Author: Martin

Collaborative MIMO for WiMAX and LTE

In two previous blog entries I focused on the limited uplink power of mobile stations and how WiMAX, UMTS/HSDPA and LTE overcome this hurdle by allowing several mobiles to transmit simultaneously. In the future, however, limited transmission power might not be the only limitation.

WiMAX and LTE will probably both use a technology called MIMO (Multiple Input / Multiple Output) which makes use of multiple antennas at both the transmitter and the receiver to transmit independent data streams on the same frequency via different directions. Especially small hand held devices, however, might not be equipped with several antennas due to their small size or due to the additional cost incurred. Thus, they can not make use of MIMO. This reduces both their own speed as well as the overall speed of the network.

The solution to this problem is called "uplink collaborative MIMO" or multi user MIMO (MU-MIMO). Here, the network can instruct, for example, two mobiles to transmit simultaneously, each on an independent MIMO path. Even though both signals are sent on the same frequency, a MIMO capable base station will still be able to pick up the signals independently from each other if the main energy of each signal arrives from a different direction. This in effect creates a MIMO channel, just that the two or more antennas do not belong to one terminal but to several. Interesting approach!

From what I can read in the press, only Nortel has so far picked up on this and has stated that it will implement collaborative MIMO in the uplink direction for both WiMAX (here and here) and LTE (here).

HSDPA Performance Of Vodafone’s 3G Network in Italy Has Me Puzzled

Back I am in Italy for a while. I’ve grown quite accustomed to the great performance of the TIM HSDPA network, which I’ve described in a number of previous posts. This time around, I set out to test the Vodafone HSDPA network in Rome and to compare it with the results achieved in TIM’s (Telecom Italia Mobile) network. The results were quite a surprise.

I had two SIM cards to test the network. For the first tests, I used my German Vodafone SIM card and a Roamer WebSession, described in more detail here, to establish an Internet connection. As already experienced in the SFR network in France, file download speeds were capped at around 45 kBytes/s. While already quite good it falls far short of 160 kBytes/s that are reachable with my category 12 Sierra Wireless 850 HSDPA card in the TIM network.

Viws
In France I was quite uncertain if and where the speed was throttled down. With the help of the Vodafone Italy network, I can now add a further piece to the puzzle which unfortunately raises more questions then it answers. To find out more, I bought a local Vodafone prepaid SIM card for direct access to the Internet and not via the GGSN of Vodafone in Germany used by the German Vodafone SIM card. To my great surprise the download speed of the file was almost the same as with the German SIM card. In the IP packet inter-spacing diagram (for an introduction of how to interpret the diagram see here), however, the download of the same file with the two different SIM cards in the same network looks completely different. As can be seen in the first graph on the right side, the file download via the German Web Session in the Italian Vodafone network shows IP packet inter-spacing mostly around the 30 ms line. A clear but not yet conclusive indication for throttling. With the Italien SIM card however, most packets of the same file are transmitted with a packet inter-spacing time of 10 ms as can be seen on the left side of the graph. So the transmission would be much faster if it were not for the randomly distributed packet inter-spacing of quite a lot of packets between 50 ms and 200 ms. To be honest, I have no idea why some packets take such a long time to arrive. I don’t think it can be RLC retransmissions as the automatic retransmission of packets discarded by the Node-B’s HARQ process usually takes around 80 to 100 milliseconds. Also these inter-spacings were not caused by IP layer retransmissions.

More clues

Performance_comparison_vodafone_vs_
I then went on to do a direct comparison of the performance of the TIM network and the Italian Vodafone network by downloading two files from different servers to exclude the possibility that the Vodafone network has a problem with the connection to one file server. The result is shown in the second graph. On the left, the download speed for file 1 and file 2 are shown for the Vodafone network. Note the constantly changing top speeds. Afterwards I replaced the Vodafone SIM card with the TIM SIM card in the wireless card and performed the same downloads in the TIM network. The result is shown on the right side of the graph. The throughput is fairly constant and much higher than in the Vodafone network. When looking into the Wireshark trace the Vodafone throughput suffers from two things. First, the random packet inter-spacing times described above. Second, I have observed IP layer retransmissions every couple of seconds which also greatly reduce the download speed. The TIM network does not suffer from any of those.

Conclusions

As I repeated the tests over several days and at different times of the day a temporary error or network overload can be excluded as the reason. There are two likely causes for the problems observed in the Vodafone network. The most probable one is that there is an incompatibility between my Sierra Wireless 850 HSDPA card and Vodafone’s HSDPA network. It’s still early days for HSDPA so I would not be surprised if this were the case. Another possible cause could be that Vodafone has a big IP routing problem somewhere in the network. A good way to verify this would be to repeat the tests with a different HSDPA card or mobile phone. If the situation improves it’s an interoperability issue. If not, well, then it could still be both.

Music Phones Are (Audibly) Getting More Popular

Ricky Cadden lists a number of good points when it comes to why Music Phones still have some way to go before they reach the same usability as Apple’s iPod. Nevertheless, people have definitely started to use their phones for listening to music. There’s no better proof than to take a ride in the Paris metro these days. Today, I took the metro twice and each time I was in a carriage in which a person listened to the music on his phone without headphones.I am slightly annoyed at this behavior. I hope this will not become a new trend…

How File Sharing Of Others Drains Your Battery

Ever thought file sharing of others could have an impact on your mobile device’s runtime on a battery charge? If not, read on:

For DSL connections I think it is quite the norm rather than the exception that subscribers are assigned a public IP address. In wireless networks things are a bit different. While some operators also assign public IP addresses when customers establish a connection to the Internet from their mobile device, others use a pool of private IP addresses. When private IP addresses are used, which is similar to the way a DSL router at home maps several PCs and notebooks to a single public IP address, wireless clients are not directly reachable from the outside world. Thus, packets destined to ports from which a subscriber has not originated a connection are rejected. In networks that assign a public IP address, however, all packets are delivered.

From a technical point of view using public IP addresses rather than private ones for mobile terminals is the right thing to do. However, there is a big practical disadvantage: If the IP address assigned to a mobile terminal was previously used by somebody for file sharing, other hosts keep sending packets to that IP address as they are not aware the IP address is now used by someone else. In terms of bandwidth usage this is not a problem as the packets are small and only occur every couple of seconds. For mobile terminals however, this means radio resources are kept assigned instead of being removed during times of inactivity (e.g. reading a web page or simply doing nothing with the device for some time). This in turn has a drastic impact on how long the device can run with a single battery charge.

I’ve seen this happening several times now while being connected via a 3G network and running Wireshark on the PC. In addition I can also see Internet worms banging on my door every now and then trying to send messages to the Windows Messaging Service (a bug closed several years ago…)

Do We Have To Use Private IP Addresses With Future Always On Devices?

Today, this behavior is probably not an issue for most people as Internet sessions tend to be brief. In next generation wireless networks, however, mobile devices will always have an IP connection to the network because voice calls will use the IP network rather than a circuit connection like today. Further down the road when not only a few users but most will use SIP/IMS etc. clients, end user devices should be able to connect with each other without the need for a gateway. With private IP addresses and NAT (Network Address Translation) this will be rather difficult to do.

With IPv6, Network address translation, private addresses and dynamically allocated public IP addresses should hopefully be a thing of the past. So will IPv6 with static IP addresses be the solution to the problem or the beginning of the next one?

List of Countries With UMTS Prepaid Internet Access

In the past few months the list of countries in which mobile access to the Internet is now available via UMTS/HSDPA networks with prepaid pay-as-you-go SIM cards has grown considerable and I’ve been reporting about it quite a bit. Time for an overview with links to the corresponding blog entries:

Germany:

  • AldiTalk (0.23 euros per MB)
  • Vodafone (WebSessions: 1.95 euros / 15 min, 9.95 euros / 2h and 14.95 euros / 24h)

Italy:

  • TIM (volume bases, 500 MB / 30 days for 20 Euros)
  • Wind (volume based, 1 GB / 30 days for 20 Euros)
  • Vodafone (time based, 1.5 euros per 15 minutes)

Spain:

  • Yoigo (capped at € 1.20 a day)

Austria:

  • Drei (0.8 euros per MB)

U.K.:


Reports from people commenting:

Poland:

  • Plusgsm (Simdata (Plus GSM): prepaid, 0.03PLN/100KB = 0.30 PLN/MB = 0.08 EUR/MB.)

India

U.S.A.:

And for those countries in which Prepaid Internet Access via UMTS networks is not yet available, the German Vodafone WebSession offer with a German Vodafone prepaid SIM might help.

Vodafone Websessions with SFR in France

Spring is one of the best times of the year to be at the Côte d’Azur in France. While the weather and landscape is great, France is really missing attractive prices for Internet access over 3G. Not really affordable for post-paid customers, nothing is available for pre-paid customers at all. A good opportunity to use my Vodafone Germany prepaid SIM card for Internet access via the WebSessions roaming offer.

Downlink Speeds

Vodafone’s partner network in France is SFR and according to their web page, they’ve got HSDPA deployed in some parts of the network. Accordingly my speed expectations where high. And indeed, when I activated the 3G connection my data card showed that HSDPA is available in the network. Speed tests performed over several days revealed however, that the downlink speed is artificially limited to around 45 kBytes/s. The limitation is certainly not the air interface as the signal strength was good and speeds I measured in Germany and Italy were much higher.

It’s hard to tell from a users point of view exactly where the bottleneck is. It could be that Voda’s Home Location Register (HLR) in Germany and SFRs SGSN in France can not exchange the QoS profile correctly which subsequently leads to the throttling of my connection. It could also be that there is a limitation on the IP link used for forwarding my packets between the SFR network and the Vodafone Germany network. It’s also possible that the SFR SGSN or the Vodafone GGSN is unilaterally limiting my speed. In practice this means that HSDPA does not give me a great advantage in the SFR network over a 3G UMTS device as the speeds are the same. Due to this I was not sure if the connection was HSDPA at all. Subsequent tests described below showed, however, that the data card really got an HSDPA and not UMTS bearer.

Uplink Speeds

After getting a 384 kbit/s uplink bearer in Italy and Germany I was also disappointed about the ‘meager’ uplink speed of only 128 kbit/s in the SFR network. It’s likely that this is no interoperability or throttling problem but a general network limitation of the SFR radio network. Either they haven’t activated the higher bearer option or it’s not yet available in the current software version of their radio network. Whichever it is they should consider upgrading or switching on the option as the difference is remarkable.

Round Trip Delay Times

One of the indicators that the data card got an HSDPA bearer and not a UMTS bearers were the round trip delay times. With the data card I got a round trip time of about 170 ms. HSDPA usually delivers a round trip time to an external host of around 120 ms (100 ms to the first hop). The additional delay is most likely due to international roaming which means that my data is tunneled from SFR into Vodafone Germany’s network before entering the Internet via Vodafone’s GGSN. With a Nokia N93 3G ‘only’ terminal I got round trip times of around 380 ms. I am not quite sure why there are an additional 200 ms of dealy as UMTS is usually only around 50 ms slower.

Radio Ressource Management

On the positive side I noted that the HSDPA radio resource management was more advanced than what I experienced in the TIM network in Italy and the Vodafone network in Germany. While the HSDPA bearer is active, the above mentioned round trip times to an external host of about 170 ms can be observed. In Cell_FACH state, which TIM and Voda’s network in Germany might not support yet, round trip delay time s were around 360 ms. This reduced activity state was only entered after around 45 seconds. After about 60 seconds the connection is put into Idle, Cell_PCH or URA_PCH state from which it takes around 800 ms to get back into active state. This is a lot quicker then the 2 seconds observed in Vodafone’s network in Germany and TIM’s network in Italy.

Skype, VoIP and IPSec

I tried Skype and my companies VoIP client over both HSDPA and UMTS and got crystal clear connections. Also, my IPSec tunnel worked fine between the notebook and my company. Very well!

Wifi competition

Except for the artificially throttled speed, my experiences in the SFR network with the HSDPA card were very positive. I should also note, however, that some Wifi operators such as Orange have moved forward a bit as well and are now offering 10 hours online for 15 euros. The 10 hours can be distributed over 30 days. For 15 euros, one can stay online for several days if the connection is only used for a couple of hours a day. For me a Vodafone 24h WebSession for 15 Euros is still better because I am online for more than 10 hours a day. Also, I need access at different locations throughout the day which is difficult with Wifi hotspots. People with less online time and stationary use, however, might find a 10h over 30 days for the same price more attractive. Also, they are not limited to 50MB of traffic per WebSession Vodafone intends to introduce in September.

P.S. For more articles on this topic, click on the HSDPA link next to the date below

Mobile Internet Advertisment in Austria

21042007786
When it comes to mobile technology, Austria is usually the place where things happen first. Not surprising therefore, that Mobile Internet offers are not only on the front page of most mobile operators web sites but also being marketed out in the open now. Take a look at the picture on the left. When I was in Austria recently, there was an advertisement of ONE for their H.U.I. mobile Internet access offer at almost every bus stop I passed. T-Mobile is also putting large advertising banners in cities to market their latest offer of €0.- basic charge and 10 cents per Megabyte of HSDPA traffic (post paid).

A reason for promoting the mobile Internet so fiercely these days might be that the operators don’t have have much to advertise for anymore in the voice domain since Mobile Virtual Network Operators (MVNO’s) have caused a landslide in prices for voice minutes.

One of the things still missing in Austria is Internet access via prepaid SIMs. Let’s hope that will follow soon as well. Some German MVNO’s have set a good example.

Are Virtual COM Ports Fast Enough for HSDPA, WIMAX And Other Wireless Broadband Technologies?

I’ve been having a lot of fun lately with my Sierra Wireless HSDPA data card for my notebook which reaches speeds I still have difficulties to believe that they are possible over a wireless network. While many people probably prefer data cards for wireless Internet access I’d rather like to use my mobile phone to connect the notebook to the Internet. This is because I use connected applications both on the mobile phone and the notebook and don’t want to have two subscriptions for mobile data access. While the data card acts like a network adapter and thus has no speed limitation on the interface between the data card and the notebook, mobile phones simulate a modem over a serial port, also called a COM port. The question I thus had was if those virtual COM ports have the same speed limitations as their real counterparts. If so, this could be a problem for mobile phones that support HSDPA, WiMAX and other future wireless broadband technologies which can deliver higher data rates than real COM ports can transport.

The maximum data rate that can be set for a real or a virtual COM port is 921.600 bits/s. This is far too slow for modern HSDPA mobiles that support downlink speeds of up to 3.6 MBit/s. For real serial ports this limit is real as the speed setting in the modem dialog translate into commands to a UART chip to send bits over a serial wire at exactly this speed. Most if not all high end phones today, however, have no serial connector anymore. Instead they use USB for communicating with a PC. In order not to modify the dial up networking software of operating systems, serial port emulations over USB are used. To the dial up network softwae it looks like the ‘modem’ in the mobile phone is connected via a serial port. The question now is whether the same speed limitations apply to these virtual COM ports over USB as well. At least in the dial up networking menus, the maximum speed that can be set for them is also 921.600 bits/s. USB is much faster than HSDPA or any other wireless wide area network. Even USB 1.1 can already handle a speed of up to 12 MBit/s, so physically no limitation exists.

Virtual_com_speed_192_kbits
As I don’t have an HSDPA capable mobile phone yet I did the next best thing and used a Nokia 6680 UMTS mobile. Instead of setting the COM port speed to 921.600 bit/s, which is twice as high as what UMTS can deliver, I set it to 19.200 bit/s which equals a maximum transfer rate of 2.4 kBytes/s. This is much less than what a UMTS connection can deliver, about 40 kBytes/s. Then, I established a UMTS connection, started a download of a big file and monitored the throughput. As can be seen in the picture on the left, the COM port speed is set to 19.2 kbit/s = 2.4 kBytes/s. The download rate, however is 40 kByte/s.

This proves that the speed setting for a virtual COM port has no influence on the actual speed of the data transfer over the virtual connection. Thus, if both the driver on the PC and the mobile phone support higher speeds over the virtual COM port, HSDPA and WiMAX speeds will be supported without a need for modifying the dial up networking software of the operating system. Very well, so I am looking forward to testing things with a real HSDPA mobile phone!