Why downlink frequency is greater than uplink frequency in mobile communications?

Question

Generally, downlink freq. > uplink freq. in mobile communications. I was able to gather some answers as to why is so. But my seniors (in my company) seem not to be satisfied with the below answers.

1. High frequency means higher attenuation. So lets suppose that BTS (as in case of GSM) transmits using low frequency on the downlink. So, that means lower attenuation. Hence, data is received as it should be (without attenuation). So, MS (mobile) has to transmit on uplink using higher freq. in order to avoid interference. But high freq. means higher attenuation. So, in order to overcome the attenuation, MS has to transmit on high power. But due to the constraint on the amount of energy available on the MS battery, this approach is not feasible. Hence, downlink frequency should be greater than uplink frequency.
2. This point is from user perspective. Generally, people download more than upload (however, its converging now). So, we require more bandwidth in DL than in UL and it can be attained only if DL freq. > UL freq. Hence DL freq. > UL freq.

I have searched almost everything (however, that's not possible). If you could help me regarding the same, it would be very kind.

Answer 1

I would say it's all a matter of power consumption and handling capacity. I will distinguish between two cases in which Uplink and Downlink (UL and DL from now on) are interchanged.

1. Satellite communication:

If we take the typical Ka-band for satellite communication, we see that the UL is around 30 GHz, while the DL lays around 20 GHz. Satellites are constrained by their weight, being the cost around several thousands of euros per kilogram. For this reason, their equipment is also limited: they don't have big batteries and get their power essentially from solar panels. Thus, they cannot use big power for transmission.

As you very well stated, the attenuation is higher at higher frequencies. Since we have a power constrain, it seems logic to transmit in the lower frequency. In the case of the UL, the so called "ground segment" consist essentially of several-dozen-meter-diameter antennas with whole buildings for the data processing and electrical supply, so the extra attenuation due to higher frequency is not as meaningful.

2. Mobile communications

In this case, the situation is just the opposite. The UL goes from the user phone to a fixed base station. Take, for example, any of the UMTS FDD frequency bands. You will check that the uplink frequencies are always smaller than downlinks'. In this case, it's the user's phone, which has a size constraint and relies on small batteries, while base stations usually have much better electric supplies with not such strigent requirements. Thus, it makes sense that the Uplink in mobile communications employs the lower frequencies, while the base station employs higher ones.

Answer 2

In Mobile communication (unlike in satellite communication where source doesn't seems to have much battery (similar to UE in mobile communication)there are 2 devices one is mobile & other is mobile tower here mobile tower is in continue/adequate supply of power while mobile power will goes zero soon so here as per the requirement load can be put on on mobile tower hence high power(some one needs to be more) high frequency higher frequency higher throughput (throughput is secondary issue) hence concluded