Android dual SIM handsets - recommendations and reviews

[rfranzq]

Originally Posted by inquisitor

A final note on the Zopo C2/ZP980's dual SIM functionality: Both SIM slots support UMTS but only alternately, so one of the SIMs will always be restricted to GSM.

Well, if you had not started this interesting thread you [and we]
would not have found out this important bit of info---and have prevented much sim-swapping.
Could this be seen as a battery-saving feature?
'It's a feature and not a bug.'

[inquisitor]

Originally Posted by rfranzq

Well, if you had not started this interesting thread you [and we]
would not have found out this important bit of info---and have prevented much sim-swapping.
Could this be seen as a battery-saving feature?
'It's a feature and not a bug.'

The fact that only one radio module supports UMTS definitely saves battery life as UMTS - despite significantly lower transmission power levels (max. 0.25W vs. 1-2 for GSM) - requires a lot of CPU power for the spreading codes calculations which results in higher power consumption by the baseband processor. Besides impairing standby performance a second UMTS radio module would also increase hardware complexity and - most importantly - production and royality expenses. Further to that simultaneous UMTS support for the second SIM would also be quite pointless to most users as the most relevant advantage of UMTS is its higher data bandwidth but establishing a simultaneous second data session is useless. Instead most users will use the second SIM only for voice and SMS for which GSM still works fine as long as GSM and UMTS coverage are overlapping. However since there are UMTS-only networks without overlapping GSM coverage as a fallback option and since more and more operators are reducing GSM capacity by refarming former GSM spectrum in favour of UMTS or LTE deployments UMTS support for the second SIM is becoming a necessity.
Has anybody actually seen any dual SIM device with dual UMTS support yet?

[DRNewcomb]

Originally Posted by inquisitor

Instead most users will use the second SIM only for voice and SMS for which GSM still works fine as long as GSM and UMTS coverage are overlapping. However since there are UMTS-only networks without overlapping GSM coverage

Japan and S. Korea come to mind.

[UKSTEVE]

I have a Star W007 (for sale - see for sale on here).

It's to stop any signal conflicts - one SIM is on 3G, the other on 2G - means they can both have best signal reception/transmission conditions. If two 3G handsets are in close proximinity, the spread spectrum nature of the signals means you get a lot more bad packets.

It's a clever solution to the problem of using two 3G transceivers in close proximity.

The Star W007 is software-switchable in this regard - i.e. you can switch which transceiver module pairs with which SIM card.

+Steve

[inquisitor]

Originally Posted by UKSTEVE

It's to stop any signal conflicts - one SIM is on 3G, the other on 2G - means they can both have best signal reception/transmission conditions. If two 3G handsets are in close proximinity, the spread spectrum nature of the signals means you get a lot more bad packets.

If it actually was a problem to establish two separate radio links on adjacent UMTS frequencies from the same handset how could DC-HSPA work which does just the same to increase bandwidth? Integrating two 3G radio modules may be a cost-related problem for budget phones but I think there's no major technical issue at all.
Also keep in mind that the 5 MHz channels of UMTS include guardbands of 580kHz at the upper and lower end. So the actual carrier bandwidth of a UMTS channel is only 3840kHz and all carriers are separated by 1160kHz which should suffice to avoid interferences.

[UKSTEVE]

DC-HSPA is - I think - only in Release 8 version at the moment Mr I - all is does is to double the data rates by doubling the bandwidth to 10 MHz (2×5 MHz).

In theory, DC-HSDPA can support up to 42.2 Mbps, but unlike HSPA, it does not need to rely on MIMO transmission. This is the key.

My understandng is that Release 8 of DC-HSDPA can only operate on adjacent carriers, whilst Release 9 will allow the paired cells to operate on two different frequency bands. I was talking to a Three tech recently who said the carrier is testing Quad Channel - QC-HSPA!

Obviously the spread spectrum nature of 3G / DC-HSPA in this context will result in packet quality degradation, but since it's one handset and one cell site controlling everything, it's not a MAJOR problem.

What I was trying to say - sorry I didn't express myself clearly - is that two 3G transceivers placed side by side would hit packet quality problems - and since we're talking about handsets that are designed cheaply and based on older chip sets, having one 3G and one 2G transceiver in the same handset avoids any problems - and it's cheaper

+Steve