This is my 1st blog where i will be sharing some basic differences between 5G NR and LTE. There can be multiple other differences also but these are some of the selected ones, which i wanted to share with everyone. I might add few more later. Also, i will try to elaborate more about individual difference with images and figures in my subsequent blogs, where i will be elaborating more about individual topics.
I hope that i have assembled good enough differences for basic understanding, including some protocol level differences too. Please go ahead, read them and do share your review comments or suggestions.
Happy reading !!
Always – on signal support
LTE (Long Term Evolution): was designed to support always on signals be it any condition or situation, leading to a lot of wastage of resources and required continuous evaluation. For Example: System information broadcast, signals for detection of base station, reference signals for channel estimation etc.
NR (New Radio): Always-on transmissions are minimized in order to enable higher network energy performance and higher achievable data rates, causing reduced interference to other cells.
LTE: Just introduced support for licensed spectra at 3.5 GHz and unlicensed spectra at 5 GHz.
NR: It’s first release supports licensed-spectrum operation from below 1 GHz up to 52.6 GHz and planning is ongoing for extension to unlicensed spectra.
Flexibility support for time/frequency resources
LTE: has majorly supported fix timing/frequency for transmission in certain situations. For Ex: Uplink Synchronous HARQ protocol, where a retransmission occurs at a fixed point in time after the initial transmission.
NR: believes in configurable time/frequency resources. It avoids having transmission on fixed resources.
LTE: dependent on cell-specific reference signals for channel estimation, which are always transmitted.
NR: For channel estimation, NR doesn’t include cell-specific reference signals, instead relies on user specific demodulation reference signals, which are not transmitted unless there is data to transmit, thereby improving energy performance of the network.
Dynamic uplink downlink allocation
LTE: Uplink and downlink allocation does not change over time. Even though a later feature called eIMTA allowed some dynamics in UL DL allocation.
NR: Supports dynamic TDD, which means dynamic assignment and reassignment of time domain resources between UL and DL directions.
Device and Network Processing time
LTE: Better than 3G but not enough considering future requirements under highly dense environment for certain applications
NR: Processing times are much shorter in NR for both device and network. For Example: A device must respond with an HARQ ACK within a slot or even lesser (depending on device capabilities) after receiving a downlink data transmission.
Low Latency Support
LTE: Requires MAC and RLC layers to know the amount to data to transmit before any processing takes place, which makes it difficult to support very low latency.
NR: This is one of the most important characteristics of NR. Let me explain this support by giving 2 examples below:
- Header structures in MAC and RLC have been chosen to enable processing without knowing the amount of data to transmit, which is especially important in the UL direction as the device may only have a few OFDM symbols after receiving the UL grant until the transmission should take place.
- By locating the reference signals and downlink control signaling carrying scheduling information at the beginning of transmission and not using time domain interleaving across OFDM symbols, a device can start processing the received data immediately without prior buffering, thereby minimizing the decoding delay.
Error Correcting Codes
LTE: uses Turbo coding for data, which are the best solution at the lower code rate (For example: 1/6, 1/3, 1/2)
NR: uses LDPC (Low density Parity check) coding in order to support higher data rate as it offers lower complexity at higher coding rates as compared to LTE. They perform better at higher code rates (For Example: 3/4, 5/6, 7/8)
Time Frequency Structure of Downlink control channels
LTE: Less flexible as it needs full carrier bandwidth
NR: Has more flexible time frequency structure of downlink control channels where PDCCH’s are transmitted in one or more control resource sets (CORESETS) which can be configured to occupy only part of the carrier bandwidth.
Service Data Application Protocol layer
LTE: Not present
NR: Introduced to handle new Quality of service requirements when connecting to 5G core network. SDAP is responsible for mapping QoS bearers to radio bearers according to their quality-of-service requirements.
LTE: Supported only 2 states: Idle and Connected
NR: Supports a 3rd state also called the RRC_INACTIVE, which is introduced to reduce the signaling load and the associated delay in moving from idle-to-active transition. In this state, RRC context is kept in both the device and the gNB.
In Sequence Delivery of RLC Packets
LTE: Supports reordering and in sequence delivery of RLC PDUs to higher protocol layers, leading to more delays.
NR: doesn’t support in-sequence delivery of RLC PDUs in order to reduce the associated delay incurred by the reordering mechanism which might be unfavorable for services that require very low latency. By doing this, RLC reduces the overall latency as packets do not have to wait for retransmission of an earlier missing packet before it is delivered to higher layers but can be forwarded immediately.
Concatenation of RLC PDUs
LTE: supported it to disallow RLC PDUs to be assembled in advance
NR: Removed this from RLC protocol to support assembly of RLC PDUs in advance, prior to receiving the Uplink Scheduling Grant.
Location of MAC Header
LTE: All the MAC Headers corresponding to certain RLC PDUs are present in the beginning of the MAC PDU.
NR: MAC Headers are distributed across the MAC PDU such that the MAC header related to a certain RLC PDU is located immediately next to RLC PDU, which is motivated by efficient low latency processing. With the structure in NR, MAC PDU can be assembled “on the fly” since there is no need to assemble the full MAC PDU before the header fields can be computed, leading to reduction in processing time and hence the overall latency.
HARQ Retransmission Unit
LTE: sends whole transport block in case of retransmission even if there is issue in only a small part of the transport block, which is very inefficient.
NR: supports HARQ retransmissions at a much finer granularity called code-block group, where only a small part of big transport block needs to be retransmitted.
Number of HARQ processes
LTE: Max supported was 8 for FDD and up to 15 processes for TDD, depending on the UL- DL configuration.
NR: Max supported is 16
HARQ in Uplink
LTE: It was synchronous HARQ as the timing of retransmission was fixed depending on the max number of HARQ Processes. There was no associated HARQ process number as in Downlink HARQ.
NR: It is asynchronous HARQ in both UL and DL as gNB explicitly signals the HARQ process number to be used by the UE, as part of the downlink control information. It was required to support dynamic TDD where there is no fixed UL/DL allocation.
LTE: Used a concept of two synchronization signals (PSS & SSS) with a fixed format which enabled UEs to find a cell.
NR: Uses a concept of Synchronization signal block (SSB), spanning 20 resource blocks and consisting of PSS, SSS & PBCH. The timing of the SSB block can be set by network operator.
Location of synchronization signals
LTE: Located in the center of transmission bandwidth and are transmitted once every 5ms.
NR: Signals are not fixed but located in a synchronization raster. When found, UE is informed on where in the frequency domain it is located. SS Block by default is transmitted once every 20ms but can be configured to be between 5 and 160ms.
Beam Forming of Synchronization signals
LTE: Not supported
Beam forming of Control channels
LTE: Not supported
NR: supported and requires a different reference signal design with each control channel having its own dedicated reference signal.
LTE: 2 different cyclic prefixes are defined, normal and extended where extended cyclic prefix was only used for specific environments with excessive delay spread, where performance was limited by time dispersion.
NR: Defines a normal cyclic prefix only, with an exception of 60 kHz subcarrier spacing where both are defined.
Subframe & Slot
LTE: with its single subcarrier spacing, number of slots in a subframe are always fixed. A frame is made up of 10 subframes each of 1ms, making the frame duration of 10ms. Each subframe carries 2 slots, so 20 slots makes a complete frame.
NR: Subframe is a numerology independent time reference while a slot is the typical dynamic scheduling unit. NR slot has the same structure as an LTE subframe with normal cyclic prefix for 15kHz subcarrier spacing, which is beneficial from a co-existence perspective.
LTE: 2 frames structures were used in LTE which were later expanded to three for supporting unlicensed spectra.
NR: Single frame structure can be used to operate in paired as well as unpaired spectra.
LTE: Uses two-dimensional resource blocks of 12 subcarriers in the frequency domain and 1 slot in the time domain, so transmission occupied 1 complete slot (at least in the original release)
NR: NR resource block is a one-dimensional entity spanning the frequency domain only, reason being the flexibility in time duration for different transmissions. NR supports multiple numerologies on the same carrier, so there are multiple resource sets of resource grids, one for each numerology.
LTE: For downlink signals, the DC subcarrier is not transmitted, but is counted in the number of subcarriers. For uplink, the DC subcarrier does not exist because the entire spectrum is shifted down in frequency by half the subcarrier spacing and is symmetric about DC. This is the subcarrier in the OFDM/OFDMA signal whose frequency would be equal to the RF Center frequency of the station. Generally, all devices have the DC coinciding with the center frequency.
NR: DC subcarrier is used as NR devices may not be centered around the carrier frequency. Each NR device may have its DC located at different locations in the carrier.
Max Supported Bandwidth
LTE: Maximum carrier bandwidth of 20 MHz
NR: designed to support very high bandwidths, up to 400 MHz for a single carrier.
LTE: There was fixed carrier spacing of 15kHz.
NR: Concept of numerology is created, keeping the base value of carrier spacing as 15 KHZ. Along with 15kHz, other supported values are 30, 60, 120 and 240 kHz to cater to different needs in different scenarios.
LTE: used normal MIMO and the maximum number of antennas in MIMO is 8 (DL) * 8(UL) using spatial multiplexing by UE Category 8
NR: Uses the concept of MIMO with an antenna array system using massive number of antennas, which can go up to 256(DL) * 32(UL).
Key Performance Indicators along with other differences:
- Peak Data Rate (With LTE-A): Downlink (1 Gbits/s), Uplink (.5 Gbits/s)
- Peak Spectral Efficiency: Downlink (30 bps/Hz) – with 8-layer spatial multiplexing, Uplink (15 bps/Hz) – with 4-layer spatial multiplexing
- Control Plane Latency: <100ms
- User Plane Latency: <10ms
- Mobility (With LTE-A): Device speeds up to 500 Km/h
- Max Supported Bandwidth: 20 MHz
- Waveform: CP-OFDM for DL, SC-FDMA for UL
- Maximum number of subcarriers: 1200
- Slot-Length: 7 symbols in 500us
- Peak Data Rate: Downlink (20 Gbits/s), Uplink (10 Gbits/s)
- Peak Spectral Efficiency: Downlink (30 bps/Hz), Uplink (15 bps/Hz)
- Control Plane Latency: <10ms
- User Plane Latency: <0.2ms for URLLC
- Mobility: Device speeds up to 500 Km/h
- Max Supported Bandwidth: 100 MHz in Frequency Range1 (400 MHz to 6 GHz) and up to 400 MHz in Frequency Range2(24.25 GHz to 52.6 GHz)
- Waveform: CP-OFDM for DL, CP-OFDM and DFT-s-OFDM for UL
- Maximum number of subcarriers: 3300
- Slot-Length: 14 symbols (duration depends on subcarrier spacing), 2,4 and 7 symbols for mini-slot
Below is the you tube link of a very basic and interesting 5G NR Webinar from a 5G expert from Ericsson (Mr. Erik Dahlman), one of the authors of the Book “5G NR _ the next generation wireless access technology”. Happy Learning!