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Improving Received Signal Strength for Dual 3G/4G Cellular/WiMax Modems


  • Improving Received Signal Strength for Dual 3G/4G Cellular/WiMax Modems

    This discusses techniques available to improve the received signal strength for dual-mode 3G/4G modems capable of Cellular/PCS 3G data and 4G WiMax. For example for the Sierra 250U modem available from Clearwire and Sprint.

    Current WiMax modems in the market implement 2xReceive MIMO diversity which means they have 2 integrated antennas and, where available, two external antenna ports. For dual 3G/4G modems one of these antenna ports, typically the primary, is used for both Cellular/PCS as well as WiMax frequency band and the secondary antenna port is used for WiMax 4G MIMO diversity only.


    Sierra 250U Antenna Port Connections

    Fig1: Sierra 250U 3G/4G Modem antenna connectors

    Due to the presence of multiple wireless protocols operating in difference frequency bands supported by the two external antenna ports its important to get the right antenna necessary for the particular band you need. For example if you only need to use it in a 4G coverage area then you don't need to have an antenna that supports 3G.

    1. Boost 3G Only: Useful for example for use in areas with no 4G. Standard cellular boosting techniques can be used. For example you can use a 1900MHz PCS Yagi antenna directly attached to the modem or use it in conjuction with a cellular amplifier/repeater either directly connected to the modem or rebroadcasting the amplifier signal a modem in close proximity Learn More

    2. Boost 4G Only: This is the most common setup for fixed sites like buildings since you already have a prior knowledge of the existence of 4G. Our most popular antenna kit includes both directional or omnidirecional antenna options to boost the 4G WiMax signal. See an example drawing on using a 16dBi panel antenna in ANT#1 port and a 12dBi omnidirectional antenna in ANT#2 port.

    3. Boost BOTH 3G & 4G:

      • Method1: Manual switchover of antennas.

        Simply get a 3G antenna and a 4G antenna, two low-loss coaxial cables and two pigtails and manually switch over antennas as needed. The benefit of this is that you can get the best performance antenna available in each band. High performance single-band antennas are generally more readily available that antennas that can support both the cellular/pcs band as well as the 2.5-2.7GHz WiMax band. Disadvantage is that you lose the benefit of seamless failover from 4G to 3G that is provided by the modem. For example if you are running fine on 4G and the 4G signal fades below the threshold where it becomes unuseable you would lose a connection requiring you to physically hook up the 3G antenna and restart to pick up the 3G signal. Then you would need to repeat this when you think the 4G signal is back.

      • Method 2: Get a wideband antenna capable of handling both Cellular/PCS and WiMax frequency bands.

        An example of such an antenna is 3G/4G wideband LPDA antenna (Log Periodic Dipole Array) which is available as an option in our pre-configured antenna kit. The advantage of this is that it eliminates the manual swithover. The disadvantage is that wideband antennas generally have lower gain than single-band antennas.

      • Method 3: Use two single-band antennas but with a diplexer to combine them into a single cable/pigtail.

        This option provides the best compromise and allows one to use very good quality single-band antennas and avoid having to do a manual switchover of antennas. For example BK-67N Cellular/WiMax 80-2170/ 2490-2690 MHz diplexer allows splitting up and combining the cellular/pcs band to the WiMax frequency band on a single coaxial cable and use attach this using a single pigtail to ANT#1 port of the Sierra 250U. Remember ANT#2 is for 4G only so a standard WiMax antenna can be attached directly to this port.

        Here is an example drawing showing how a cellular/wimax diplexer can be used to allow boosting the cellular 3G using a cellular amplifier without affecting the 4G WiMax boosted independently.

        View BK-67N diplexer/filter datasheet

    NOTE: as with all other instances where an outdoor antenna is routed indoors a properly grounded lightning surge device is STRONGLY recommended. See some example setup drawings here.


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