How LTE Deals With Limited Uplink Power

In a previous blog entry I’ve been looking at how WiMAX and HSDPA allow several mobiles to simultaneously use the uplink. This is necessary as the power output of a terminal is much lower than that of a base station. Due to this restriction a single terminal can not use the total uplink bandwidth of a channel. The only way to compensate for this is to allow several mobiles to transmit at the same time. After writing this article a reader asked how LTE (Long Term Evolution), the successor of HSDPA/HSUPA, deals with this. So here we go:

In downlink direction, LTE is based on Orthogonal Frequency Division Multiplexing (OFDM) technology, quite similar to WiMAX (802.16e). While WiMAX uses OFDMA (Orthogonal Frequency Division Multiple Access) modulation in uplink direction, it was decided by 3GPP to go a different way for LTE. Here, SC-FDMA (Single Carrier – Frequency Division Multiple Access) will be used. It took me quite a while to figure out the basics of SC-FDMA but I think I’ve finally got the basics right and have posted the results of my research here.

So why doing it differently?

It looks like while OFDMA has many advantages it suffers from bad Peak to Average Power Ratio (PAPR). Again, I didn’t find an easy to understand explanation of PAPR and it’s implications on the web. Therefore I decided to do some of my own research and I am very thankful to a number of readers who have helped in the process. The results are presented here.

Deep Inside The Network: How UMTS And WiMAX Deal With Limited Uplink Power

Lately, I’ve been thinking a bit how different wireless systems deal with the fact that the power output of a mobile phone is much lower than the power output of the base station. In practice this means that uplink data rates per mobile phone can not reach the same level as in the downlink. Most systems today use a different frequency ranges for uplink and downlink (FDD, frequency division duplex) with the same bandwidths. This means that if only a single mobile can transmit in uplink direction at a time, bandwidth is wasted due to the power limitation.


3G networks use Code Division Multiple Access (CDMA) in both uplink and downlink. This means that several mobile phones can send their data at the same time to the base station, each with a different code. The base station knows the code of each terminal and is thus able to extract the simultaneous data streams from the single incoming signal. This way, the data rates of all mobiles can be added up and the uplink is used very efficiently, despite the limitation in uplink power. A single mobile is not able to fully use the available bandwidth due to the power limit. If several terminals communicate with the base station, however, as is usually the case, the uplink frequency band can be used to its limit. This method applies to both 3G UMTS and 3.5G HSUPA (aka E-DCH) as they both use dedicated bearers.


The WiMAX air interface uses Orthogonal Frequency Division Multiplexing (OFDM) in both uplink and downlink direction. Basically, the OFDM approach splits the total available bandwidth into independent sub-channels and data is sent simultaneously over these sub-channels. As UMTS/HSPA terminals, WiMAX terminals are also power limited and therefore face the same problem. Contrary to the code division approach described above, WiMAX assigns different sub-channels in the uplink to different terminals. Thus, each terminal can focus it’s power on fewer sub-channels. In other words a terminal can put more power in a sub-channel if it doesn’t have to use all of them. Other sub-channels not used by the terminal are assigned to other terminals. This means that several terminals in effect communicate with the base station in uplink direction simultaneously.

The comparison shows that both UMTS and WiMAX have interesting ways to ensure that several mobile terminals can communicate with a base station simultaneously in the uplink direction to counter the restricted power output and to use uplink resources efficiently. The way it is done, however, is quite different.

Insight Into Who Backs WiMAX And Who Opposes It

Ericsson recently announced that they will stop their WiMAX development and that they will instead accelerate their LTE development. The Register has taken up on this and has published a very interesting article by Wireless Watch on which companies are pushing WiMAX and which companies are rather opposed. So if you are interested in the technical and political quarrels between 3G, 3.5G, 3.9G, 4G, UMTS, HSPA, WiMAX, LTE and UMB this one is a must read.

The Big Supporters:

  • Motorola
  • Nortel
  • Samsung
  • Huawei
  • ZTE

The Big Reluctant Followers:

  • Alcatel-Lucent
  • Nokia Siemens Networks

The Big Opposers:

  • Ericsson
  • Qualcom

My personal opinion: I think it’s good to have different technologies out there in the market that compete with each other. It speeds up development and it offers new starters in the wireless operator world possibilites which have not existed so far. As I discussed in more detail here, I think the consumer will benefit from this no matter in which direction the market will go.

WiMAX Base Station Prices and Coverage Ranges

The past week saw an interesting meeting of WiMAX operators, consulting companies and equipment vendors in Germany. Not a lot was reported from it except for this very interesting article on Heise. The technical details given in the report are quite interesting:

WiMAX Range

Haven’t we heard of incredible WiMAX base stations coverage ranges of 70 kilometers before!? Now that first networks are being rolled out the numbers suddenly sound a lot more realistic. The presenter of DBD, one of the WiMAX operators in Germany was saying that the coverage radius of a typical base station in urban areas is around  500 to 900 meters. In rural areas they are planing cells with a radius of 4 km. I think that this is quite a realistic number now and quite similar to the coverage areas of GSM and UMTS/HSDPA base stations today!

Customers per Base Station

The DBD presenter is quoted in the Heise article that the company is aiming for 200 users per base station at network launch and that they are seeking 400-500 users per base station in the long run. Again, numbers which are similar to GSM/UMTS networks. In a typical GSM deployment, each base station servers about 2000 subscribers. This number is much higher than the one given by DBD but I think they are nevertheless comparable as lot of those people are not using their phone a lot and thus do not require a lot of bandwidth and do not spend a lot of money. The 400-500 users DBD says it would like to have per base station would probably not be occasional users but such with a broadband Internet contract for,let’s say, 20-40 euros a month similarly like DSL or cable customers.

Base Station Prices

Heise quotes Alvarion saying that their next generation Micro WiMAX base station will be available for less than 10.000 euros. Prices of UMTS base stations are reported by Unstrung to be less than $24.000 these days. It’s a bit difficult to compare the two numbers number as the report says nothing about how many transceivers, bandwidth, capacity such a base station would offer. Also, I found a comment in the UMTS base station price article linked above quite interesting which mentions that today the cost for installation, site acquisition costs, etc. is much more dominant than the cost of the base station itself.

Market Strategies

For areas in which DSL and cable are already available, WiMAX operators will have to think hard about what they can offer customers that DSL and cable can’t to make people come over to them. The Heise article once more quotes the DBD manager saying that they will try doing this by providing telephony service on their network. While this is certainly a good approach it’s nothing new and nothing that isn’t already offered by DSL and cable operators today. So from my point of view they should concentrate more on the mobility of their solution and market that accordingly. Especially for students, commuters and mobile workers, a high speed Internet access both at home and while on the go in one package and for one price will make more sense than DSL at home and WiMAX/HSDPA/EVDO on the go. This limits the competition to UMTS and EVDO networks.

DBD will start deploying 802.16e kit beginning in 2008 and will also retrofit their existing network. While this will require exchanging customer terminals it will also provide them with the ability to offer notebook adapters and (hopefully) even notebooks with built in WiMAX chips from Intel. This should also lower site installation costs as mobile WiMAX will depend less on installing external directional antennas at the customer premises.

Other recent blog entries on WiMAX:

3GSM: Talking About WiMax

I find it a bit strange that at this years congress there’s lots of talk and demos on WiMAX. I wonder if the organizers are very happy about this as the exhibition is focused on GSM and UMTS. WiMAX on the other hand, once ready for the market, is direct competition for established UMTS operators. Most infrastructure vendors like Motorola, Lucent and Nokia are showing some kit and there is even a dedicated WiMAX booth in hall 1. From the demos I think it’s quite clear that 802.16e hardware is still in it’s early phases. Sprint is exhibiting on the WiMAX booth and, just like Intel, they are showing Samsung WiBro handsets and PCMCIA cards. While WiBro is a close cousin of WiMax 802.16e, it’s not the ‘real’ thing. Other companies showing their kit on the WiMax booth are Motorola, Samsung and Nokia. However, it’s mostly chartware for now.