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CP (Cyclic Prefix) Length in 4G/5G OFDM?
The reasons for defining two cyclic-prefix lengths for LTE are two fold: A longer cyclic prefix, although less efficient from a cyclic-prefix-overhead point of view, may be beneficial in specific environments with extensive delay spread, for example in very large cells. It is important to have in miRead more
The reasons for defining two cyclic-prefix lengths for LTE are two fold:
- A longer cyclic prefix, although less efficient from a cyclic-prefix-overhead point of view, may be beneficial in specific environments with extensive delay spread, for example in very large cells. It is important to have in mind, though, that a longer cyclic prefix is not necessarily beneficial in the case of large cells, even if the delay spread is very extensive in such cases. If, in large cells, link performance is limited by noise rather than by signal corruption due to residual time dispersion not covered by the cyclic prefix, the additional robustness to radio-channel time dispersion, due to the use of a longer cyclic prefix, may not justify the corresponding additional energy overhead of
- The cyclic prefix should not only cover the main part of the actual channel time dispersion, but also the timing difference between the transmissions received from the cells involved in the MBSFN transmission.
See lessa longer cyclic prefix.
CP (Cyclic Prefix) Length in 4G/5G OFDM?
The cyclic-prefix length should cover the maximum length of the time dispersion expected to be experienced. However, increasing the length of the cyclic prefix, without a corresponding reduction in the subcarrier spacing Δf, implies an additional overhead in terms of power as well as bandwidth. In pRead more
The cyclic-prefix length should cover the maximum length of the time dispersion
expected to be experienced. However, increasing the length of the cyclic prefix,
without a corresponding reduction in the subcarrier spacing Δf, implies an additional overhead in terms of power as well as bandwidth. In particular, the power loss implies that, as the cell size grows and the system performance becomes more power limited, there is a trade-off between the loss in power due to the cyclic prefix and the signal corruption due to time dispersion not covered by the cyclic prefix. This implies that, although the amount of time dispersion typically increases with the cell size, beyond a certain cell size there is often no reason to increase the cyclic prefix further as the corresponding power loss would have a larger negative impact, compared to the signal corruption due to the residual time dispersion not covered by the cyclic prefix .
The different cyclic-prefix lengths can then be used in different transmission scenarios:
- Shorter cyclic prefix in small-cell environments to minimize the cyclic-prefix overhead.
- Longer cyclic prefix in environments with extreme time dispersion.
See lessWhat is RB/PRB means in LTE/NR?
In 5G NR the term RB (Resource Block) was introduced to mean 12 consecutive subcarriers in frequency domain. Where in LTE a resource block was defined as 12 subcarriers in the frequency domain and 6 or 7 symbols (depending on CP length) in time domain. Motivation for defining RB as a frequency domaiRead more
In 5G NR the term RB (Resource Block) was introduced to mean 12 consecutive subcarriers in frequency domain. Where in LTE a resource block was defined as 12 subcarriers in the frequency domain and 6 or 7 symbols (depending on CP length) in time domain.
Motivation for defining RB as a frequency domain allocation only is to have flexible allocation of the radio resources on OFDM symbol level. When we say a UE is allocated with 5 RBs of PDSCH resources we only mean 60 subcarriers in frequency, we don’t know it’s time allocation, it can be 5 OFDM symbols or 13 OFDM symbols.
PRB (Physical Resource Block) is defined with the introduction of bandwidth parts. PRBs follow a numbering index on the overall resource grid which depends on the numerology and relative to the start of the bandwidth part. When we say 10 PRBs we mean actual physical allocation inside a BWP.
VRB (Virtual Resource Block) is defined as contiguous resource allocation. VRB doesn’t mean where in BWP/frequency data is allocated. These virtual resource blocks can be mapped to PRBs in contiguous or distributed manner.
See lessMotivation to use wider subcarrier spacing in 5G NR
According to my understanding there are two main reasons to use wider subcarrier spacing in 5G NR: Complexity 5G uses a nominal channel bandwidth in FR1 of 100MHz and in FR2 of 400MHz. Larger channel bandwidth means higher number of subcarriers. For example in 100 MHz bandwidth with 15 kHz subcarrieRead more
According to my understanding there are two main reasons to use wider subcarrier spacing in 5G NR:
- Complexity
- Lower latency
See less5G uses a nominal channel bandwidth in FR1 of 100MHz and in FR2 of 400MHz. Larger channel bandwidth means higher number of subcarriers. For example in 100 MHz bandwidth with 15 kHz subcarrier spacing there will be close to 6500 subcarriers. For 6500 subcarriers we need FFT/IFFT size of 8192 which increase complexity of the system.
Therefore widening the subcarrier spacing would allow us to use a larger bandwidth by reducing the absolute number of subcarriers and therefore confine the FFT size with a feasible limit.
Wider subcarrier spacing allows shorter symbol duration to cope up with the requirements for lower latency in URLLC. Also, with wider subcarrier spacing and shorter symbol duration the ability to analyze the radio signal affected by the doppler shift, carrier frequency offset due to mobility of transmitter/receiver increase. It is easy to mitigate these effects in higher subcarrier spacings.
RLC transmission modes in 5G
Depending on the reliability and latency required for the data transported in the logical channels, the RLC channel can be configured with 3 different transmission modes: Transparent Mode (TM): In this mode, the RLC protocol layer forwards the data packets from PDCP without attaching any header. TheRead more
Depending on the reliability and latency required for the data transported in the logical channels, the RLC channel can be configured with 3 different transmission modes:
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Each of these modes can both transmit and receive data. In TM and UM, a separate entity is used for transmission and reception, but in AM a single RLC entity performs both the transmission and reception.
Example:
See lessTM Mode: SRB0, paging, broadcast system information
AM Mode: Error correcting through ARQ, Duplicate detection
UM Mode: DRBs
Difference between Search space and Coreset?
CORESET = COntrol REsource SET. As the name says it is a set of resource for the control channel. It means the physical resources configured for the control channel like: number of PRBs, number of symbols. Coreset is the set of time-frequency resources allocated for the control channel ( PDCCH). ForRead more
CORESET = COntrol REsource SET. As the name says it is a set of resource for the control channel. It means the physical resources configured for the control channel like: number of PRBs, number of symbols.
Coreset is the set of time-frequency resources allocated for the control channel ( PDCCH). For example: OFDM symbols 1 & 2, PRBs from 12 to 84 is a control region of 72 PRBs * 2 symbols.
SEARCH SPACE: As the name says it is the space to search for. Area where UE will be searching for PDCCH. Searchspace has to be subset of Coreset since that is the area allocated for control information. Search space is defined as starting REG, starting symbol, aggregation level and the type of DCIs it will carry.
Single CORESET can have multiple search space. These Search Space can overlap with each other to efficiently utilize the CORESET resources.
Since CORESET is large and it is difficult for the UEs to search in entire CORESET region, so there is search space to help UEs find out their DCIs with minimum number of blind decoding.
See lessWhat is the difference between LDPC and Polar
LDPC is used for data channels and Polar is used for control channels
LDPC is used for data channels and Polar is used for control channels
See less