High UL RSSI in LTE and Tropospheric Ducting

Received Signal Strength Indicator (RSSI)

RSSI comprises the linear average of the total received power (in [W]) observed only in OFDM symbols containing reference symbols for antenna port 0, in the measurement bandwidth, over N number of resource blocks by the UE from all sources, including co-channel serving and non-serving cells, adjacent channel interference, thermal noise etc

Uplink RSSI is referred as the uplink Received Signal Strength Indicator of the total wideband received power within the entire channel bandwidth. Having too low or too high UL RSSI will contribute to poor quality in the network. Too low RSSI could simply mean the cell is “deaf” and cannot “listen” to any UE. Possible reasons are components failure, incorrect parameter settings, incorrect equipment installation, etc. On the other hand, too high RSSI is equally undesirable. This case is more often seen. Due to the high noise interference, it could likely lead to frequent drop calls, call setup failure and poor voice quality. Cells with UL RSSI > -90 dBm can be considered as having high UL interference.

High UL RSSI reason in LTE

There are multiple reasons for high UL RSSI in LTE and below are the frequent reason you can observe in your network

Cells with UL RSSI > -90 dBm can be considered as having high ULinterference.

For different OEMs these thresholds are different

Huawei:-Cells with UL RSSI > -80/-85 dBm

ZTE:-Cells with UL RSSI > -80/-85 dBm

Nokia:-Cells with UL RSSI > -95 dBm

Ericsson:-Cells with UL RSSI > -105 dBm

Samsung:-Cells with UL RSSI > -110 dBm

Interference is any kind of barrier of distraction in the process of communication. in LTE/Telecom radio signals are interfering with each other due to multiple reasons

High UL Interference Reason

Interference can be either external or internal.

1.  External interference comes from outside network

2.  Internal interference comes form within system/network

Below are source/reason of respective interference

1. External interference

    1.1Jammer

    1.2 UBR radiating within same freq as your carrier

    1.3 Tropospheric ducting

    1.4 Different PLMN with same colliding set of freq that you are radiating

2. Internal interference

    2.1 Wrong Antenna setting

    2.2 Channel Card fault

    2.3 Channel card sampling for emf radiation with inaccurate frequency

    2.4 Time and Phase clock collision if priority is same

    2.5 GPS fault (TDD)

    2.6 Abnormal traffic surge

Troposphere

The troposphere is the lowest layer of earth atmosphere [troposphere is derived from the Greek tropos (meaning "turn, turn toward, change")and sphere (as in the Earth)] and is also where nearly all weather conditions take place with total average height 13KM above earth 

Tropospheric ducting

Tropospheric ducting is a type of radio propagation that tends to happen during periods of stable, anticyclonic weather. In this propagation method, when the signal encounters a rise in temperature in the atmosphere instead of the normal decrease (known as a temperature inversion), the higher refractive index of the atmosphere there will cause the signal to be bent. Tropospheric ducting affects all frequencies, and signals enhanced this way tend to travel up to 800 miles (1,300 km) (though some people have received "tropo" beyond 1,000 miles / 1,600 km), while with tropospheric-bending, stable signals with good signal strength from 500+ miles (800+ km) away are not uncommon when the refractive index of the atmosphere is fairly high.

Tropospheric ducting Impact

When radio wave signals are propagated through the atmosphere, their transmission capacity is greatly influenced by atmospheric conditions at that point. Within the atmosphere, radio waves can refract, reflect, diffract, and even be ducted. In Telecommunication specially low power signals (High Frequency) refraction tends to further lengthen the ray path thereby increasing the propagation time, which in turn lead to late arrival time of the signal at the receiver circuit(Propagation delay)

Also, wind, air temperature, and water content of the atmosphere can combine to either extend radio communications or to greatly attenuate wave propagation making normal communications extremely difficult. So, due to the non homogeneous nature of the atmosphere, little changes in atmospheric constituents can produce dramatic changes in ability to communicate

Tropospheric Ducting Impact on RAN Performance

1. Ducting Effect is found in morning & in evening

2. 93%~94% of total drop increment is contributed by the morning slab & evening slab. Evening slab is dominating because of its high traffic generation characteristics. But ducting severity is no less in morning slab 

Overland paths are usually the strongest at sunrise and weakest at mid afternoon. Generally ducting happens at the beginning & ending of winter. Water paths are usually the strongest at mid afternoon and weakest at sunrise. Combination land-water paths may peak at various times depending on the local weather conditions.

3 types of Bad Quality Drops (Uplink, downlink & both link) are contributing more than 50% of total drop increment. Out of them Bad Quality- downink is the most dominant, contributing about 35% of total.

The Signal Strength what is sufficient in case of low interference scenario to retain a call, suddenly become insufficient when interference increases

Below are sample example of how Tropospheric ducting causing change in UL RSSI in 4G