Introduction:
3GPP LTE technology defines a data shared channel (PDSCH - Physical Downlink Shared Channel) to carry both users' data traffic and control signaling/messages that provides life line to User Equipments (UEs) for the day to day operations. The mantra (or telltale) here is to ensure optimum resource utilization for data traffic to increase cell throughputs. Better efficiency of scheduling with shared channel and conservation of vital (UE's) battery power is enabled with the use of PDCCH (Physical Downlink Control Channel).
In LTE, all UEs that expect any data (system information, random access response, paging, common control messaging and user specific data or control messages) on the DL has to monitor the PDCCH first. The PDCCH informs the UEs about the DL resource allocation information. The allocation information includes the number of Resource Blocks (RBs), Modulation and Coding Schemes (MCS), Multiple Input Multiple Output (MIMO) schemes and UL power control command with an indication for Channel Quality Index (CQI) report.
Since both the PDDCH and PDSCH share the available resources in every subframe (Transmit Time Interval -TTI of 1ms duration), the number of simultaneous users (a measure of capacity) who can be served in a cell will be limited by the availability of PDCCH resources. Looking the other way, PDSCH throughput is inversely proportional to the PDDCH size (or resources). That is, the smaller the resources reserved for PDCCH the larger the resources available for PDSCH which means higher throughput is achieved in the TTI. Another scenario is the case where PDCCH occupies larger resources due to the requirement of higher number of users to be served which means large capacity. Now, PDSCH is left with lesser resources to carry data leading to lower throughput. From the above discussion we see that LTE provides us with a handle to leverage either capacity or throughput every TTI as the scenario may warrant. The key to this kind of leverage is through the use of the PCFICH (Physical Control Format Indicator Channel). The PCFICH provides the information about the PDCCH resources (number of OFDMA - Orthogonal Frequency Division Multiple Access symbols) in the present TTI or subframe. Figure 1 provides a pictorial representation of the resource utilization in a generic subframe by PDSCH, PDCCH and PCFICH in LTE.
Figure 1: A generic subframe in LTE
Till now our discussions were limited to only time domain. When we shift our focus to frequency domain (i.e., subcarriers) yet another PDCCH constraint will be seen. That is the PDCCH resources are made up of OFDM symbols on the time domain and subcarriers in the frequency domain. The number of subcarriers available for communication is dependent on the available (deployed) bandwidth. LTE supports scalable bandwidth deployments of 1.4MHz, 3MHz, 5MHz, 10MHz, 15MHz and 20MHz with 6, 15, 25, 50, 75 and 100 RBs, respectively. To achieve the same capacity in two (2) different deployment bandwidths (say 5MHz and 10MHz), the PDCCH may have to span further OFDM symbols in lower bandwidth leading to more degradation in cell throughput. This leads to yet another dimension to the design of a radio network. LTE supports a variable number of OFDMA symbols (maximum 3 in case of higher bandwidth and 4 OFDMA symbols in case of less bandwidths like 1.4MHz deployment) to take care of all the scenarios discussed so far. Physically, PCFICH carries this information (number of OFDMA symbols that constitutes the PDCCH channel for the present subframe) to all users monitoring the DL. In summary, LTE UE needs to first decode the PCFICH to decode PDCCH and then listen to PDCCH for PDSCH resources and then decode the data on PDSCH.
PCFICH Details - What, where and how
PCFICH carries CFI (Control Format Indicator) that informs 4 values 1, 2, 3, or 4 suggesting that PDCCH occupies 1, 2, 3 or 4 OFDM symbols in the present subframe. Figure 2.below provides more information about PCFICH and what CFI carries.
PCFICH is mapped to 4 REGs (labeled p, q, r and s in Figure 2) which are equidistant and spread across the bandwidth in frequency domain. These four REG locations are a function of PCI and the deployed bandwidth. In time domain, it is mapped to the first OFDMA symbol of the subframe (see Figure 1). LTE defines a REG (Resource Element Group) as 4 useful REs (Resource elements) or modulation symbols grouped together. But with 2 MIMO Reference signals associated, 6 REs are grouped into a REG in the first OFDMA symbol of the subframe (Figure 2 shows sample position of Reference signals marked as x).
Fig 2: PCFICH - Where and What
The Math of PCFICH has
4REGs = 4 time 4 useful REs or 16 modulation symbols
With QPSK modulation PCFICH always carry 32 bit of information which are mapped to CFI values 1 to 4 as shown in Figure 2.
PCI Planning
In PCI planning we usually consider the Primary Sync Signal (PSS) and Secondary Sync Signal (SSS) where every cell has a unique PCI. A PCI is a combination of one of the three unique (orthogonal) PSS sequences and one of 168 cell group identity (or SSS) sequences that make a range of 0 to 503 unique identities. To minimize interference, the thumb rule is to ensure that neighboring cells shall not transmit the same PSS. This ensures that sync signal and reference signals do not interfere with each other.
PCFICH issues with PCI planning
Now let's focus on PCFICH issues while planning the PCIs. A careful observation of LTE standards shows that PCFICH location is a function of two variables – the PCI and the bandwidth deployed. In popular deployments of 10Mz, we see that first OFDM symbol of every subframe has 100 REGs (@ of 2REGs per RB). Likewise we can have 12, 30, 50, 150 and 200 REGs for bandwidths 1.4MHz, 3MHz, 5MHz, 15MHz and 20MHz respectively. Also PCFICH spans four (4) locations across the bandwidth and are equidistant. But PCIs are 504 (five hundred and four), so cells with different PCI are bound to have overlapping PCFICH locations. Hence we see two scenarios arising, first a set of PCIs for a given bandwidth will have exactly same PCFICH positions and secondly a group of cells that have one or more overlapping PCFICH locations. If the PCIs within a set happens to be assigned for neighbor cells, the UEs at cell edges will experience interference while decoding PCFICH. Any problems in reading PCFICH leads to a situation where UE is not reading either or both PDCCH and PDSCH thereby reducing the cell edge throughputs.
First let us see the scenario of neighboring cells with PCIs from a set that has the same PCFICH positions are very same. Figure 3 shows one such case where three 10MHz neighbor cells with PCIs 0, 25 and 50 (from the same set) deployed. PCI planning with synchronization and reference signals point of view looks perfect. i.e., PCIs 0, 25 and 50 have Primary Synchronization signals 0, 1 and 2 respectively. Also the DL cell specific reference signal positions do not overlap, minimizing the interference. Hence we see that interference is better managed. But there's an element of surprise when you analyze interference due to PCFICH.
With 100 REGs in a 10MHz deployment, only 25 (=100/4) unique PCFICH locations (with four equidistant REGs) are possible. Here we see cells with PCIs separated by a distance of 25 has the same PCFICH positions (REGs 0, 25, 50 and 75). This will drastically reduce the cell edge throughputs.
Figure 3: Scenario showing neighbor cells have same PCFICH location.
Now let us look the other scenario where neighbor cell have one or more PCFICH location in common as seen in figure 4. Here we consider three 5MHz cells having few PCFICH positions overlapping being deployed. Like in the previous case with 5MHz deployed, a total of 25 * 2 = 50 REG location are possible within the bandwidth. With 4 equidistant locations we can have only 12 unique PCFICH positions. Figure 4 shows three neighbor cells having PCIs 0, 12 and 25. When we consider the cell with PCI 0 and 12 or 12 and 25, we see that two REG positions 72 and 222 are overlapping that decreases the probability of decoding PCFICH, while PCIs 0 and 25 have all the four positions in common making it even worse to decode PCFICH. When the PCIs have a difference of 13, we see other two REG positions 0 and 150 overlapping. Hence it is not advisable to have PCIs with multiples of 12/13 or 25 in neighborhood.
A summary of above discussion can be seen in Table 1 below. Extra care must be taken while deploying neighbor cells with PCI distances such that those mentioned in the table are not used leading to better cell edge throughputs. Figure 5 below shows an example scenario that abides by PCFICH plan to ensure a better cell throughput.
Figure 4: Scenario showing neighbor cells with few (1 or more) overlapping PCFICH locations
Table 1: PCFICH Separation distance for various LTE deployments
Figure 5: Scenario of PCI plan with PCFICH check
Conclusion
Maximum throughputs can be achieved by careful cell planning considering the PCFICH locations additionally. The cell planning should see that neighboring cells in the cluster or across clusters do not have a distances mentioned in Table 1. PCFICH positions have a randomized interference due to individual REGs of cells overlapping with other cell individual REGs.
3GPP LTE technology defines a data shared channel (PDSCH - Physical Downlink Shared Channel) to carry both users' data traffic and control signaling/messages that provides life line to User Equipments (UEs) for the day to day operations. The mantra (or telltale) here is to ensure optimum resource utilization for data traffic to increase cell throughputs. Better efficiency of scheduling with shared channel and conservation of vital (UE's) battery power is enabled with the use of PDCCH (Physical Downlink Control Channel).
In LTE, all UEs that expect any data (system information, random access response, paging, common control messaging and user specific data or control messages) on the DL has to monitor the PDCCH first. The PDCCH informs the UEs about the DL resource allocation information. The allocation information includes the number of Resource Blocks (RBs), Modulation and Coding Schemes (MCS), Multiple Input Multiple Output (MIMO) schemes and UL power control command with an indication for Channel Quality Index (CQI) report.
Since both the PDDCH and PDSCH share the available resources in every subframe (Transmit Time Interval -TTI of 1ms duration), the number of simultaneous users (a measure of capacity) who can be served in a cell will be limited by the availability of PDCCH resources. Looking the other way, PDSCH throughput is inversely proportional to the PDDCH size (or resources). That is, the smaller the resources reserved for PDCCH the larger the resources available for PDSCH which means higher throughput is achieved in the TTI. Another scenario is the case where PDCCH occupies larger resources due to the requirement of higher number of users to be served which means large capacity. Now, PDSCH is left with lesser resources to carry data leading to lower throughput. From the above discussion we see that LTE provides us with a handle to leverage either capacity or throughput every TTI as the scenario may warrant. The key to this kind of leverage is through the use of the PCFICH (Physical Control Format Indicator Channel). The PCFICH provides the information about the PDCCH resources (number of OFDMA - Orthogonal Frequency Division Multiple Access symbols) in the present TTI or subframe. Figure 1 provides a pictorial representation of the resource utilization in a generic subframe by PDSCH, PDCCH and PCFICH in LTE.
Figure 1: A generic subframe in LTE
Till now our discussions were limited to only time domain. When we shift our focus to frequency domain (i.e., subcarriers) yet another PDCCH constraint will be seen. That is the PDCCH resources are made up of OFDM symbols on the time domain and subcarriers in the frequency domain. The number of subcarriers available for communication is dependent on the available (deployed) bandwidth. LTE supports scalable bandwidth deployments of 1.4MHz, 3MHz, 5MHz, 10MHz, 15MHz and 20MHz with 6, 15, 25, 50, 75 and 100 RBs, respectively. To achieve the same capacity in two (2) different deployment bandwidths (say 5MHz and 10MHz), the PDCCH may have to span further OFDM symbols in lower bandwidth leading to more degradation in cell throughput. This leads to yet another dimension to the design of a radio network. LTE supports a variable number of OFDMA symbols (maximum 3 in case of higher bandwidth and 4 OFDMA symbols in case of less bandwidths like 1.4MHz deployment) to take care of all the scenarios discussed so far. Physically, PCFICH carries this information (number of OFDMA symbols that constitutes the PDCCH channel for the present subframe) to all users monitoring the DL. In summary, LTE UE needs to first decode the PCFICH to decode PDCCH and then listen to PDCCH for PDSCH resources and then decode the data on PDSCH.
PCFICH Details - What, where and how
PCFICH carries CFI (Control Format Indicator) that informs 4 values 1, 2, 3, or 4 suggesting that PDCCH occupies 1, 2, 3 or 4 OFDM symbols in the present subframe. Figure 2.below provides more information about PCFICH and what CFI carries.
PCFICH is mapped to 4 REGs (labeled p, q, r and s in Figure 2) which are equidistant and spread across the bandwidth in frequency domain. These four REG locations are a function of PCI and the deployed bandwidth. In time domain, it is mapped to the first OFDMA symbol of the subframe (see Figure 1). LTE defines a REG (Resource Element Group) as 4 useful REs (Resource elements) or modulation symbols grouped together. But with 2 MIMO Reference signals associated, 6 REs are grouped into a REG in the first OFDMA symbol of the subframe (Figure 2 shows sample position of Reference signals marked as x).
Fig 2: PCFICH - Where and What
The Math of PCFICH has
4REGs = 4 time 4 useful REs or 16 modulation symbols
With QPSK modulation PCFICH always carry 32 bit of information which are mapped to CFI values 1 to 4 as shown in Figure 2.
PCI Planning
In PCI planning we usually consider the Primary Sync Signal (PSS) and Secondary Sync Signal (SSS) where every cell has a unique PCI. A PCI is a combination of one of the three unique (orthogonal) PSS sequences and one of 168 cell group identity (or SSS) sequences that make a range of 0 to 503 unique identities. To minimize interference, the thumb rule is to ensure that neighboring cells shall not transmit the same PSS. This ensures that sync signal and reference signals do not interfere with each other.
PCFICH issues with PCI planning
Now let's focus on PCFICH issues while planning the PCIs. A careful observation of LTE standards shows that PCFICH location is a function of two variables – the PCI and the bandwidth deployed. In popular deployments of 10Mz, we see that first OFDM symbol of every subframe has 100 REGs (@ of 2REGs per RB). Likewise we can have 12, 30, 50, 150 and 200 REGs for bandwidths 1.4MHz, 3MHz, 5MHz, 15MHz and 20MHz respectively. Also PCFICH spans four (4) locations across the bandwidth and are equidistant. But PCIs are 504 (five hundred and four), so cells with different PCI are bound to have overlapping PCFICH locations. Hence we see two scenarios arising, first a set of PCIs for a given bandwidth will have exactly same PCFICH positions and secondly a group of cells that have one or more overlapping PCFICH locations. If the PCIs within a set happens to be assigned for neighbor cells, the UEs at cell edges will experience interference while decoding PCFICH. Any problems in reading PCFICH leads to a situation where UE is not reading either or both PDCCH and PDSCH thereby reducing the cell edge throughputs.
First let us see the scenario of neighboring cells with PCIs from a set that has the same PCFICH positions are very same. Figure 3 shows one such case where three 10MHz neighbor cells with PCIs 0, 25 and 50 (from the same set) deployed. PCI planning with synchronization and reference signals point of view looks perfect. i.e., PCIs 0, 25 and 50 have Primary Synchronization signals 0, 1 and 2 respectively. Also the DL cell specific reference signal positions do not overlap, minimizing the interference. Hence we see that interference is better managed. But there's an element of surprise when you analyze interference due to PCFICH.
With 100 REGs in a 10MHz deployment, only 25 (=100/4) unique PCFICH locations (with four equidistant REGs) are possible. Here we see cells with PCIs separated by a distance of 25 has the same PCFICH positions (REGs 0, 25, 50 and 75). This will drastically reduce the cell edge throughputs.
Figure 3: Scenario showing neighbor cells have same PCFICH location.
Now let us look the other scenario where neighbor cell have one or more PCFICH location in common as seen in figure 4. Here we consider three 5MHz cells having few PCFICH positions overlapping being deployed. Like in the previous case with 5MHz deployed, a total of 25 * 2 = 50 REG location are possible within the bandwidth. With 4 equidistant locations we can have only 12 unique PCFICH positions. Figure 4 shows three neighbor cells having PCIs 0, 12 and 25. When we consider the cell with PCI 0 and 12 or 12 and 25, we see that two REG positions 72 and 222 are overlapping that decreases the probability of decoding PCFICH, while PCIs 0 and 25 have all the four positions in common making it even worse to decode PCFICH. When the PCIs have a difference of 13, we see other two REG positions 0 and 150 overlapping. Hence it is not advisable to have PCIs with multiples of 12/13 or 25 in neighborhood.
A summary of above discussion can be seen in Table 1 below. Extra care must be taken while deploying neighbor cells with PCI distances such that those mentioned in the table are not used leading to better cell edge throughputs. Figure 5 below shows an example scenario that abides by PCFICH plan to ensure a better cell throughput.
Figure 4: Scenario showing neighbor cells with few (1 or more) overlapping PCFICH locations
Table 1: PCFICH Separation distance for various LTE deployments
Figure 5: Scenario of PCI plan with PCFICH check
Conclusion
Maximum throughputs can be achieved by careful cell planning considering the PCFICH locations additionally. The cell planning should see that neighboring cells in the cluster or across clusters do not have a distances mentioned in Table 1. PCFICH positions have a randomized interference due to individual REGs of cells overlapping with other cell individual REGs.
No comments:
Post a Comment