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Comment by hayksaakian | Hacker News Reader

maybe this sounds simple to you, but it probably goes over the heads of 99.99% of people (and i'd venture to say 90% of technical people).

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I believe the majority of Hacker News readers could run OpenBTS on an Ettus as a weekend project, provided they had a little systems programming experience. I think the greater problem is expense: an Ettus USRP is into the 4 figures, although you could probably make something similar with the HackRF. [1]

[1] https://greatscottgadgets.com/hackrf/

HackRF is only half-duplex though, so I'm not sure you actually can.

Cellphones are only half duplex too, though.

Didn't know that! Do they switch between transmitting and receiving constantly to keep up a 2-way data stream?

I'm not danellis and I'm not skilled in signals processing, but assuming LTE is multiplexed with TDD, yes. The radio is switched throughout time between receiving and transmission. See https://en.wikipedia.org/wiki/Duplex_%28telecommunications%2... for how TDD works.

I've only worked with GSM, not anything newer, but the phone is assigned a timeslot in a repeating frame, and the uplink and downlink timeslots are offset so that the phone doesn't have to transmit and receive at the same time. As I understand it, that makes the antenna and amplifier design much simpler.

Although now that I've said that, I don't recall what happens when it's using multiple channels for packet data.

True for 2G, but for 3G and 4G most deployments are using full duplex (FDD) where reception and transmissions can happen concurrently and use different frequencies.

The only exception for 3G is China with its TD-SCDMA standard. Everywhere else, 3G is FDD.

For 4G, there is a TDD variant. It's mostly used in China and on some specific high bands (2.3/2.5 GHz around Wifi, 3.5 GHz). But in western countries when one use 4G it's FDD.

Cellphones are half duplex but the basestation is generally full-duplex. A half-duplex basestation could probably handle no more than one cellphone. OpenBTS and YateBTS, as well as most commercial basestations, have design patterns that require tight timing capabilites that in turn lend themselves to running on full-duplex basestations.

Ah, yes, you're right, of course. I don't know why I was thinking of the phone side. (You can use two HackRFs, though.)

OpenBTS and YateBTS need a full-duplex radio to function. So far only Ettus radios and bladeRFs [1] have those capabilities. The bladeRF is also the cheapest option, costing in the low 3 figures (with coupon code HKRNWS). Full-disclosure, I make the bladeRF.

[1] http://nuand.com/bladeRF

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