Wireless Waffle - A whole spectrum of radio related rubbish
H comes after E, E comes after G...signal strength
Friday 4 April, 2014, 09:57 - Much Ado About Nothing
Posted by Administrator
ww explains it allWhen using your mobile phone, smart phone or tablet have you ever noticed that next to the signal strength bars (usually found at the top of the screen), there is often a letter (or two) that seem to change almost at random as you move around, and even sometimes when you aren't moving at all? Have you ever wondered what these letters are there for and what the implications of them changing from one letter to another are? Well wonder no more because the answers are about to follow, as Wireless Waffle explains it all...

Letter(s)MeaningTypical Connection SpeedExplanation
G or GPRSGPRS10-20 kbpssignal bars gThis is a 2G service and is the slowest connection you can get. It's often achingly bad.
E or EDGEEDGE50-60 kbpsThis is also 2G and the second slowest connection - theoretically up to 384 kbps but almost never this fast. Think 'dial-up' internet (if you can remember back that far).
3G3G80-100 kbpsThis is the original 3G mobile system and is good (compared to GSM) but still not brilliant.
HHSPA0.5-2 MbpsA truly broadband wireless connection with good real-life connection speeds.
H+Evolved HSPA (or HSPA+)2-8 Mbpssignal bars h+An even faster connection, might even be termed 'zippy'.
4GLTE5-20 MbpsThe fastest connections available today.
RRoaming-Beware - this means you are connected to a network outside your home country and data costs could be astronomical! The R is sometimes shown in a triangle.
XNo signal-On some phones, an X appears above the signal bars if there is no signal at all.

Note that the typical connection speeds given above are those that are generally achieved in real-life. Though in theory the technologies used can offer faster connections, much depends on how many users are in a cell and what they are doing, how close to the centre of the cell you are, whether you are stationary or on the move, and a whole host of other factors.

Arrows (sometimes coloured, and sometimes integrated into the signal bars) pointing up and down are also illuminated. This just shows whether you are downloading (the downward arrow) or uploading (the upward arrow) data to the mobile network.

youtube bufferingIn addition, the number of bars shown on your signal meter will also affect how good your connection is. So a '3G' connection with all signal bars lit might be better than a 'H+' connection with only one bar lit. However a full strength signal may not necessarily mean a fast connection as most phones show the 'strength' of a signal and not the 'quality' It's quite possible to have a full strength signal that's suffering lots of interference and thus is bad quality.

What does any of this matter? It doesn't really, but if you are wanting to view a YouTube video and your phone is showing 'G' or 'E', the chances of you getting a fast enough connection are virtually nil.
1 comment ( 105 views )   |  0 trackbacks   |  permalink   |   ( 3 / 120 )

Marconiji and Elastic-Magneto Wavessignal strength
Tuesday 1 April, 2014, 08:04 - Much Ado About Nothing
Posted by Administrator
gujarat science 4 bookIt has been widely reported with great delight across India and in the world-wide media that the Level 8 Social Science school text books being given to children in the Indian state of Gujarat contain many inaccuracies. Examples of these heinous inaccuracies include 'factlets' such as these gems:
  • It was Japan that dropped a nuclear bomb on America during World War II (and not vice versa).
  • Carbon Trioxide (CO3) has increased due to the cutting of trees (CO3 is highly unstable but can be made by blowing ozone at dry ice).
  • That after the partition of India in 1947, a new nation called 'Islamic Islamabad' was formed whose capital was Khyber Ghat (and not that Pakistan was formed with a capital Islamabad).
Wireless Waffle has managed, through a contact living in the state, to get hold of a copy of the Level 4 Physics book used in classrooms across Gujurat. Like its social science bretheren, it is also chock-full of errors and mistakes. For example, it claims that:
  • marconijiIndian inventor Guglielmo Marconiji (pictured right) discovered the elastic-magneto wave and his brother, Luigi, is the source of the character in the SuperMario game.
  • Elastic-magneto waves travel at the speed of steam, which is generally taken to be 100 km per hour.
  • The first radio station in India went on-air in 1748 and was called 'Bhapa Stesana Bharati' (Indian Steam Radio), as it used steam engines to generate the elastic-magneto waves. The signals from this station will not reach the moon until a week next Tuesday, if the timetable doesn't change.
  • Phones work by transferring ear molecules between those speaking. Conference calls are a modern invention because of the need to clone ear molecules to be heard in several places at once.
Laughable as these 'factlets' may seem, there are many, much worse, part-truths that are espoused by those who ought to know better... Here's looking at you, ITU!
add comment ( 93 views )   |  0 trackbacks   |  permalink   |   ( 3 / 122 )

The full Gravity of the situationsignal strength
Friday 14 March, 2014, 14:28 - Spectrum Management, Much Ado About Nothing
Posted by Administrator
For those who have not yet seen the Oscar winning film, Gravity, please note that although there are no spoilers (in the traditional sense) in the ensuing text, for those who are regular readers of Wireless Waffle, reading what is to follow before seeing the film may leave you with the same level of bemused bewilderment that it did us, and may spoil your enjoyment of the film! So a spoiler that's not a spoiler.

The movie begins with a few astronauts on a space-walk (a.k.a EVA) to add some new equipment to the Hubble space telescope. They are communicating with each other using radios built into their space suits and also, at the same time, are able to communicate seamlessly with ground control in Houston.

Following the 'disaster' around which the film's premise is based, the astronauts lose communication with Houston, and, for that matter, any ground based people. According to the film, this has been caused by the loss of the communication satellites that were handling the signals. In addition, their space suit radios seem incapable of communicating with each other over ranges of just a few hundred metres.

Let's first examine the space suit radios themselves. According to a document provided by NASA the range of the communication system between suits is just 80 metres in the worst case (though could be much greater). Given that this system took US$20 million to develop and operates in a 'free space' environment, the poor coverage performance is lamentable. However, it appears that this element of the story might just be feasible. Note that the Russians use a much more off-the-shelf technology for EVA voice communication that has a much greater range!

Could a ground station communicate directly with a space-suit. Based on NASA's paper, and on typical UHF communication systems, no. But with a little ingenuity, for example the use of a high power transmitter and high gain antenna on the ground, it is not beyond the wit of man.

space shuttle legoAs for a loss of Earth-space communication being caused by the loss of a communications satellite, there are satellites used to relay data from space to Earth (for example, the TDRSS), however full data communications with a space shuttle could also be accomplished directly from the shuttle to a network of ground stations at S-band frequencies around 2.2 GHz (and voice-only communication at VHF and UHF frequencies). Although in theory, these ground stations could be connected back to Houston via satellite, the chances are that there would be a terrestrial, fibre-based connection that could do the job just as well. So whilst passing over such a ground station, there is no reason why Earth-space communication could not have been re-established. Of course a space vehicle (such as the shuttle) is then needed to relay these signals to any astronauts on EVA.

tdrss space links

There is then a moment when one of the astronauts finally receives a signal from the ground but it appears to be from a Chinese man whose dogs and baby can be heard to be barking and crying (respectively) over the air. Whilst tuning into these transmissions, the astronaut in question says 'you're coming in on an AM frequency'. What is an AM frequency when it's at home? AM is a modulation scheme and not a frequency. And why would someone sitting at home in China be using any kind of frequency that is shared with Earth-space communications? And the communication seems to be full duplex as the astronaut can communicate with the man on the ground concurrently with listening to his transmissions. None of this makes much sense.

landing module re entryAnd lastly, communication with the ground is finally re-established when a landing module descends into the atmosphere. The range of the types of frequencies used for Earth-space communications at atmospheric altitudes is limited by the curvature of the earth, (e.g. at 30,000 feet, the range of communications is roughly 300 km). This would mean that those on the ground would have needed to be aware of the location where the landing module was coming down (odd that they could do this if they had had no communication with the landing module until that point) and thus be in the neighbourhood for communications to be possible.

Whilst the film may have excelled for its special effects, the way in which the radio communications were portrayed will be a real 'spoiler' for anyone who knows anything about the radio technologies or radio propagation. Still, science-fiction, by its definition, doesn't need to be scientifically accurate!
add comment ( 265 views )   |  0 trackbacks   |  permalink   |   ( 3 / 167 )

'Loafsat' to solve Internet Censorship Problemssignal strength
Saturday 1 March, 2014, 09:03 - Satellites
Posted by Administrator
outernet logoA consortium led by the Media Development Investment Fund (MDIF), and calling itself Outernet is planning to launch hundreds of small satellites (at 30x10x10 cm at their largest, they are about the size of a loaf of bread) to 'broadcast' the Internet. The idea is that selected portions of the internet will be broadcast using UDP-based WiFi multicasting (as well as, potentially, DVB and DRM).

Stepping aside from the political questions about who will decide which portions of the Internet will be broadcast - and which will not - there is the much bigger question of whether or not it is even possible to broadcast WiFi successfully from a satellite. There are several technical issues to overcome:
  • The satellites, presumably in low earth orbit, will be several hundred kilometers above the planet, so the path loss will be significant.
  • They will have to overcome interference from terrestrial WiFi networks on the same channel.
  • The low earth orbit means they will not be stationary in the sky, leading to Doppler shift on the received signal.
The power of the transmitter on the satellites is not known, but we can backwards calculate what it would have to be in order to deliver a service. WiFi typically needs to receive a signal of around -90dBm (1 picoWatt of power) in order to function, and preferably more (especially for faster connection speeds), but let's take that as the baseline.

loafsat 1At a frequency of 2450 MHz, the free space path loss over 500 km (a typical height for low earth orbit satellites) is just over 154 dB. In reality, atmospheric absorption will increase the path-loss, as will clouds and rain, but let's assume it's a relatively clear, low humidity day. The satellite will therefore have to have a radiate a power of 154 - 90 = 64 dBm in order to achieve the necessary signal level on the ground. This is a power of just over 1.5 kiloWatts. At a satellite height of 150 km (about the minimum possible), path loss is around 10 dB less, meaning it would have to radiate a power of 150 Watts.

7dBiIf the transmit antenna has a gain of 10 dBi, which is very feasible, the transmitter power requirements end up being 150 Watts at a height of 500 km and 15 Watts at a height of 150 km. Note that no transmitter is 100% efficient, and the satellite would have to have receivers and control systems too, so the power requirements would be greater than that which the transmitter alone requires. If it is also assumed that the satellite is over the dark side of the Earth for some proportion of time, and has to rely on batteries, the solar power generation requirements increase, or alternatively the satellites would have to switch off at night.

Of course, high gain antennas could be used on the ground, but this would then require special equipment for the satellite to be received and would go against the concept of receiving the signal on 'smartphones and tablets'.

It is not possible to easily generate 150 Watts of power on a satellite the size of a loaf of bread. A typical satellite solar panel can generate around 300 Watts per square meter of area. The total surface area of the 'loaf' would be 0.14 square meters, meaning it could potentially generate 42 Watts of power if all faces were covered in solar panels and were in full sunlight (which is, of course, impossible as at least one face would be in shadow).

Of course, the 'loaf' could have its solar panels unfold after it is launched to make a bigger panel, so the 0.5 sq metres required to generate 150 Watts might just be possible. But this would still only provide power when the satellite was in daylight. To be powered at night it would need to generate at least double the power (one lot for the transmitter and another to charge the battery) and contain a battery capable of holding the charge. This would again be difficult on a satellite of this size.

The above transmitter power calculations assume that there is no interference on the channel. If standard WiFi channels are to be used, then depending on the location it could be expected that there would be other signals around causing interference. Assuming that the main use of the satellite will be in areas where there are no other forms of Internet connection, we could take it that there would not be WiFi interference, and so arguably we could look upon the satellite kindly and disregard this effect.

On the Doppler shift issue, at 2450 MHz, the received frequency of a low earth orbit satellite will vary by around +/- 50 kHz as it passes overhead. The IEEE standard for WiFi specifies a maximum frequency error of +/- 25 parts per million (ppm) for the 2.4 GHz band. This equates to roughly +/- 60 kHz meaning that the Doppler shift of the satellite leaves it just within acceptable frequency tolerances.

So, in conclusion:girls and loafsat
  • If the 'loaf' was at 150km height it might just be able to generate enough power to transmit a WiFi signal that is strong enough to be received on the Earth. At a height of 500 km, extending solar panels would be necessary. For use at night even larger panels, plus batteries would be needed.
  • Any terrestrial interference in the band would largely obliterate the satellite signal, so it would only really be receivable in remote areas (which is, after all, it's main intention).
  • Doppler shift is just within acceptable tolerances.
So is it technically feasible. The Wireless Waffle answer is 'just about'. But if it was launched, it would increase WiFi interference levels over the majority of the planet, especially outdoors.

And you can bet that if it did work, those Governments that censor Internet access would find ways to jam the signal either terrestrially or by building their own 'loaf sat', increasing WiFi interference further. The loaf-sat-wars may be just about to get toasty...
add comment ( 117 views )   |  0 trackbacks   |  permalink   |   ( 3 / 200 )

| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | Next> Last>>