging IC, suspect the IC or a shorted rail. If current is zero, check the port and protection device. Do not conclude 'bad battery' until the charging path is measured under load.
Traps
Shorting a lithium battery; probing an antenna with a DC ohmmeter; flashing mismatched firmware; changing an IMEI; assuming a charged battery is a good battery under load.