Category: MIMO

MIMO Testing Challenges

Over at Betavine Witherwire there's an interesting post on the challenges of consistently testing multi-antenna devices which will shortly appear on the market. The author of the post mentions that even without MIMO, 3G network capacity could increase by 50% if all devices are equipped with multiple receive antennas and sophisticated noise cancellation algorithms. Obviously that also translates in higher throughput per device. Consequently, network operators are likely to be very interested in these developments and accurate testing of the performance enhancements is a must.

While many tests with mobile devices today are performed with the air interface simulated over a cable, that won't work that easily anymore for MIMO and receive diversity as the antennas in the device are effectively bypassed. It's the antennas and their location and shape inside the device, however, that will make the big difference. More details in the post linked to above.

So I wonder if it's possible to model the impact of the antennas by simulating their characteristics in addition to the signal path with a simulator box that sits on the cable between a real base station and the mobile device)!?

A formidable challenge and I look forward to what the guys in 3GPP RAN4 come up with.

Verizon’s first Draft Specification for LTE Devices

Unstrung has an interesting pointer to Verizon's first draft spec (v0.9) which details what they want devices to be capable of to allow them on their future LTE network. While they are mainly referring to relevant 3GPP specification documents, there are some nuggets of information in there that took my attention:

  • The 700 MHz band they intend to use seems to be band 13 in 3GPP talk. I wonder if that encompasses the complete 700 MHz band in the US or just a part for it!? In other words, will band 13 devices also be usable in other LTE networks (e.g. that of AT&T)? According to this post on Gigaohm, the total bandwidth of the 700 MHz that was auctioned off was 18 MHz per direction (uplink/downlink), so the answer is probably yes.
  • The channel bandwidth they will use: 10 MHz (double that of HSPA today but short of the theoretically possible 20 MHz as per the standard. Again, by looking at the link above, that's obvious because that's all there is available.
  • Devices must have RF connectors for testing purposes for all antennas. Haven't seen that in years on mobile devices. RF designers will have fun…
  • Devices will be assigned an IPv6 address when attaching to the network. IPv4 addresses shall be requested by the device if an application requests an IPv4 connection. The IPv4 address shall be released once no applications are executed on the device that require it. Interesting requirement, looks like an IPv4 address is not initially assigned to the mobile device by the network. This should not be a big deal, S60 for example already has a dual IPv4/IPv6 stack today.
  • Mobile device timer for moving from Connected to Idle state: The document says that the LTE standards say that the move from Connected to Idle state (on the radio layer, this has nothing to do with the IP address assignment) is controlled by the network. The standard leaves it open for the mobile to also initiate such a state change, for example if the device detects (by whatever means) that no applications currently wants to send and receive data. At this point, however, Verizon does not mandate devices to implement this. An interesting side note: Today, HSPA only knows a network initiated state change in the standards. In practice, however, there seem to be some devices that also trigger it from the devices side with a bit of an unorthodox signalling message exchange. Looks like standards people have learnt from that and included this feature in the LTE specs from day one.
  • No mention is made of dual mode CDMA/LTE capabilities. I wonder if that means that they expect that first devices will be LTE only? That wouldn't make a lot of sense to me. I can't imagine people would be very happy using a CDMA USB dongle and a separate LTE dongle, depending on where they are.
  • Verizon expects that first devices are data only, no voice capabilities. A pity, but who knows, they might yet discover the benefits of VOLGA.

Note that the current version is only a draft, there are still many unfinished chapters.

4G and Peak-Rate Marketing

Moray Rumney, Lead Technologist at Agilent Technology is quite outspoken about the negative impact of what he calls "Peak-Rate marketing in telecommunications", i.e. the gap between proclaimed (theoretical) data rates of wireless systems and realistic data rates and capacity achieved in practice. I fully agree with his arguments and will also discuss this topic in my next book. In the latest Agilent Measurement Journal, Moray looks at the topic again, this time from the point of view of how Femto cells could positively influence the data rate and capacity equation in the future. His argument is that the effect of adding a femto layer (Wifi or 3G/4G femto cells) in an overall network architecture increases throughput and overall capacity by orders of magnitude while increasing theoretic peak data rates of macro cells does relatively little in comparison. An article not to be missed, it starts on page 52! Also interesting from a wireless point of view is the article starting on page 25 about resolving design issues in HSPA mobile devices. Earlier issues of the journal can be found here.

3GPP Moves On: LTE-Advanced

LTE is not yet even deployed and the 3GPP  Third Generation Partnership Project) is already  thinking about how to further evolve the technology. A main driver is probably the ITU (International Telecommunication Union), who will in due time release their requirements for so called IMT-Advanced 4G wireless systems.

It is quite certain that in terms of bandwidth, LTE and all other beyond 3G wireless systems such as the current WiMAX 802.16-2005 (802.16e) specification will fall short of the ITU requirements, which will probably be in the range of 100 MBit/s to 1 GBit/s. A very ambitious goal. Earlier this month, 3GPP hosted an IMT-Advanced workshop in Shenzen to which 170 representatives of network vendors and network operators from all over the world attended.

Not much has been reported about it yet in the news or on blogs, so one could think they are working in the shadows. But far from that, the 3GPP website reported on it here and all papers presented during the workshop and a report can be downloaded from here. Tons of information! Compared to other standards bodies that keep their proceedings to themselves, it is great to see 3GPP is so openly distributing their information. They set a good example!

The following bullets list some of the first ideas for LTE-Advanced presented during the meeting to comply with the likely requirements of IMT-Advanced. During the meeting it was decided to officially gather and approve them in 3GPP TR 36.913 over the coming months:

  • LTE advanced shall be backwards compatible to LTE (i.e. like HSPA is backwards compatible to UMTS)
  • Primary focus should be on low mobility users in order to reach ITU-Advanced data rates.
  • Use of channel bandwidths beyond 20 MHz currently standardized for LTE (e.g. 50 MHz, 100 MHz).
  • Increase the number of antennas for MIMO beyond what is currently specified in LTE
  • Combine MIMO with beamforming.
  • Further increase in Voice over IP capacity
  • Further improved cell edge data rates
  • Improved self configuration of the network

Very ambitious goals, given that vendors are still working on the challenges of LTE. But then, what would the world be without ambitious goals?

Thanks to Zahid Ghadialy and his post on his ‘3G and 4G Wireless Blog’ for the pointer!

What next for mobile telephony?

… asks Moray Rumney, Lead Technologist over at Agilent in the latest edition of the Agilent Measurement Journal (3/2007). In his article, Moray takes a look at which factors contribute to the ever increasing wireless transmission speeds and explains where the limits are and why the 300+ MBit/s promised by LTE and other technologies in a 20MHz channel will remain a theoretical promise rather then becoming a practical reality. He then goes on to describe what is possible with the given physical limits and presents his thoughts about how to address capacity issues in the future. An absolute must read!

The journal is available here and the article can be found on page 32.

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!

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.

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).