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External Wimax antennas NOT for extending coverage beyond coverage map


  • External Wimax antennas NOT for extending coverage beyond coverage map

    Can outdoor antenna allow reception outside clear's wimax coverage area? Would this antenna/modem combination allow me to receive Clear's Wimax signal from outside their current coverage area? If so, approximately how away from the nearest Clear Wimax transmitter can I be and still receive signal?
    No this is not the intent of these high gain WiMax antennas. We don't expect it to extend coverage outside coverage areas but rather to improve it in areas within coverage areas experiencing weak signals. These antennas are especially useful in instances where the signal is very weak inside the building but good outside due to the RF shielding effect especially of buildings with thick walls or metallic structures. This is a tricky question actually. Since we get this quite often let me take the time to explain why in some detail. It's difficult to predict RF wireless propagation behavior at any frequency and for any protocol but particularly in populated areas with lots of obstructions and interference from other users. It get's even trickier for 802.16 WiMax which uses MIMO to mitigate multipath effects in a densely populated area. The reason is that each area is different and has different interference and obstructions. Now it could very well extend the coverage but if it did it would not be by much outside the "intended" coverage area and even then the received signal strength may likely be too weak and unreliable to be usable. Remember also that coverage maps are not exact but rather are predictions based on some statistical model and you could empirically find an area that is outside official coverage zone but with a sufficiently strong signal that is easily 'boostable' by these antennas. Note that even if you were able to solve the RSS (received-signal strength) and EIRP (effective isotropic radiated power) problem outside official coverage zone, other unmodifiable network parameters inherent to the network topology may act to prevent reliable speeds/performance such as latency to the BTS which increases with distance (recall the ACK timeout issues for 802.11 WLAN networks when a WiFi client is too far from AP). Further recall that the electromagnetic field intensity in free space diminishes inversely to the square of the distance from the transmitter so it doesn't take a large distance past the coverage edge for the Cellular/WiMax signal to become totally unusable.
    how then can I determine if these antennas will work for me?
    Consider for example that you are using our Omni-directional 12dBi WiMax Antenna or the more popular Directional WiMax Panel Antenna which provides 16dBi of directional gain together with the Zyxel MAX206M2 Clearwire WiMax CPE modem which specifies a receiver sensitivity** figure of -99dBm when using QPSK modulation.
    If your received signal strength is currently -102dBm then you would not be picking up a sufficiently strong Clear 4G Wimax signal for a reliable link but by adding this antenna which has 16dBi of directional gain *** then even after cable/connector losses your received signal would be above the receiver sensitivity. E.g for a 20ft LMR-400 cable with 1.4dB loss at 2.6GHz and 1dB connector losses including lightning surge device final RSSI would approach -88.4dBm (-102dBm + 16dBi - 1.4dB - 1dB) Same goes for transmitted power (or more correctly the EIRP= Effective Isotropic Radiated Power). So even if you are currently connected but experiencing slow speed, improving your received signal could bring it above the receiver sensitivity required for a better modulation mode like 16QAM/ 64QAM which would improve speed!
    So the surest way to determine if these antennas will work for you is to actually get your hands on a CPE/ USB modem and during the trial period measure the RSSI/CINR which is available from the communication manager software that comes with the modems. If the calculated original RSSI (negative dBm value) plus antenna gain minus cable/connector losses doesn't give you a strong enough signal then you are out of luck. Alternatively, if your outdoor signal at the location where you could mount an antenna is already very good and your speed is fantastic at this outdoor site then these antennas will clearly work for you and in fact you need not worry about RSSI/CINR readings (provided of course you get a strong enough antenna, keep cable lengths reasonabe & where necessary take advantage of MIMO by having two external antennas. But as discussed extensively in our forums it's hard to predict precise RF radiation behavior especially for a MIMO multi-antenna protocol such as WiMax which is why we offer a 30-day return period with no restocking fees on these antennas.
    Therefore in summary to determine if stronger external antenna could improve your performance you need to determine if: (RSSI Before Adding Antenna) + (Antenna Gain) - (Cable Loss) - (Connector Loss) > (Receiver Sensitivity) NOTE:
    • RSSI before adding antenna is a negative number in units of dBM e.g -92dBm
    • LMR-400 cable has a loss of about 6.9dB/100ft
    • Assume about 0.1dB loss for each additional connector and 0.5dB loss for addition of lightning protection
    • Receiver sensitivity depends on modems chipset used but typically for the fastest modulation mode (fasted speed) you needs to be in the -70's dBm or better.
    • Note CINR (Carrier-to-Interference-Plus-Noise ratio) not mentioned above is also important. We have seen reliable communication with values lower than 10dB but generally needs to be above 10dB. A good RSSI and low CINR could point to too much co-channel or adjacent interference at the receiver site. One experiment to troubleshoot this would be to turn off 2.4GHz radios e.g 802.11b/g/n WiFi systems or to orient highly directional antennas differently. In fact this is one motivation for narrow bandwidth antennas (covering the ~2.5-2.7GHz Wimax band with sharp rolloff outside this band), narrow beamwidths (provided can pick up a sufficiently strong signal in oriented direction) and large front-to-back ratios (>20dB) since these keep interference power low.
    ** Recall for any wireless radio not just WiMax (e.g Cellular, WiFi, etc):
    • receiver sensitivity is the minimum signal that could be reliably received by the radio at a specified Bit error rate (the lower,i.e the more negative, the better)
    • this receiver sensitivity figure is spec'd for QPSK which is the lowest modulation mode (lowest bit rate) so for 16QAM and 64QAM we expect a larger (worse) receiver sensitivity.
    *** Factory installed standard MAX206M2 internal integrated antennas have 5dBi directional dipole so a 16dBi antenna is a significant difference if you remember that dBi is a logarithmic scale = 10 x log(Gain)

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