The opinions expressed here are the author’s only and do not necessarily represent those of CTVR.
We have finally got around to trying out one of the Huawei 3G wireless broadband modems. These are white mouse-like USB devices that operators including Vodafone, O2 and 3 are marketing at the moment in a bid to capture revenue from those who want internet connectivity on the move or simply want an alternative to dial-up but cannot, for one reason or another, use wireline broadband. These little white devices all come from the same manufacturer however the operator logos, price plans and basestation networks differ.
Our model is the Huawei E220 high-speed downlink packet access (HSPDA), which claims a top speed of 3.6Mbps. The nice thing about this is that it is powered from the USB port and contains all the software needed to get going; it’s advertised as an autoinstall procedure but it can take a bit of prodding to install correctly. Normally, USB ports can supply up to 0.5A when requested. The modem package we received included a two-USB to single cable connector enabling the modem to be powered from two USB ports. This can be very handy because sometimes USB ports can fail to deliver enough current to keep power-hungry devices happy.
Of course, the actual download speed is dictated by the number of concurrent users per basestation sector. For non-HSPDA-enabled basestations, the connection falls back to general packet radio services (GPRS), which has a top rate of 236.8kbps. A quick speed test in the front square of Trinity College Dublin resulted in 2.6MB/s…perhaps the operator was watching. On a daily return train journey from Mullingar, Co. Westmeath to Dublin city, the unit switched between UMTS and GPRS and handed over between basestations fairly well. It is still not as seamless as one might like; the connection was dropped several times as it switched between HSPDA and GPRS. The lowest reported rate experienced when connected to a non-3g enable basestation in Mullingar was marginally over 53kB/sec Despite these issues, it is proving very handy for getting through part of the daily barrage of emails before getting into the office. On the evening train, the problems are more to do with the lack of elbow movement on the packed train rather than technological ones.
3 Ireland and Vodafone charge for exceeding a usage cap (which includes both uploads and downloads) whereas O2 currently employ a ‘fair usage’ policy. This makes better sense for the rare occasions when the cap may be exceeded (if you are lucky enough to enjoy a good and consistent rate).
O2 Ireland have three 5MHz UMTS frequency division duplexing (FDD) paired carriers, or spectrum segments, and one 5MHz UMTS time division duplexing (TDD) unpaired carrier. The downlinks from the basestations occur in the frequency range from 2155MHz to 2170 MHz. The uplinks from the modem use the frequency range between 1965MHz-1980MHz. We decided to take a closer look at what goes on behind the scenes in the modem, specifically by taking a look at the physical random access channel (PRACH), which is used to transmit the random access channel (RACH) preamble. Upon start-up, this is the first transmission from the modem as it attempts to find and connect to, a basestation.
Using an Anritsu MS2781A Signature spectrum analyser, we were able to locate the specific uplink frequency used by the modem. We then placed the modem close to the antenna connected to the analyser. A zero-span trace of the estimated centre frequency of the uplink channel, which was triggered when the detected power exceeded the noise floor of the system, allowed us to capture the following time-domain plot of the random access channel transmissions.
This shows a series of pseudo-random QPSK bursts (preambles) of 1ms duration gradually increasing in power until the power is sufficient enough to enable a basestation to receive it. What is impressive is that when the basestation finally detects the modem, it replies and the modem begins transmitting a longer connection message all within about a 4ms period.
For latency-intolerant applications e.g. some on-line multiplayer games, this would not the best solution, however a couple of short Skype calls of acceptable quality were experienced. For this operator at least, P2P client operation was possible.
