If I were to ask you how fast you can go on LTE, you’d probably say 300 Mbps. That’s the number most often quoted as the peak downlink data rate for LTE. (Well, technically it’s 299.552 Mbps, but let’s not quibble over a few kbps.) The problem with that number is that it makes a number of assumptions about how the LTE network is configured; for example, it presumes that the LTE channel is 20 MHz wide and that 4x4 SU-MIMO is being used for the downlink transmission. But what other assumptions are hiding behind that number?


A Thoughtful Experiment

Let’s perform a quick thought experiment. Suppose an LTE user is downloading a very large file, and is watching the progress display on his device. What would it take for the data speed shown on the display to be as close to 300 Mbps as possible?

Consider the situation. The user has established a data session with an FTP server somewhere, and packets are flowing through the Internet, to the LTE network, over the radio interface, to the user’s device. The pipeline between the server and the user needs to be as large as possible (in order to maximize the data rate), and that pipeline needs to be kept filled (in order to sustain that data rate). What has to happen for this to be the case?

I’ve already mentioned a couple of things: we need a 20 MHz LTE radio channel, and 4x4 SU-MIMO. But let’s put a more comprehensive list together.

  • The wider the channel, the more subcarriers are available to carry data. 20 MHz is the largest LTE channel bandwidth available in Release 8.
  • Those extra subcarriers are of no use if they are given to some other user. Our user must be the only active user on the channel. Note that this has the beneficial side effect of reducing the size of the PDCCH, which leaves more room for the PDSCH in the subframe.
  • The more data the user can receive per TTI (transmission time interval), the higher the net data rate will be. SU-MIMO allows the network to transmit multiple transport blocks in parallel to a single user; 4x4 SU-MIMO provides the highest potential throughput in Release 8.
  • The user’s UE must be capable of handling a 4x4 SU-MIMO transmission and 300 Mbps, which means it must be a Category 5 device.
  • Each resource element must contain the maximum amount of data bits (and the least amount of overhead). A CQI 15 transmission uses 64QAM modulation (6 bits per symbol) with minimal error correction (about 5.55 data bits per symbol, on average).
  • In order to use a CQI 15 transmission and still achieve the required packet error rate target, the downlink radio conditions (as seen by the user’s device) must be sufficient to support that rate. The radio channel must have a strong signal with minimal noise and interference; in general, the user will have to be physically close to the cell for this to be true.
  • Related to this, each Physical Layer transmission must work on the first try; if Hybrid ARQ or RLC retransmissions are needed to recover transmission errors, that will increase the time it takes to successfully send the packet (which reduces the data rate), and take resources away from other packets (which increases delay, and hence reduces the data rate). The UE must be able to send a HARQ Ack after every transmission, which requires excellent downlink conditions.
  • There must be no other bottlenecks in the data path. Usually we’re just concerned about the capacity of the backhaul facilities between the eNB and the S-GW, but every node and every link along the way must be able to support a sustained 300 Mbps data transmission. That includes everything all the way out to the FTP server, which is outside the LTE network.
  • The application itself must be capable of generating enough data to keep the pipe full. That means that the FTP server must send data to the user at 300 Mbps. Moreover, it must never let the eNB buffers get empty, otherwise there will be a gap in the packet flow, which reduces the net data rate. In other words, the server must generate 300 kilobits of data (37.5 kilobytes) every millisecond.

There may be a few other bits and pieces, but you get the idea. Achieving the maximum advertised data rate is not a trivial task.


So You’re Telling Me There’s A Chance

Can it be done at all? Absolutely. In a closed lab environment where the RF is clean and access is controlled, where the server is configured properly and is located right beside the cell, the (potential) capabilities of LTE have been demonstrated many times.

Will the average user see anything resembling 300 Mbps? Absolutely not. The chances that a random individual will happen to be in the right place at the right time, during exactly the right thing in the right way, are vanishingly small. To be fair, this statement applies equally well to any other wireless data technology, not just LTE; it is a rare event when a normal subscriber achieves a data rate even a third of the advertised maximum. Interestingly enough, it’s generally not the RF conditions or the traffic load on the cell that slows things down; it’s the application the subscriber is using, which rarely generates more than a trickle of data.

So what’s the moral of this story? Don’t worry about the number on the box, and enjoy a data experience that’s simply better than what you had before.