00:00Hi, in the last session, we talked about 12 connectivity and we seen that how it can improve
00:06the user throughput and how it can improve the user experience for better throughput
00:11to fast load balancing and improved mobility robustness.
00:15Now in this session, we will explore the concept of flexible numerology in 5G, we will understand
00:22that why it is important and we will see that how it works within the network.
00:27Let's begin by looking at the LTE frame structure and then we will compare it with the 5G NR
00:33frame structure to understand the differences and improvements.
00:37Similar to LTE, the NR frame in 5G is also 10ms long.
00:43So this frame length is common for both the technologies.
00:46Today, LTE supports carrier bandwidth of up to 20MHz and it uses a fixed OFDM numerology
00:55with consistent parameters across the network.
00:58LTE employs a 15kHz spacing between OFDM subcarriers and this is standard across the whole LTE network.
01:06LTE frames are divided into two slots.
01:09Each slot has 0.5ms and each slot contains 6 or 7 OFDM symbols.
01:16So in LTE, the symbol duration is approximately 66us which is same across all LTE deployments.
01:24Now when designing a new era interface like 5G, it's important to carefully select an
01:30appropriate symbol duration to optimize the performance.
01:33But before that, let's understand two important concepts that affect symbol duration.
01:39These concepts are delay spread and coherence time.
01:43Both of them are important concepts in wireless communications.
01:47Delay spread refers to the time difference between when the first and last significant
01:52signal reflection arrives at the receiver due to multipath propagation.
01:57So if you send an impulse in the ear, then there is a concept called multipath effect.
02:03So when this impulse is transmitted, it takes various parts and reflections to reach at the
02:09receiver.
02:10And because they are reflecting from different surfaces and follow different paths, this one
02:15signal impulse reaches at the receiver in multiple pulses.
02:19And because of different paths, they arrive at the receiver with slightly different times.
02:25This variation in the arrival times is named as delay spread which is a key factor in wireless
02:31communication.
02:32Larger cells usually experience higher delay spread.
02:36So a delay spread is the gap of duration between the first receiving impulse and last receiving
02:41pulse.
02:42Now let's discuss coherence time.
02:45So coherence time refers to the time period during which the characteristics of a communication
02:50channel remains relatively constant.
02:53It is the duration over which the channel response is stable.
02:57Means it does not change significantly.
03:00Higher frequency bands and faster moving user equipments or UEs typically result in shorter
03:06coherence time due to rapid change in the channel conditions.
03:10Now the concept is to ensure the reliable communication, the symbol duration should be
03:15shorter than the coherence time and significantly longer than the delay spread.
03:20And from the 5G spectrum, we've seen that 5G supports multiple frequency bands, including
03:26multimeter bands.
03:28Now 5G is expected to support a wide range of frequency bands and various cell sizes and
03:35fast moving user equipment.
03:37Now because of this diversity, a single symbol duration is insufficient to meet all these
03:42needs of different scenarios.
03:44So we are needing additional flexibility to select the symbol duration.
03:48And to address this, 5G NR introduces flexible numerology.
03:53Now that allows various symbol durations to match the different use cases and network
03:58conditions.
04:00Like LTE, 5G NR also have a 10 millisecond frame structure.
04:04But now it has added more flexibility in the subcarrier spacing.
04:08Means unlike LTE, 5G NR supports a range of subcarrier spacing.
04:14That is from 15 kHz to 240 kHz.
04:17So that provides more adaptability for the different scenarios.
04:22Although this picture here only shows up to 120 kHz.
04:26But please remember that NR can support up to 240 kHz subcarrier spacing for even more
04:32flexibility.
04:33Each NR subframe is still limited to 14 OFDM symbols.
04:37Just like we have LTE.
04:39But 5G can adjust the subcarrier spacing.
04:43So the number of symbols per frame is fixed at 14.
04:46But the duration of each slot decreases as the subcarrier spacing increases.
04:51So it allows faster data transmission.
04:54And hence, larger subcarrier spacing helps in reducing latency as well.
04:59Because it allows data to be transmitted in a shorter slot duration.
05:04Which is crucial for time sensitive applications.
05:07This kind of flexibility brings a better configuration for different types of deployments.
05:12According to their most suitable properties.
05:15For example, outdoor microcells operating on the lower frequency bands like 700 MHz can
05:22use 15 kHz subcarrier spacing.
05:24Which is better suited for the environments with high multipath interference.
05:29Mid-sized indoor deployments can use 30 kHz or 60 kHz subcarrier spacing.
05:35Due to their smaller cell size and the need of efficient spectrum uses.
05:40Although mmWave deployments which offer extremely high bandwidths.
05:45They require careful selection of subcarrier spacing.
05:49So if you use smaller subcarrier spacing in these high frequency bands.
05:53Then the FFT computation or Fast Fourier Transform computations can be more complex
05:58and challenging.
06:00In addition, they also have smaller cell sizes.
06:03So a higher subcarrier spacing suits them better.
06:07Okay, so that's it for today.
06:08I believe now you have some basic understanding on 5G NR, flexible numerology and how to use
06:14it in network for better performance and for better user experience.
06:18This was also one of the major updates in 5G NR network.
06:22Compared to the legacy 4G ALT network.
06:25In the next session, we will be talking about mini slots, preemptive scheduling and self-contained
06:31slots in 5G.
06:32So stay tuned for the updates.
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