HSDPA – Page 5 – WirelessMoves

Continuous Packet Connectivity (CPC) Is Not Sexy – Part 2

In a previous post I’ve given a broad overview of a 3GPP release 7 work item called "Continuous Packet Connectivity" (CPC). This feature or rather this set of features aim to improve user experience by enhancing battery lifetime, reaction time after idle times and to increase network bandwidth in situations with many simultaneous voice over IP and other real time service users. Rather than introducing a bold new concept, CPC very much works "under the hood" by improving functions that are already present. Part 2 and 3 of "CPC Is not sexy" now take a closer look at the individual features:

A new UL DPCCH slot format configurable by Layer 3 in a semi-static way (Section 4.1 of 3GPP TR 25.903):

In UMTS networks, information is sent in both uplink and downlink in virtual channels. For a connection several channels are used simultaneously since there is not only user data sent over a connection but also control information to keep the link established, to control transmit power, etc. Currently, the radio control channel in uplink (the Uplink Dedicated Control Channel, UL DPCCH) is transmitted continuously even during times of inactivity in order not to loose synchronization. This way, the terminal can resume uplink transmission of user data without delay whenever required.

The channel carries four parameters (for details, see 3GPP 25.211, chapter 5.2.1):

Transmit power control (TPC)
Pilot (Used for channel estimation of the receiver)
TFCI (Transport Format Combination Identifier)
FBI (Feedback indicator)

The pilot bits are always the same and allow the receiver to get a channel estimate before decoding user data frames. While no user data frames are received, however, the pilot bits are of little importance. What remains important is the TPC. The idea behind the new slot format is to increase the number of bits to encode the TPC and decrease the number of pilot bits while the uplink channel is idle. This way, additional redundancy is added to the TPC field.

As a consequence the transmission power for the control channel can be lowered without running the risk of corrupting the information contained in the TPC. Once user data transmission resumes, the standard slot format and higher transmission power is used again.

UL HS-DPCCH gating/discontinuous transmission (DTX) in 2 cycles (based on section 4.2 of TR 25.903 ) connected with a F-DPCH gating in DL and an implicit CQI reporting reduction in UL (see section 4.4 of TR 25.903)

CQI reporting reduction: To make the best use of the current signal conditions in downlink, the mobile is required to send information back to the network about how well a transmission was received. The quality of the signal is reported to the network with the Channel Quality Index (CQI) alongside the user data in uplink. The proposed concept has the goal to reduce the transmit power of the terminal while data is transferred in the uplink but not in the downlink by reducing the CQI reporting interval.

UL HS-DPCCH gating (gating=switch off): When no data is transmitted in both uplink and downlink the UL DPCCH for HSDPA is switched off. Periodically it is switched on for a short time to transmit bursts to the network in order to maintain synchronization. This improves battery life for applications such as web browsing. The solution can also improve battery consumption for VoIP and reduces the noise level in the network (i.e. more simultaneous VoIP users)

F-DPCH gating: Terminals in HSDPA active mode always receive a Dedicated Physical Channel in downlink in addition to high speed shared channels which carries power control information and Layer 3 radio resource (RRC) messages, e.g. for handovers, channel modifications etc. The Fractional-DPCH feature puts the RRC messages on the HSDPA shared channels and the mobile thus only has to decode the power control information from the DPCH. At all other times the DPCH is not used by the mobile (thus it’s fractional). During these times, power control information is transmitted for other mobiles using the same spreading code. Consequently, several mobiles use the same spreading code for the dedicated physical channel but listen to it at different times. That means that fewer spreading codes are used by the system for this purpose which in turn leaves more resources for the high speed downlink channels.

Your head is still not spinning? Great, then watch out for part 3 of this mini-series which explains UE DRX and HS-SCCH-less reception!

If WiMAX Becomes a 3G (IMT-2000) Standard, What’s Left for 4G?

Now that 3G systems such as UMTS are under full deployment, the industry is looking forward to what comes next. While some say that WiMAX is a 4G system, the IEEE and the WiMAX forum think that 802.16e is rather a 3G technology and have asked the ITU (International Telecommunication Union) to include this standard into its IMT-2000 specification (International Mobile Telecommunications 2000). This specification is generally accepted as being the umbrella defining which standards are to be considered 3G.

This is mainly a political move since in many regions of the world, frequencies are reserved for 3G IMT-2000 systems. If WiMAX were included in IMT-2000, and it looks like it will be in the near future, some frequency bands such as the 2.5 GHz IMT-2000 extension band in Europe could be used for WiMAX without changing policies.

So what remains for IMT-Advanced, the ITU umbrella name for future 4G technologies?

Currently there is still no no clear definition by ITU of the characteristics of future 4G IMT-Advanced systems. The ITU-R M.1645 recommendation gives first hints but leaves the door wide open:

It is predicted that potential new radio interface(s) will need to support data rates of up to approximately 100 Mbit/s for high mobility such as mobile access and up to approximately 1 Gbit/s for low mobility such as nomadic/local wireless access, by around the year 2010 […]
These data rate figures and the relationship to the degree of mobility (Fig. 2) should be seen as targets for research and investigation of the basic technologies necessary to implement the framework. Future system specifications and designs will be based on the results of the research and investigations.

When WiMAX is compared to the potential requirements above it’s quite clear that the current 802.16e standard would not qualify as a 4G IMT-Advanced standard since data rates even under ideal conditions are much lower.

3GPP’s Long Term Evolution (LTE) project will also have difficulties fulfilling these requirements. Even with the recently proposed 4×4 MIMO, data rates in a 20 MHz carrier would not exceed 326 MBit/s. And that’s already a long stretch since putting 4 antennas in a small device or on a rooftop will be far from simple in practice. If WiMAX is accepted as a 3G IMT-2000 technology, how can LTE with a similar performance be accepted as a 4G IMT-Advanced technology?

Additionally, one should also not forget that IMT-2000 systems such as UMTS are still evolving. UMTS is a good example. With HSDPA and HSUPA, user speeds now exceed the 2 MBit/s which were initially foreseen for IMT-2000 systems. But development hasn’t stopped here. Recent new developments in 3GPP Release 7 and 8 called HSPA+, which will include MIMO technology and other enhancements, will bring the evolved UMTS technology to the same capacity levels as what is currently predicted for LTE on a 5 MHz carrier. HSPA+ is clearly not a 4G IMT-Advanced system since it enhances a current 3G IMT-2000 radio technology. Thus, HSPA+ categorized as a ‘enhanced IMT-2000 system’.

Maybe that’s the reason why the IEEE 802.16 working group is already looking forward and has started work on 802.16m with the stated goal of reaching top speeds of 1 GBit/s.

When looking at current research it’s clear that the transmission speed requirements described in ITU-R M.1645 can only be achieved in a frequency band of 100+ MHz. This is quite a challenge since such large bands are few. Thus, I have my doubts whether these requirements will remain in place for the final definition of 4G IMT-Advanced.

Does It Really Matter If A Technology Is 3.5G, 3.9G or 4G?

While discussions are ongoing the best one can do is to look at HSPA+, WiMAX, LTE and other future developments as "Beyond 3G" systems. After all, from a user point of view it doesn’t matter if a technology is IMT-2000, Enhanced IMT-2000 or IMT-Advanced as long as data rate, coverage and other attributes of the network can keep up with the growing data traffic.

A whitepaper produced by 3G Americas has some further thoughts on the topic.

As always, comments are welcome!

Continuous Packet Connectivity (CPC) Is Not Sexy – Part 1

Currently, the 3GPP Standards body is giving the final touches to a set of features which are together referred to as Continuous Packet Connectivity (CPC). Several papers mention CPC but I haven’t found a single one so far who could really tell in simple words why these features are necessary and what they actually do. The reason for this is simple: While features like MIMO, spatial multiplexing, beamforming, etc. etc. are broad new concepts (and sound sexy…) CPC consists of a couple of deeply embedded features enhancing existing functionality. Twisting a couple of bits here and a couple of bits there is not very sexy and also not very understandable out of the box.

The Situation Today

With HSPA (HSDPA and HSUPA), mobile devices now have a multi megabit data bearer to both send and receive their data. As devices do not send data all the time there are the following activity states which require more or less interaction with the network:

Active: In this mode, the mobile uses HSDPA High Speed Downlink Shared Channels (HS-DSCHs) and an HSUPA Dedicated Uplink Channel (E-DCH).

During Short Periods of Inactivity (< around 10s): The network keeps the high speed channels in both uplink and downlink direction in place so the mobile can resume transferring data without delay. Keeping the high speed channels in place means that the mobile has to keep transmitting radio layer control information to the network which has a negative impact on battery life and also decreases the bandwidth for other devices in the cell. 10 seconds is certainly a compromise which is not always ideal since during a web browsing session, for example, it takes the user longer in many cases than this time to click on a new link.

During longer periods of inactivity (< around 30s): When no data is transfered for longer than a couple of seconds, the network puts the device on slow channels (RACH in uplink , FACH in downlink). This has the advantage that the mobile does not have to send radio layer control information back to the network anymore. This saves battery capacity to some extent. However, the mobile still has to observe the downlink channel to catch incoming data transmissions which also requires some energy. If the mobile wants to resume communication or in case data arrives for the device from the Internet, the network starts sending/receiving the data on the slow channels and starts a procedure to put the device back on the fast channels. However, this procedure takes in the order of 1 to 2 seconds so the user notices a delay when requesting a new web page for example. This delay is quite undesired.

Even longer periods of inactivity (> around 30 seconds): After about 30 seconds, or 60 seconds in some networks, the Radio Network Controller decides that it’s unlikely that the mobile will send or receive any more data for some time and thus puts the connection in Idle state. In this state the mobile does not have to send control information to the network and also does not have to listen to downlink transmissions except during periodic slots in which paging messages are broadcast. These paging messages are important to inform devices of incoming calls or of new data packets. For most of the time the mobile can now completely switch of the receiver and only activate it to receive paging messages and to scan for other cells of the network. If the mobile wants to transmit data again the radio layer has to request a channel again from the network. This takes even longer than the upgrade from a slow channel to a fast channel and results in an even longer delay before a web page starts loading. (Note: I won’t consider Cell-PCH and URA-PCH states for now)

The mobile keeps it’s IP address in all states, i.e. also in Idle state. Therefore, these state changes are transparent to applications and the user except for the delay when upgrading to a faster channel once data is transfered again.

Desired Improvements

Continuous Packet Connectivity aims at reducing the shortcomings described above by introducing enhancements to keep a device on the high speed channels (i.e. in active state) as long as possible while no data transfer is ongoing by reducing the negative effects of this, i.e. reducing power consumption and reducing the bandwidth requirements for radio layer signaling during that time.

CPC Enhancements

CPC introduces the following new features to reach these improvements:

In Uplink:

A new UL DPCCH slot format
UL DPCCH gating/discontinuous transmission
Implicit CQI reporting reduction

In the Downlink:

F-DPCH gating in DL
Discontinuous reception (DRX) at the UE
A so called HS-SCCH-less operation
Modified HS-SCCH for retransmission(s)

Unless you regularly attend 3GPP RAN meetings, this list probably won’t tell you much. But don’t despair, I’ll publish part two of "CPC is Not Sexy" soon in which I will describe these features in understandable terms.

How To Top-Up A Vodafone Prepaid SIM for Websessions

A final piece of information for users of Vodafone Germany Prepaid SIMs for Websessions has so far been missing: How to top-up without scratch cards that can only be bought in Germany!?

Here’s a link to an online service here that allows to top-up German prepaid SIM cards. The pages itself are in German but pretty easy to figure out for non natives as well. To top-up a Vodafone SIM card, select "Vodafone CallNow", create an account by typing in your eMail address, specifiy the SIM card’s phone number, select the amount to top-up and enter your credit card details. After a minute the selected amount is on the SIM card and can then be used for WebSessions. All quite straight forward and works well.

Deactivating the Vodfone Websession Compression Proxy

I am quite happy with Vodafone Germany’s Web Session offer that gives me fast 3G Internet access in most European countries and in some countries overseas. I’ve reported about this extensively here. One of the things that bothered me, however, was the automatic compression of pictures in web pages. This reduces the amount of data to be transmitted but in the times of HSDPA that’s not necessary anymore. When buying a PCMCIA card and the required software from Vodafone for the service there is an option in the software to deactivate the compression. If you buy a standalone prepaid SIM card however, things are a big more tricky.

One way to get around the compression is to use a VPN software that tunnels all traffic and thus Vodafone’s transparent HTTP proxy can not touch the pictures. In some circumstances, such a solution is not practicable or not even available to all users. So I searched a bit on the web to see if there are ways to deactivate the proxy without the Vodafone software. And indeed, there is! Here and here are two links to the original German articles that describe how the proxy can be instructed not to compress the picture. In essence this is done by including extra HTTP header lines in each page request which are picked up by the proxy and tell it not to compress the images. According to one article, this works for Vodafone Germany and also for E-Plus, another German operator.

To get these extra header lines into a request, an add-on called "Modify Headers" is required for Firefox. The add-on can be installed into the browser right from the Mozilla Add-On Web Page. Once installed, a new menu entry called "Modify Headers" is available in the "Tools" menu of Firefox. In the configuration tab, select "Always On: Enable Modify Headers when this window is closed". Afterwards, two new header fields have to be added manually. In the "Headers" tab, one new header called "Cache-Control" has to be created and another one called "Pragma". Both headers have to be set to contain "no-cache". That’s it!

Restart Firefox and the nasty compression is gone. If you go to pages that have previously been loaded, they are probably still in the local cache and thus still look ugly. In that case, press "STRG" or "SHIFT" together with the reload button of Firefox and the images are refreshed to their non compressed state. Below are two screen shots of HTTP request packets traced with Wireshark that show how HTTP headers look before the tool is switched on and afterwards when they include the two additional header lines.

HSDPA On A High Speed Train – Part 2

In the previous blog entry (here) I have started to report my experiences while using Vodafone Germany’s 3G HSDPA network on board of a high speed train from Saarbrücken to Frankfurt. On this line the 3G network experience is quite positive and for the general remarks see my first entry. In this second entry I’ll now show some information retrieved from Wireshark traces I took during the ride. They reveal stuff that is very difficult to simulate in the lab without special equipment. In short, a treasure chest for the TCP researcher.

All figures shown in this blog entry were made at a train speed of about 200 km/h and come from a trace of a 6MB file download. Figure one on the left shows the throughput during the file download. Total transmission for the file was about 75 seconds, top throughput about 1.5 MBit/s and average throughput about 800 kbit/s. During the file download three transmission outages can be seen at 25s, 43s and 64s. Each of them lasted about about 2.3 seconds. These timeouts where either caused by a handover or by very bad network coverage at these times.

The trace behind the graph reveals that despite the outage no TCP packets where lost so obviously the RLC layer of the radio network recovered all packets. On the TCP layer, however, such prolonged outage times without acknowledgments provoked a TCP timeout resulting in the automatic retransmission of about 15 kBytes of data (11 packets). This is shown in figure 2 on the left. Packet 2796 marked in black is the last packet received before the outage at 23.52s. Communication resumes at 25.76 seconds and it can be seen that no packet is missing by looking at the ACK numbers. The green packets that follow must thus have been saved by the RNC in the radio network. Starting with packet 2809 the sender suddenly retransmits packets (the black block in figure 2) that have already been received and ACK’d after the outage. However, the ACK’s were not received by the sender of the data in time which provoked the TCP timeout and the automatic retransmission.

Figure 3 on the left shows the packet interspacing diagram for the file download which tells a lot about HSDPA HARQ (Hybrid Automatic Retransmission Requests) operation on layer 2 of the air interface between the Node-B and the mobile. I was quite surprised to see that even at such high user speeds most packets where delivered without retransmission, and only about 20 – 30% going through one retransmission. This is quite similar to the traces I made when not moving as shown for example in this blog entry.

Summary

The trace discussed above shows impressively that high speed Internet access on board high speed trains without any on board equipment is possible. The radio network used for the test was certainly not optimized to give the best coverage along the railway. Nevertheless, overall throughput and recovery behavior on the TCP layer are impressive.

3G and HSDPA Internet Access On A High Speed Train

So far I’ve tested HSDPA all across Europe and have enthusiastically reported the great results on this blog (see here). Usually, I use HSDPA in a nomadic fashion, i.e. while being at home, in hotel rooms, on customer sites, etc. This is simple for the network, not much mobility management, no handover, stable radio environments, thus not much of a challenge. But how does HSDPA perform on a high speed train? I didn’t know until recently when I took a German ICE high speed train on the brand new LGV Est Européenne (Ligne à Grande Vitesse) from Paris to Frankfurt.

From a radio point of view the line is kind of black and white. On the French side, UMTS / HSDPA radio coverage is almost non existent. Even while still in Paris, my mobile frequently lost the 3G network once we got out of the train station. Once on the German part of the track, however, I had HSDPA coverage about 70% of the time during the 2h trip between Saarbrücken and Frankfurt. During the rest of the time, my connection fell back to the 2G network and there were only very few places without any network coverage at all.

The Network Under Test: Vodafone Germany’s 3.5G network

The Test Equipment: No fancy stuff, just a notebook and a Motorola V3xx HSDPA category 6 mobile phone, bought back in Rome a couple of weeks ago.

The Result:

During the 2 hours I ran a lot of throughput tests and downloaded around 75 MB of data. The Train speed during most of the tests ranged between 150 and 200 km/h. Very surprisingly, speed did not seem to have a great impact on the data rates. No matter how fast the train was going I always got peak data rates of about 1.5 MBit/s while radio coverage was good.

As there was no dedicated 3G radio coverage for the track there were of course also periods during which radio coverage was poor. Here, data rates dropped but were still at a respectable level of around 250 – 500 kbit/s.

I was also very positively surprised of the handover performance. Shortly before a handover occurred, radio conditions usually got quite bad so the file downloads slowed considerably. Then there’s a gap of around 1 or 2 seconds before the situation improves and the transfer speeds recovered within a few seconds. Downloads of 6 MByte files had an average throughput according to Wireshark statistics of 850 kbit/s with peak data rates of around 1.5 MBit/s. Not a single download failed!

To get a better feeling for the handover behavior I checked the link stability during handovers by sending pings to the network. Packet loss was minimal and seldom were two ping responses lost in a row which would have pointed to a prolonged network outage. To see how many packets are lost during handover I set the ping timeout to 500 ms. Here, single packet loss started to increase which points to a connection interruption during handovers or multiple failed RLC retransmissions during bad radio conditions. Most packets that were reported as lost due to the reduced timeout where nevertheless delivered, just a bit too late to be counted as a valid response. A Wireshark trace revealed that almost all ping responses eventually made it back to the notebook. This test indicates therefore, that HSDPA handovers take between 0.5 and 1 seconds. Sounds almost too good to be true. When analyzing some of the Wireshark traces in which I recorded the throughput tests, however, I could see that at some points the radio connection was lost for about 2.5 seconds (more about this in the next episode). Whether this was due to handovers or simply very bad radio conditions is difficult to say. But even if this can be attributed to handovers only it’s not too bad for a start. It’s probably also important to point out that it’s still early days for HSDPA and optimal handover performance was surely not very high on the R&D agenda so far.

File downloads and ping experiments are not the typical network usage so I also tested sending and receiving eMails and web browsing. I have to say I felt little to no difference in page download times compared while moving at high speed in a train compared to sitting at my desk at home.

Also worth mentioning is the software stability of the Motorola V3xx mobile. While most other mobiles I used in the past have sooner or later become confused by the many handovers and 2G/3G network changes with an active Internet connection, the V3xx was rock stable. Not a single reboot was required during the whole trip and the mobile even performed 2G to 3G network reselections during file transfers.

Apart from the good HSDPA performance, Vodafone has made a good job engineering their network between Saarbrücken and Frankfurt. During the two hours the Internet connection did not drop once (e.g. due to missing datafill on the SGSNs for intersystem handovers). This is rather exceptional as on other lines, like for example between Munich and Stuttgart, my Internet connection usually drops a couple of times. So whoever did the network verification along that track, please Vodafone, send him to optimize the Munich – Stuttgart line as well. And while you are at it, install additional 3G base stations along the line, I’d really appreciate the same performance as between Saarbrücken and Frankfurt.

Summary

Before doing the tests I was a bit skeptical about the outcome. The good results, however, speak for themselves and certainly answered a lot of questions concerning high speed Internet access on high speed trains. The results also indicate that dedicated 3G train line coverage would fill the gaps observed and result in a very smooth user experience independent of train speed and also without any on board equipment such as 3G/Wifi bridges.

Stay tuned for the technical deep dive once I have analyzed the Wireshark trace I took in more detail.

Vodafone WebSessions Tested in A1’s 3G Network in Austria

As a frequent traveler I often use Vodafone Germany’s Websession offer which lets me connect wirelessly to the Internet in most countries in Europe and also in some countries overseas using 3G UMTS or 3.5G HSDPA. I’ve first reported about the details of the offer here and also posted reports of how well it performs in Italy, France and Switzerland in the meantime. This blog entry takes a look at how the offer performs in Austria:

A1’s 3G network (Mobilikom) coverage area throughout Austria is excellent and even in areas without 3G coverage, EDGE capable GSM base stations deliver throughput good enough for work and play. While in 3.5G HSDPA coverage, I reached top speeds of about 2 MBit/s when downloading three files simultaneously.

Single file download top speeds where at about 800 kbit/s. As in previous cases I am still a bit puzzled to why that happens as round trip delay time for the file download was around 230 ms. Together with a TCP window size of 65k, the throughput of a single TCP session should be 2.2 MBit/s. Note: For background information on the effect of the TCP window size and round trip delay times on throughput see here.

Nevertheless, 800 kbit/s per file is more than what I observed in Italy and France where bandwidth is throttled to around 500 kbit/s overall, independently of how many files are downloaded at the same time. Looks like Vodafone A1 does something differently with Vodafone Germany then the roaming partners in Italy (Vodafone Italy) and France (SFR).

So all things taken together the Websession performance in Austria is quite convincing, too.

Wireless Internet Access: Consumer Theory and Reality

Due to my recent reports on high speed wireless Internet access via prepaid SIMs in Italy (like here, here and here) I recently got an eMail from somebody who will go there for some time asking for my advice. At first, I wanted to write a short and crisp reply saying “no problem”. Once I started typing, however, I realized there are actually more than just a few things to consider. So here’s my response:

[…]

The Theory

You know I’d really like to give you the following answer: Yes, no problem, go ahead, buy the HSDPA card, go to Rome, pick up a prepaid SIM and you are all set. Or even better, just take your notebook to Rome, visit a TIM shop and they’ll sell you a prepaid SIM, a data card for a reasonable price, and install it on your notebook while you are in the shop. Reality, however, is a bit more complex. Not because it has to be but because of a less than ideal way of how things are handled by the parties involved.

The Reality

Buying a Mobile Phone or PC Card

Buying an HSDPA Express card in the US and bringing it to Italy should work. Before you buy however, make sure of two things: For once, the card must not be locked to a specific network it must be open to all. Therefore, buying an HSDPA card from a network operator will not work as they are usually locked. […] Second, you should make sure the card supports the European UMTS band, which is 2100 MHz. The US uses different frequency bands so if the card is limited to them it won’t work in Europe. Third, you should also make sure you can get software updates via the web page of the manufacturer. It’s not uncommon that cards get pushed out the door with an unstable software version at the beginning so being able to update it is important.

Getting a SIM and Activating Mobile Internet Access

So let’s say you have a card and you’ve arrived in Italy. TIM definitely has the best HSDPA network for your purpose so I advise you to go for one of their prepaid SIM cards. Try to find a TIM shop with a helpful and friendly shop assistant and buy a prepaid SIM. Once you’ve got it, put it into a normal GSM phone and make a phone call which gets connected. This way the card is activated and only after that is it possible to put some more money on the account in order to enable the data option. Note: Just calling another party which does not pick up does not work, the call needs to be connected. Don’t ask me why. To top up, buy a top up card and be prepared to read the Italian instructions. In the TIM network you can top up your prepaid SIM by buying a scratch card and sending the secret digits via SMS to the network. Some shops also offer top ups by giving them the telephone number of the line. Works nice as well. Once there is enough money on the prepaid SIM you can activate the data offer. I think the offer is called WEB FACILE 500 MB so ask the shop attendant in the TIM store how to activate this option. Afterwards, happy surfing.

House Keeping

I don’t think TIM warns you when you are close to having used up the 500 MB or when the 4 weeks for which it is valid expire. God only knows why. So you have to check every now and then how much is left on your card by calling the TIM voice server and go to menu 3. If you are close to your limit, put some more money on the SIM card and extend the option. I am not sure how to do this as I tried as described and it didn’t work. An Italian friend of mine then called the TIM hotline and after 20 minutes of heated discussion in Italian they did it manually.

Another option is to buy a prepaid SIM card of WIND. They also seem to have an HSDPA network in Rome now. Their network performance is not as good as TIM’s when I was there but it probably also will do the job. However, you’ll get more bits for your buck 🙂 Their offer is called WIND MEGA NO LIMIT 15000.

In Building Coverage

As long as you have a window in the room and are not underground it should be all right. It’s still a bit of a gamble but you should be fine, Rome is well covered.

Misc Stuff

Other options: In case you can’t find an unlocked HSDPA card to buy in the US you can buy an HSDPA capable card or phone in Italy. If you buy a card it’s probably locked to the operator. USB adapters are another interesting alternative because you can place them in a good spot without moving the notebook if coverage is less than ideal. Phones can be bought unlocked, you might have seen that I choose to do this when I was there and bought a Motorola V3xx with a branding from TIM. As it turned out it worked fine in all networks except for TIM’s. Again, completely beyond me.

So I hope I haven’t discouraged you from going ahead with your plan. It can be done and if you have an Italian friend who knows a thing or two about computers and maybe also something about how to connect wirelessly to the Internet you should get it working without too much trouble.

Looking at it from the bright side I think one could say that there is lots of room for improvement. All it takes is the will and a bit of work from network operators…

Hope this helps,
Martin

So to me, how things could be (an not unrealistically so) sounds a lot nicer than how things actually are. As I said, there’s a lot of room for improvement…

WiMAX II – 802.16m – Chasing the Ghost

Looking at presentations from a recent LTE meeting I found it quite interesting at how many of them mention WiMAX 802.16m. I haven’t heard much about 802.16m yet but since they all refer to it I thought it might be time to find out a bit more about it.

It seems to be a bit early for that search however. First announced in early 2007 the only facts so far known about 802.16m is that the IEEE would like to create a standard as much backwards compatible as possible to the current version of the WiMAX (802.16e or 820.16-2005) but with peak data rates of up to 1 GBit/s (that’s around 1.000 MBit/s).

Compared to current systems deployed in live networks today such as HSDPA with a theoretical top speed of 14 MBit/s and about 2 MBit/s with a Cat-6 HSDPA mobile today in live networks, these numbers are staggeringly impressive. So how can such data rates be achieved? As not much is known so far, let’s speculate a bit.

Between today and WiMAX II, there’s systems such as WiMAX and LTE which promise faster data rates than those available today by mainly doing the following:

Increase the channel bandwidth: HSDPA uses a 5 MHz channel today. WiMAX and LTE have flexible channel bandwidths from 1.25 to 20 MHz (Note: The fastest WiMAX profile currently only uses a 10 MHz channel today for the simple reason that 20 MHz of spectrum is hard to come by). So by using a channel that is four times as broad as today, data rates can be increased four times.

Multiple Input, Multiple Output (MIMO): Here, multiple antennas at both the transmitting and receiving end are used to send independent data streams over each antenna. This is possible as signals bounce of buildings, trees and other obstacles and thus form independent data paths. Both LTE and WiMAX currently foresee 2 transmitting and 2 receiving antennas (2×2 Mimo). In the best case this doubles data rates.

Higher Order Modulation: While HSDPA uses 16QAM modulation that packs 4 bits into a single transmission step, WiMAX and LTE will use 64QAM modulation under ideal transmission conditions which packs 6 bits into a single transmission step.

By using the techniques above, LTE and WIMAX will be able to increase today’s 2 MBit/s to about 20-25 MBit/s. That’s still far away from the envisaged 1.000 GBit/s. To see how to get there let’s take a look at what NTT DoCoMo is doing in their research labs, as they have already achieved 5 GBit/s on the air interface and have been a bit more open at what they are doing (see here and especially here):

Again increase of the channel bandwidth: They use a 100 MHz channel for their system. That’s 4 times wider than the biggest channel bandwidth foreseen for LTE and 20 times wider than used for today’s HSDPA. Note that in practice it might be quite difficult to find such large channels in the already congested radio bands.

12×12 MIMO: Instead of 2 transmit and receive antennas, DoCoMo uses 12 for their experiments. Current designers of mobile devices already have a lot of trouble finding space for 2 antennas so a 12×12 system should be a bit tricky to put into small devices.

A new modulation scheme: VSF spread OFDM. This one’s a bit mind bogelling using CDMA and OFDM in combination. Wikipedia contains a description of something called VSF-OFCDM which might be a close brother.

A four times wider bandwidth with six times the number of antennas results in a speed increase factor of 24. So multiplying 25 MBit/s * 24 results in 600 MBit/s or 0.6 GBit/s. That’s still a factor of 8 away from what DoCoMo has said they have achieved, so I wonder where that discrepancy comes from!? I guess only time will tell.

Summary:

For the moment, the wireless world’s pretty much occupied with making LTE and WiMAX a reality. Pushing beyond that is not going to be an easy thing to do in the real world as bands that allow a single carrier of 100 MHz will be even harder to find than for the 20 MHz envisaged for LTE. Also, cramming more than 2 antennas into a small device will also be a formidable challenge.

More about 4G, LTE and WiMAX can be found here.