The handover test system has multiple RF paths with variable attenuators on each path to fade signal strength in each path individually to simulate a mobiles handover scenario in a Lab. The attenuation level setting can be controlled manually using the front panel keypad and display or remotely over the Ethernet or RS-232 interfaces.
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To understand the handover test system in-depth, we recommend you to learn more about the following components associated with it.
In this RF Handover test system article, we will cover,
Due to the presence of obstacles in the environment the signal strength sent out off by a base station is always get attenuated, to overcome this, the cellular networks are divided into different cells consisting of transceiver antenna in each cell.
The mobile handover is to maintain the continuous connection between the base station to a mobile device when it moves from one cell coverage area to another. The handover or handoff are of two types Soft handover and Hard handover.
The Handover can occur as an Intra-system or Inter-system.
Intra system handover: It occurs between sectors of the same base station which will have multiple cells within the same base station coverage area.
Inter-system handover: It occurs between one base station to a completely different base station for fast-moving mobile devices.
Vertical handover: It occurs between wireless LAN networks and mobile cellular networks for greater accessibility. The vertical handover will have the advantage of both the networks I.e. WLAN technologies offer higher data rates over smaller areas and cellular networks offer lower data rates over large coverage areas.
The soft handover was also known as ‘make-before-break is a seamless call transition from one cell to another, where calls become unreliable due to the signal fading. During the soft handover, the connection from the current cell is disconnected only after a steady connection is established to the target cell.
The hard handover was also known as ‘break-before-make’ is a process of establishing the connection to the target cell only after the mobile connection from the source is broken. Only one cell channel is needed to enable a hard handover and allow more efficient use of channels in a cellular network. Hard handover does not require parallel processing of several channels for maintaining connectivity.
Handover is the step by step process of disconnecting from one cell and connecting to another cell by offering continuity in the connection to the mobile device.
Step 1: Once the signal strength of the currently connected cell is lowered near the handover set limit. The handover system will automatically list out all the neighbouring potential target cells for handover.
Step 2: Identify the signal strength in each target cell and set the power level point on which handover is required.
Step 3: Monitor the signal transmission strength of the current base station and the listed nearby target cell.
Step 4: Once the targeted cell signal strength is reached the set limit, it will connect to the target cell by either smooth or hard handover without any disturbance in user connection and overall accessibility.
Variable attenuators are the prime RF and Microwave part of the handover test system to control the signal strength to simulate the fading effect of each cell. The basic handover system will have 2 channels I.e two-variable attenuators ( Either Programmable or manual), each to represent respective cells.
The first base station ( Transceiver TxRx1) is connected to the input of the first variable attenuator (A1) and the output of the attenuator is connected to the transceiver port of the mobile device (DUT) through one port of RF power Divider/Combiner (PD) to establish the connection between the TxRx1 and the DUT in a closed environment. Similarly, the second base station ( Transceiver TxRx2) is connected through the variable attenuator two (A2) to the DUT via another port of the Power divider/ combiner (PD).
In the initial stage first variable attenuator (A1) is kept at minimum attenuation level and the second attenuator(A2) at the maximum attenuation level. As the A1 has minimum attenuation all the signals transmitted from the TxRx1 will reach the DUT ( After compromising the internal insertion loss of the attenuators) and DUT (Mobile) will connect to the TxRx1. Then set by step the attenuation level of the A1 start to increase and the attenuation level of the A2 will decrease.
As the attenuation of the A1 increase, the signal strength from the TxRx1 reaching the DUT will correspondingly reduce and as the A2 attenuation get reduced, the signal strength reaching the DUT from the TxRx2 increases. As the signal strength of the connected TxRx is reduced to the set handover point of the mobile unit, the handover process will start.
Mobile units start to search for other TxRx which will have higher signal strength. As the attenuation of channel 2 ( TxRx2 to DUT) is reduced, the signal strength of this channel is correspondingly increased and the DUT switch to channel 2 by disconnecting from channel 1 ( TxRx1 to DUT).
Once the attenuation of the A1 reaches to maximum, the signal from the TxRx1 will be completely isolated to the DUT and at a time attenuation of the A2 will be minimum and the full strength signal reaches the DUT. This process will continue by reversing the attenuation levels in both the attenuators and the reverse handover will execute ( Disconnected from TxRx2 and connected to TxRx1) at the handover power level point of the mobile unit.
By using multiple attenuators we can simulate multiple fading channels to test the handover where multipath scenarios test is required. For example handover testing of radar processing units, mesh radio communications systems etc. By using suitable MIMO ( Multiple Input and Multiple outputs) switching circuits along with the multipath attenuator assemble, we can test and optimize mobile network multipath switching performance.
Applications of Handover Test Systems covers the handover testing of mobile units, testing of Radar TxRx performance, testing of tactical radio communication systems etc. A few of the applications are listed below.
A handover test system is used for the handover features testing of the mobile technologies such as LTE, LTE-Advanced, 5G NR, Wi-Fi etc. A handover test system is used to fade the signal strength and simulate the handover requirements as in the actual scenario for the mobile devices for the R&D applications.
The Handover test systems are used to simulate the actual wireless communication fading scenario to test and identify the performance of the systems and network as in real-world operation, in the Lab during the performance test.
Multi-channel handover test systems are used for the simulation of the air, territorial and other man-made interfaces in tactical radio communication systems for testing in the Lab during the development stage.
Handover test systems are basically classified as Programmable and manual on the base of attenuation control and as Full fan-out and Limited fan out on the base of the internal configuration of the attenuation channels.
The programmable handover test system used solid-state programmable attenuators with an internal control card to set the attenuation of each channel. Attenuation setting can be done either locally using the front panel keypad and the display or remotely over the Ethernet or RS232 using the control commands or GUI. These units work in a DC power supply or in AC with an AC to DC converter.
In Manual handover test systems, manual step attenuators are used in each channel for setting the various attenuation levels either as full fan-out or limited fan-out configurations. The attenuation levels of each attenuator are set either by a control nobs or the toggle switch in the front or back panel of the assemble. The advantage of manual handover test systems is AC power is not required for its operation.
Full fan-out handover test systems are used to test multiple networks access points and multiple handsets (Mesh network testing). These configurations will have individual attenuators in every RF path (Every input to the output) and power divider/combiners on both halves of the matrix to fad up/down the RF signal between each access point and handset under test. All the attenuators will be able to set various attenuation levels to meet the various test scenarios.
Limited fan-out handover test systems are used to test a handset between multiple network access points. This configuration is constructed with a variable attenuator at the input port of all the channels and all the signals are combined into a common path by using a power divider/combiner and fed to the DUT. In this configuration, RF signal from each access point can fade individually and feed to the handset for handover testing among the network access points. Multiple DUT (Handsets) will not be tested at a time in this configuration.
When selecting RF and Microwave Handover test systems for the handover testing of mobile units, the frequency of operating, Signal fading level and the number of channels is the prime spec needed to consider as per the test requirements.
The frequency range indicates the operating frequency range of the handover test systems. It is based on the frequency of operation of the internal variable attenuators and associated RF components like power dividers/Combiners in it. The frequency of operation needs to test based on the DUT technology test requirement.
For example, DUT based on 2G technology requires a handover test system covering 3GHz operation, if it is for WiFi testing it required up to 6 GHz and if it is for 5G technology it required a very high frequency of operation says up to 40 GHz.
Configuration indicates full fan-out or limited fan-out need to select as per the test requirement. A full fan-out configuration will be costlier than a limited fan-out configuration. If multiple DUT to Multiple access points needs to test, it is ideal to select a Full fan-out configuration.
50 ohm and 75 ohms impedance variable handover test systems are available. Need to select as per the impedance of the DUT. For Tele-communication applications it will be 50 ohms and for SATCOM it will be mostly 75 ohms handover test systems need to select.
The attenuation range needs to select on the base of the DUT handover set points. Need to select an RF Handover test system that will have a minimum 10 dB higher attenuation level setting than the handover set point of the DUT.
Attenuation accuracy indicates the deviation in the attenuation setting value in the handover test system to the actual attenuation in the channel, excluding the insertion loss. Higher the accuracy (Lower figure) better for the operation. Attenuation accuracy will reduce as the attenuation level increases I.e. for example attenuation accuracy of ± 0.50 dB up to 1-10 dB, ± 1.00 dB 11-15 dB, ± 1.50 dB for 15- 30 dB and so on.
Insertion loss indicates the additional channel loss of the handover test systems above the attenuation accuracy. Insertion loss will increase as the frequency of operation increases. Lowe the insertion loss better for the operation. Even the best system will have 2 to 3 dB insertion loss even at the lower frequency range of operation.
Switching speed indicates the time required to switch from one attenuation level to the next attenuation level during the fading. Higher switching speed helps to simulate the fading more effectively. Higher switching speed is better for the handover test requirements of advanced technologies like 5G.
RF power input level indicates the maximum input power level we can be fed into the handover test system, without fail. For normal operation +30 dBm average is fine enough to test various technical requirements. There are customized handover test systems are available which will handle higher power by adding high power fixed attenuators in front of each variable attenuator for specific test requirements.
Control indicates the Manual( Front panel Keypad ) and Remote (Ethernet, RS-232 etc) control system of the test systems to set the various attenuation levels for the testing. The display indicates the front panel LCD display or the remote display in the GIU for its operation. Need to select as per the test requirements.
RF Connectors for the handover test systems are available with various options with any of the standard RF connectors available in the industry. Customized configuration as per the customer requirements will be provided by most of the manufacturers. Specific connector requirements need to define during the time of ordering.
The operating temperature range indicates the range of external temperature on which it will operate without having any variation in any of its other electrical parameters. Need to select as per the required environmental conditions.
Most of the handover test systems will work on DC and AC power supplies. Handover test systems are available in various physical sizes and packages like box assemble, rack mount assemble, benchtop assemble etc. Need to select as per the test requirement.
Handover Test Systems are used to test the performance of the wireless device at various network RF power levels. These test systems are designed using attenuators and will have "Handover test" functionality written into the control cards firmware. The handover test system will have a minimum of two-variable attenuators will be able to set attenuation, manually or by using the programme on one RF path from minimum to maximum attenuation level and simultaneously step down the attenuation level of the other attenuator from maximum to the minimum value for the handover testing of the DUT.
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