This report on Telecom: Italy fits to my recent blog post on T-Mobile upgrading their wireless backhaul to meet the rising demand for higher data rates. The report says that Telecom Italia will connect about 1700 'antennas' of their wireless base stations to new fiber infrastructure which will also be used to bring faster fixed line Internet connections to homes and businesses. The wording is a bit strange since base stations are connected to a backhaul fiber and not individual 'antennas'. I wonder how many antennas they count per base station!? There are usually 3 sectors per base station, each having its own antenna. That would make it around 600 base stations. But that's just a guess on my side.
Unstrung recently reported that T-Mobile has started to deploy RAD's Ethernet over DSL solution for backhauling 3.5G traffic from their UMTS / HSPA base stations. I wondered in the past how soon we would see something like this happening since current 2 MBit/s E-1 line rental costs are prohibitively high and several are required for the bandwidth requirements of a 3.5G base station. The article says deployment starts in Germany, where T-Mobile is the incumbent and has surely made a favorable deal with their fixed line branch, T-Com, who is in the process of deploying a VDSL overlay network besides the already existing ADSL/ADSL2+ network. As VDSL only works over short distances, T-COM deploys curbside VDSL cabinets every several hundred meters. With 52 MBit/s in downlink and 11 MBit/s in uplink, a VDSL link offers more than enough bandwidth for a base station with multiple sectors. Backhaul from the cabinet is also not a problem since they are connected to the core network by fiber. The article doesn't say if the base station continues to use E-1 links for voice traffic or if all data is backhauled via the DSL link.
I had to hold back with this blog entry a bit because I wanted to get permission first to write about what I would say was the most interesting demo I’ve been invited to during the 3GSM / Mobile World Congress:
Lots of WiMAX demos where shown at this years congress and it’s good to see that 802.16e mobile devices have now reached PC-card card sizes and are close to general availability. It’s also nice to see that when the antenna is just a couple of meters away you can see data rates beyond 10 MBit/s. However, that tells you only little about how the system performs in practice when the base station antenna is a couple of blocks away on top of a building and there is interference from neighboring base stations. To go the extra step, Intel and Motorola have teamed up to show how their kit works in a real environment during this years show.
Network Setup
In just a few days, Intel has put up four Motorola WiMAX base stations on rooftops in central Barcelona which were connected to the core network via 50 MBit/s microwave backhaul equipment from Dragonwave. Each base station was equipped with 3 sectors, each on its own 10 MHz channel in the 2.5 GHz band. In total they had three channels available for the network so each base station used the same set of frequencies. The distance between the base stations was about 2 kilometers which is a bit more then what you would see in an inner city network deployment. They couldn’t choose the sites themselves and had to be happy with what they got. On the upside, there is less interference from neighboring cells then there would be in a public network since there were only 4 cells and thus there is no interference from cells further away.
Indoor Experience
Sitting comfortably in the lobby of a hotel in Central Barcelona, I first had a chat with the technical project manager responsible for the network setup. Very good to have somebody with a technical background to talk to. During our discussion I got a first impression of the network performance as there were two notebooks connected to the network, one via a WiMAX PC-card adapter and the other via a CPE (Customer Premises Equipment) box the size of a DSL or cable modem. Despite sitting in the ground floor lobby, the base station being a couple of rooftops away on the other side of the hotel, the probably heat insulated and RF absorbing windows and just using the built in antennas of the devices we still got a data rate exceeding 2 MBit/s via both the CPE and the PC-card adapter. Note that both were SISO (Single Input Single Output) devices. As even this speed is far beyond what you can make use of while surfing the web we streamed a couple of video streams being sent live from WiMAX connected vehicles touring the city. The resolution of the stream was around 320x240 pixels and with a frame rate of 30 fps and the video streams were crisp and clear. One of the notebooks also had an engineering monitor software package on it to observe lower layer performance of the PC card and it was interesting to see how the card goes through the different modulation and coding schemes from QPSK to 64-QAM as reception conditions changed.
Outdoor Experience
Later on we went outside and used Segways to speed up and down the streets with a notebook attached to it to see how the network copes with mobility. Again the video stream performance was flawless and we streamed a U.S. TV station over the Internet which is quite bandwidth hungry. But even this does not require a bandwidth beyond 5 MBit/s which was obviously not the limit of the network. When asked what the highest throughput is that can be observed in the network I was told that it is around 13 MBit/s with 64-QAM and about 1.5 MBit/s at the cell edge with QPSK ½ modulation and coding despite the fact that the cells are too far away from each other. Interesting numbers showing the direction in which we are headed once 2x2 MIMO is added and proper cell sizes are used.
Here's a video taken and produced by Marc Wallis and Michael Ambjorn of Intel/Motorola respectively:
(copyright by M. Wallis / M. Ambjorn of Intel/Motorola)
Conclusion I came away very impressed from the demo as the speeds were amazing. We didn’t loose the connection to the network even once during the one and a half hours sitting in the hotel and touring the city. That says a lot about the software stability of the PC-card and the network. Thanks a lot to Intel for the VIP tour invitation it was definitely the best demo I have seen during the Congress.
With mobile networks getting faster and faster a growing pain for network operators is the backhaul connection between the base station sites and the next element in the network. Today, T-1 or E-1 connections are used with a line rate of 1.5 and 2 MBit/s. With HSDPA being put in place today, backhaul capacity requirements of 3G base stations now reach 10 MBit/s or more. This means putting additional T-1 or E-1 lines in place. While this might still work today for HSDPA speeds despite the associated rising costs it certainly won't work tomorrow for WiMAX, LTE and other Beyond 3G technologies that require backhaul capacities of 60 MBit/s per base station and more.
So the big question is what comes after T-1/E-1 connections over copper, fiber or microwave!? The common answer these days seems to be:
IP over Ethernet with the capability to carry legacy GSM (TDM) and UMTS/HSDPA (ATM) links in IP pseudo-wires alongside native IP traffic generated by native WiMAX and LTE base stations.
But how do you connect the base station sites to Carrier Ethernet Networks? Can the last mile be done over copper, is fiber required or is next generation microwave an alternative? Questions over Questions :-)
