Why -48V in BTS?

Why we use -48 V DC to operate BTS why not +48 V?
What is the problem if we use positive voltage?

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The question can be splitted into 2.

Why DC?

Because in Telecommunications, alternating current (AC) affects voice communications, generating noise and other problems.

Why 48?

Negative 48 volts direct current (-48 VDC) is typically found on fixed lines when they are inactive. And when there’s a call, an alternating current (AC) of 90 volt-amperes (90 VAC) is applied, causing the phone to ring.

-48VDC therefore comes from the beginning, the so-called public switched telephone networks (PSTN), where the direct current of volts goes to channels, multiplexers, etc.

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