Assignment 8: Cell Phones, Iridium, and Your Final ProjectOverviewThis week we will spend a little time of cell phones, and how they work. Then we will spend some time looking at Iridium, the satellite phone company. This is a nice example of many features of 3G cell phone systems. Finally, we'll talk about topics for your final project. Cell PhonesMobile phones have be around since before WWII. These systems used a relatively low frequency, and only a few channels. This meant that only a few simultaneous connections could take place at once. Originally these were bulky and heavy, but by the early 1970's handheld models appeared. Calls were very expensive due to the limited network capacity. Cellular Phone SystemsThe key to breaking the limitation of only a few frequencies was the concept of cellular phones. The idea is to divide the coverage area into small regions, or cells. Each cell uses a different frequency than its neighbors, so they don't interfere. However, cells that are two cells away reuse the original frequency again. Cells of three frequencies can then tile a very larger area. The idea is illustrated below No adjacent cells share a frequency, but only three frequencies are needed to cover everything. In practice, more frequencies will be used, because the cells are actually layed out to cover where people are (like along highways), and that doesn't tile as perfectly. Most cell towers use directional antennas, because the FCC allows the output power to be 500 Watts for a directional antenna, compared to 100 Watts for an omnidirectional antenna. A typical directional antenna has three faces, which can be placed at the corners of the cell to produce relatively uniform coverage.
The area of the cells is determined by the frequency if the power is fixed. Early cell phones used frequencies in the 450 MHz range, which goes a long way. Cells were large, and not many calls could be made in a geographic area. As frequencies get higher the signal goes less far, the cells get smaller, and the capacity of the network increases. One example from Wikipedia is shown in the table below
We get 16 times more capacity at 2100 MHz as at 450 MHz. This is a tremendous benefit in urban areas. The rapidly coming 5G systems will be at 28 GHz and 34 GHz. These will be even more local, much higher signal bandwidth, exploit multiple transmit and receive paths (MIMO), and have tremendously higher capacity as a result. Cell Phone RadioCell phones are again a digital packet radio system. The packet data is encoded either using spread spectrum techniques (CDMA) or using multiple frequency shifted carriers (OFDMA, a multi-user variant of OFDM), just like the techniques we saw in WiFi last week. In Wifi, everyone on a subnet talks on a single frequency. For cell phones, the uplink and downlink are separated. They may be on different frequencies (common in the US) or they may share a frequency by multiplexing (common in China). Multiplexing has the advantage that usually the bulk of the traffic at any instant will be one way, or the other. The multiplexing can dynamically favor the higher data rate direction, and make better use of the spectrum. Starting with 3G systems (2000's) the control and data parts of the cell phone signal were encoded as IP, just like WiFi. The voice signal was still switched separately due to concerns about latency in packet switched networks. With 4G (the 2010's) everything went to IP including voice. This followed the same transformation on wired lines. Effectively the 4G phone system is WiFi with a special radio transport layer. Remarkably, that means you can use WireShark to watch the traffic on a cell phone network! 3G Networks3G networks were interesting to capture with your SDR. However, there don't seem to be any active in our area now. If you find one, let me know! Each network had an accurate frequency source which could be used to calibrate the frequency of the crystal in your rtl-sdr. Beyond that, there is a control channel that is responsible for housekeeping like setting up and taking down calls, and assigning frequencies. This is transmitted in the clear, so that if you can capture and decode the 3G signal, you can feed the result into wireshark, and look at who is connecting to who, and what the traffic looks like. Unfortunately, I don't know how to do this with a 4G net. One place where 3G still is used is Iridium. This is the satellite phone system originally launched in 1998. This operates in the 1.61885 to 1.6265 GHz band, which is within the range that your rtl-sdr can acquire. You will need an antenna tuned to that frequency (a patch antenna is good), and a low noise amplifier (LNA) right next to the antenna in order to avoid losing too much signal in the cable. This youtube video describes how the Iridium system works, and how the signals can be acquired and decoded. This is done a German group. Some of what they show is illegal here (decoding pager messages, for example). Note that this was done with the original Iridium satellites that are now being replaced with IridiumNext. I had expected that many of these issues would have been fixed, but that doesn't seem t be the case. Certainly anyone using the legacy modes is still vulnerable. Final ProjectThe final project will be very much like the weekly assignment. You can work in groups of two to three if you'd like. Ideally there would be about five groups. Aim for about 15 slides, and about 20 minutes, with another 10 minutes for questions. You will present these during the last week of class. If you do work in groups, include a slide about what each person contributed (don't show this in the presentation). For this week, look over the topics, and choose one. Make 2 slides about what the topic is, and why it is interesting. Upload your slides here We'll go over them in class on Tuesday. You will have Thursday to work with your group, look up resources, and ask questions about your projects. Possible Topics
|