Monday, September 14, 2009

LTE: Physical Layer

I couldn't control my desire to learn the LTE physical layer, so I pushed everything aside and started reading 36 series specs. The Radio network of LTE looks fairly simple at a glance but the complexity increases as we go deep, just like any other system.

LTE Uu interface is what I am looking at. eNB behaves like a relay mapping the radio network to the IP network. The IP side consists of an interface towards MME over S1_MME and towards SGW over S1_U. The radio side communication also has two planes, user plane and control plane.

LTE User Plane.jpg

The above figure shows EUTRAN user plane. As we see we have a MAC layer, RLC and PDCP. Individual protocols shall be dealt with later. The user plane looks fairly simple as data from UE goes to eNB and eNB maps this data over GTP tunnel and sends it to SGW over S1_U. MAC, RLC and PDCP are at Layer 2 in UE and eNB.

However control plane adds few more things. L3 comes into picture for NAS signaling. This NAS signaling is carried all over to MME inside RRC signal.

Over PDCP we have RRC layer which is responsible for paging,RRC connection management, mobility functions etc etc. RRC is terminated in eNB. But NAS is terminated in MME. NAS is responsible for EPS bearer management, Authentication, security etc. Attach Request is a NAS signal which is carried all the way to MME.
Stepping few layers below we have PHY which is physical layer. This is where the actual engineering is. The whole concepts of high speeds come into picture because of sophisticated physical layer. Its no secrete what technologies are used here. OFDMA with 64 QAM and 2x2 MIMO is the most discussed combination for LTE. How does this combination give us such high speeds?
QAM : Quadrature Amplitude Modulation
Going back to engineering basics, we have a simple modulation scheme called PSK. Phase shift keying, which is analog to digital modulation scheme(transmitter side). In PSK we have 1 bit per symbol .0 and 1. Each bit is associated with a Phase shift. with 4 Phase shifts we can transmit 2 bits per symbol. As with 64 QAM we shall be able to transmit 6 bits per symbol. If we look at this scheme in the given bandwidth, by changing the modulation scheme, we are able to transmit more and more bits. This is resulting in increase of data rates.
Time to look at Shannon's theorem :


As I said above, changing the modulation scheme gives you more throughput. However hight modulation schemes can be only be used when the signal to noise ratio is high. From above theorem, channel capacity is bandwidth multiplied by logarithm of SNR. Higher the SNR higher is the channel capacity which means more throughput.

Second factor which increases channel capacity is bandwidth. Now bandwidth is directly proportional to symbol rate. Higher the symbol rate then higher is the bandwidth. But again, increasing the symbol rate doesn't increase the channel efficiency as channel bandwidth is fixed because available spectrum is finite. So there is a trade off between symbol rate and channel throughput. The basic idea is keeping on increasing the symbol rate(modulation scheme) doesn't always improve the efficiency. So considering these factors I think 64 QAM should be a suitable choice for LTE.

OFDM : Orthogonal Frequency Division Multiplexing
With above in mind lets head to OFDM. The theory behind OFDM is little confusing. Lets understand the below figure (FDMA).
Consider we have X amount of spectrum. This can be divided into channels of each Y amount of bandwidth. Each channel is separated by Guard band to avoid interference. This is basic idea in normal multiplexing schemes. I believe in CDMA we identify each channel by a code (?). So what is happening is we have equally spaced channels occupying the entire bandwidth. Note that these channels are non overlapping. Each channel has a subcarrier(?).
In OFDM: With OFDM systems, it is possible to increase throughput in a given channel without increasing channel bandwidth or the order of the modulation scheme. This is done using digital signal processing methods that enable a single channel to be created out of a series of orthogonal subcarriers. As below figure illustrates, subcarriers are orthogonal to one another such that the maximum power of each subcarrier corresponds with the minimum power (zero-crossing point) of the adjacent subcarrier. In a typical system, the bit stream for a channel is multiplexed across various subcarriers. These subcarriers are processed with an inverse Fourier transform (IFT) and combined into a single stream. As a result, multiple streams can be transmitted in parallel while preserving the relative phase and frequency relationship between them.
This way we can include more number of subcarriers in a given bandwidth thus increasing the overall system throughput.
MIMO : Multiple Input Multiple Output
The Shannon's theorem above is assumed to have 1 transmitter and 1 receiver antenna. If we consider multiple antennas then the theorem could be modified as
Thus in theory increasing the antennas will effectively increase the channel capacity without any change in available bandwidth. Now what we can do with MIMO is increase SNR by transmitting a unique bit stream using multiple antenna in the same channel. This is called Spatial Multiplexing.
With MIMO systems, the bit stream is multiplexed to multiple transmitters without changing the symbol rate of each independent transmitter. Thus, by adding more transmitters, we can increase the throughput of the system without affecting the channel bandwidth.
Thus the combination of OFDMA, MIMO and QAM will give us more bandwidth and higher data rates in LTE. The source for this post comes from various places and it would be stupid of me to post the names of the text books. Next, the above is my understanding of the system, kindly correct me if there are any mistakes. Will appreciate it.
Hope, this was helpful, more to come soon and comments are greatly welcomed.

1 comment:

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