Transmitted signal quality specify how much the transmitted signal deviated from an ideal signal in terms of frequency domain, timer domain or modulation property. The impairments on the transmitted signal are introduced by the transmitter radio parts having nonlinear properties e.g power amplifier, Digital Front End (DFE) and Analog Front End (AFE) etc.
In the specifications, the signal quality can be assessed for a Base Station or Mobile Terminal by measuring following
- Frequency Error (Fe)
- Modulation Quality (EVM)
- Time Alignment Error (TAE)
In this post we will discuss for Frequency Error.
Frequency error is the measure of the difference between the actual BS transmit frequency and the assigned frequency. The same source shall be used for RF frequency and data clock generation. 3GPP specification has set the minimum requirement for frequency error for different classes of Base stations considering Macro, Micro and Pico cell deployments.
The requirement is applied at the reference points defined for difference BS types e.g. for Type 1-C Base station requirement shall be applied at the antenna connector supporting transmission in the operating band and for Type 1-H this requirement shall be applied at each TAB connector supporting. Below is the requirements specified as per 3GPP TS 38.104 for frequency error in terms of ppm (parts per million).
- Wide Area BS: ±0.05 ppm
- Medium Range BS: ±0.1 ppm
- Local Area BS : ±0.1 ppm
ppm can be easily convertible to Hz using following formula.
- Frequency Error [Fe] = Carrier Frequency (fc) in Hz * (ppm/10^6)
Let’s take an example, consider carrier frequency fc as 850MHz operating band n5 and calculate the Fe for different types of Base stations.
- Wide Area BS (Fe): 850 * 10^6 * (0.05/10^6) =850 * 0.05 = ± 42.50 Hz
- Medium Range BS (Fe): 850 * 10^6 * (0.1/10^6) =850 * 0.1 = ± 85 Hz
- Local Area BS (Fe): 850 * 10^6 * (0.1/10^6) =850 * 0.1 = ± 85 Hz
From above calculations we can see the for Wide area base station which is related to macro cell deployment scenarios have more strict requirement for frequency error and relaxed for Macro and Pico cells.