MIMO

Hello all,

So we meet again.

As you know, the initial focus on 5G NR(as of rel-15) has been on increasing the data throughput (enhanced mobile broadband or eMBB). There are different ways to achieve these high data rate requirements. One is obviously the increased spectrum. The more spectrum you have, the more data you can transmit. That's why mm-wave is important for 5G. Another method is to use rich digital modulation techniques. For example, 64-QAM can be used to send 6 bits in one symbol while 256-QAM can be used to send 9 bits in one symbol. As we move to higher digital modulation techniques, we can send more data per symbol (of course, this is not as easy as it sounds. Higher modulation techniques increase the UE complexity. It will also result in an increased bit error rate if the symbol is not estimated correctly at the receiver). Then there are frequency reuse concepts like MIMO, beamforming, etc.

To achieve the eMBB requirements, RF concepts like massive MIMO & beamforming are stressed more in 5G. I would like to discuss these concepts but I thought it's better to first discuss the basic MIMO before we move to 5G specific features.


MIMO
MIMO is short for multiple in, multiple out. The "In" and "out" always refer to the transmission channel. MIMO is a method for increasing the capacity of a radio link using multiple transmission and receiving antennas by exploiting multipath propagation. MIMO is fundamentally different from smart antenna techniques developed to enhance the performance of a single data signal, such as beamforming and diversity.

If someone asks the question, "How communication happens between a transmitter and a receiver?", one will typically say something like below that there is a transmitter with antenna sending the signal. The signal transmits through the channel and there is a receiver that the same frequency, receives and then decodes the signal.

Suppose, consider a case where there are multiple antennas at the transmitter(say M) and multiple antennas at the receiver(say N). Each antenna at the transmitter is used to send different data blocks on the same spectrum. Will the receiver will be able to decode it? One could say that it may not. Since both antennas are using the same bandwidth spectrum, the interference can be high so that the receiver won't be able to decode the signals. But in MIMO, at the expense of additional hardware and computation complexity, we can increase the number of parallel data streams with the same available bandwidth. That means, with MIMO, we can actually increase the channel capacity.

A key feature of MIMO systems is the ability to turn multipath propagation, traditionally a pitfall of wireless transmission, into a benefit for the user. In such a MIMO system, there are M x N signal paths from the transmit antennas to the receive antennas, and the signals on these paths are not identical. The signal component at each antenna at the receiver includes the signal component from all transmit antennas. Since the MxN multi-paths are different, the channel fading seen at each of the receiver antenna too is different. The signal at the receiver can be unscrambled only when it has good transmit diversity (due to man-made objects or natural obstacles). This diversity in the arrived signal makes it possible for the DSP to unscramble the signal, decode the transmitted data blocks and then makes it into a parallel stream. In some cases, the receiver can send the channel estimation info to the transmitter to adjust the transmission. Such kind of system is called a closed-loop MIMO system.

We should then ask the question - for a MIMO system with M transmit antennas and N receive antennas, how many such parallel data streams are possible? the answer is clear - it is the minimum of (M, N). So, at maximum, under ideal conditions (with a rich multipath environment), it is possible to have min(M, N) times the channel capacity of the SISO system.

Is it always possible to have channel capacity multiplied by min(M, N)? The answer is NO. For MIMO to work, it should have multiple data streams that are uncorrelated in nature. In the case of the MxN system, if there exists LOS between transmitter and receiver, then the no of orthogonal data streams that can be transmitted will come down to 1 as the paths between tx and rx antennas are almost same i.e. the correlation is high. Basically, as the multipath between Tx & Rx decreases, the no of uncorrelated data streams also decreases. The beauty of MIMO is that we are using the naturally available multipath condition(which is considered as a pitfall) to our advantage.

I tried to avoid mathematics to explain the MIMO concept. If you really want to know how exactly it works with mathematics, Please refer to the below IEEE document & chapter 5 in the below book.

Reference: From theory to practice: an overview of MIMO space-time coded wireless systems
Book: https://books.google.co.in/books?id=G5C5ii8O_y0C&printsec=frontcover#v=onepage&q&f=false

5G NR OFDM numerology & frame structure

Hello all,

In this post I would like to discuss about the OFDM numerology implemented in 5G NR and the corresponding frame structure.

For those of you who are not aware, I am laying out an overview of OFDM implemented in 4G as below. If you are familiar with it, you can skip to next section.

OFDM in 4G:

• OFDM is a form of frequency division multiplexing technique that uses a large number of narrow band closely spaced sub-carriers to carry data.
• What is sub-carrier?
• it is basically a sinc shaped waveform. A short pulse in time domain is equivalent sinc shaped waveform in frequency domain. It is better understood with the below diagram.
• The subcarrier spacing equals the inverse of the symbol period. => Subcarrier Spacing = 1/T. 

• In LTE, the sub-carrier spacing is chosen as static - which is 15kHz. How they arrived at 15kHz is kind of tricky. It is based on the coherence bandwidth & coherence time of the channel. The goal is to effectively combat frequency selective fast fading. 
• Coherence bandwidth is a statistical measurement of the range of frequencies over which the channel can be considered "flat“.
• Coherence time is the time duration over which the channel impulse response is considered to be not varying.
• So, basically we need a waveform which has a low sub-carrier spacing over which the fading can be considered as flat and at the same time the symbol duration should be less than coherence time so that only slow fading occurs. For 4G frequencies, after a lot of simulations, they arrived at 15kHz.
• In OFDM, these sub-carrier are tightly placed together so that the zero crossings of one carrier will coincide with the highs of adjacent carriers. That's how they maintain orthogonality. 
• OFDM signal generation. The snippet has been taken from a QCOM paper. 

OFDM numerology in 5G:

Similar to 4G, 5G too uses OFDM. But 5G supports multiple subcarrier spacings. Unlike 4G, 5G is expected to support wide bandwidths (upto 400MHz). If we stick to the same 15kHz spacing, it needs an IFFT which should support 20k+ (check the diagram above for OFDM signal generation). This places a lot of computing burden and the system too becomes complex. There are also applications which need comparatively less symbol duration (low latency applications like real time gaming etc). So, a new parameter μ is introduced as part of 5G OFDM numerology. Based on this parameter, subcarrier spacing changes and accordingly the symbol duration. As the subcarrier spacing increases, symbol duration decreases and vice versa.

So, as μ ranges from 0 to 4, subcarrier spacing can change from 15kHz, 30kHz, 60kHz, 120kHz & 240kHz. Only the 60kHz subcarrier supports extended cyclic prefix.

There is one more reason why this increased sub carrier spacing is needed. As the mobile is moving with respect to transmitter, some Doppler frequency shift is seen at gNB. This Doppler frequency shift increases with frequency of operation. For 5G frequencies (say for FR2 which ranges from 24GHz to 52.6 GHz), the Doppler frequency shift is comparatively high. Since OFDM is prone to such frequency errors, measures are needed to combat this increased Doppler frequency shift. The increased sub carrier spacing helps here.

Also, when I was reading this, I asked this question to myself but couldn't get a proper answer. What could be the reason for enabling extended CP only for the subcarrier spacing 60kHz? If anyone of you know, please do let me know in comments.

Frame structure:
From the above sinc pulse, you can observe that the subcarrier spacing has relation to symbol duration. Basically, as the subcarrier spacing increases, symbol duration decreases. So, the OFDM numerology has dependence on frame structure. Here is an interesting diagram I found from a keysight whitepaper.

As you can see, symbol duration decreases with increase in subcarrier spacing. This results in more number of symbols with less duration in one subframe. the lower TTI helps lower latency requirement.

• One radio frame has a duration of 10 ms and is divided into 10 subframes with the duration of 1 ms each.
• 1 Slot = 14 OFDM symbols (normal CP, otherwise 12 symbols). Forget about the slot in LTE literature. The Slot in 5G means different than that of 4G. 
• Concept of Mini slot: 
• The motivation for a mini-slot is to support URLLC traffic.
• Mini-slots have the duration of 2, 4, or 7 OFDM symbols and can start at arbitrary symbols relative to the symbol 0 of each sub-frame.
• Slots and Mini-slots are the basic scheduling unit. However, 5G NR also supports scheduling time of a partial slot - the How part, we'll discuss later.
• And then there are self-contained slots to support both DL & UL in one slot. 

Will add more details in future posts. 

5G - an introduction

Hello all,

I recently started learning about 5G and would like to share my understandings as I continue to learn new concepts. This is my first blog post. So, please don't mind if there are some spelling/grammatical mistakes. Please do let me know in comments if you have a question/comments on my understanding.

What is 5G?

First we should ask the question "what is 5G?". As you may be aware, the International Telecommunication Union (ITU) lays out the requirements for every cellular generation. As for 4G, the requirement has been to support 1Gbps  data throughput. Till 4G, the focus of cellular requirements were tied to only data throughput. But for 5G time frame, new industries & use cases emerged which demand for higher data throughput as well as improvements in other parameters. On one side, 5G is expected to support high data throughput and on the other side, it is expected to support billions of delay tolerant & low data throughput IoT devices. And then there are applications which need the data transfer to be with ultra-reliability with ultra-low-latency during ultra-mobility. Any technology that can be said as 5G needs to support all these use cases. Please note that, we'll only be discussing 5G developments by 3GPP in this blog.

As we discussed, 5G needs to support versatile use cases that will be deployed in a number of ways. 3GPP divided these developments in three folds and calls it NR*. Following is the pictorial representation of these requirements taken from Qualcomm's whitepaper. If you want to go through it in full, visit this.

*Why the name NR? - I think, just like for 4G, 3GPP came up with the name LTE as way of evolution to reach 4G standard. For 5G, the term NR is introduced which is abbreviated as "New Radio".

Deployment options of 5G:
Like the earlier cellular generations, 5G cant be deployed just like that considering its complexity and cost involved. So, 3GPP provided two deployment options for 5G. They are called NSA (non-standalone mode) and SA(standalone mode). In NSA mode, LTE/NR acts as an anchor carrier for dual connectivity mode and connects to EPC or 5G core network. In SA mode,  NR is the only carrier and 5GC/EPC as the core network. Operator are expected to first start with NSA mode and then slowly move to SA mode .Each deployment type has different options which operators are free to choose from.

That is it for today for introduction. Will try to add new articles soon.