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OFDMA System tab

Whether you want to simulate OFDMA UL (uplink) or OFDMA DL (downlink), the upper part of the workspace display will allow you to select the system you want.


It is used to define the necessary parameters for the modeling of OFDMA systems. The OFDMA interface has been split into 2 tabs (when victim) or 3 tabs (when interferer):

Click on the below links to get directly to the following sections of this page:

Please consult the CDMA/OFDMA commonalities in terms of cellular topology, Pathloss and Effective Pathloss, Reference Cell etc...

General settings

7 panels characterised the OFDMA system. The below graphic represent the OFDMA UL when a victim.


The only difference with OFDMA DL is the following panel.


Identification

  • Library: allows to import/export the receiver characteristics from/to the library to/from the workspace.
  • name and description: You can name and write some description of the victim system you want to simulate.

OFDMA general settings

They are similar for the OFDMA DL and UL.

Parameter Description
OFDMA Link component The type of OFDMA System. There are considerable differences between modelling of uplink and a downlink in OFDMA system. See the end of this page for more detailed explanation of the differences.
SINR Minimum Lower boundary of SINR to take into account in the simulation. In DL, any UE with a C/I lower than the SINR minimum will be disconnected right away. In UL, the UE will get tagged with a disconnect flag. For a specific threshold (Maximum allowed disconnection attempts) of disconnection, the UE is removed from the cell. Note that this input is only valid for OFDMA DL, in OFDMA UL it is not used in the calculation.
Max subcarriers per Base Station Number of available Resource Blocks (RBs) per BS
Number of subcarriers per mobile Number of RBs per UE. Note the ratio of Max subcarriers per Base Station/ Number of subcarriers per mobile gives the number of active users per serving BS.
Handover MarginSpecifies the maximum difference, in dB, between the links in users active list. The actual active-list selection is based on pathloss calculations.
Minimum Coupling Loss (dB) The minimum path loss. It is used in the calculation of the effective path loss
System bandwidth Specified in MHz
Receiver Noise Figure Equipment-specific noise figure of receiver, specified in dB
Bandwidth of RBs Specified in MHz
OFDMA LTE Link-to-system level mapping The traffic (i.e. bit rate) per UE is a look up table used to map throughput in terms of spectral efficiency (bps per Hz) with respect to calculated SINR (= C/(I+N)) (dB) (Signal to Interference-plus-Noise Ratio) level. A drop-down selection of Link level data look-up 2 dimensions functions from Library. The OFDMA Link level data has the same formats for uplink and downlink but with different values. This link level data (bitrate mapping) is user selectable and can be modified depending on the simulation to perform. You are responsible to choose an appropriate set of data.

The achieved bit rate is calculated as follows:

Receiver settings

The content of this panel depends whether OFDMA system is a victim or an interfering system.

  • OFDMA is a victim, you will have to set the blocking mask.

Transmitter settings

The content of this panel depends whether OFDMA system is a victim or an interfering system.

This panel is available only if OFDMA UL selected.

Parameter Description
Maximum allowed disconnection attempts When the number of disconnection attempt is greater than this threshold, then the mobile is disconnected. This means that the UE is removed from the served UE list of that BS, The BS is marked with “got spare capacity” and the UE is added to the disconnected UE list.
Minimum transmit power of mobile Minimum transmit power used in the power control.
Maximum allowed transmit power of mobile Transmit power of the UE
Power Scaling Threshold Used in the calculation of the path loss limit for the power control. It is a limit threshold compared to the value of the CDF used in the power control

This panel is available only if OFDMA DL selected.

Parameter Description
Base Station Maximum transmit power Specified in dBm

OFDMA Capacity

The panel is similar for the OFDMA DL and UL.

Parameter Description
Users per BSDefines how many mobiles per cell should be generated in the system. For each base station (BS), each UE will be added to served UE list of that BS. Depending on the propagation or handover conditions, a UE will either remain connected to the BS or will be disconnected. THIS IS NOT the number of active users. The number of active users is the ratio of Max subcarriers per Base Station/ Number of subcarriers per mobile as described in section OFDMA general settings

Pathloss Correlation

The panel is similar for the OFDMA DL and UL.
The concept of a simple correlation model for shadow fading has been widely adopted in LTE co-existence studies mostly employed in uplink case. The propagation attenuation is modelled as the product of the path loss and the shadow fading. The shadow fading is well approximated by a log-normal distribution (ref. 12 of the SEAMCAT Handbook (January 2010)). Let z denotes shadow fading in dB with zero mean and variance σ2. Then the shadow fading of path from one UE to the i-th BS is expressed as zi=a*x + b*yi , where a2+b2=1 and x and yi are independent Gaussian distributed variables, both with zero mean and variance σ2 . yi and yj for are independent as well. Figure 9 presents how to set-up the pathloss correlation in SEAMCAT (only available for OFDMA).




Thus, the correlation coefficient of the shadow fading from one UE to two different BSs, i.e., the i-th and j-th BS, is . In most LTE studies, is assumed [TR36.942]. For cellular systems with three-sector antennas, the shadowing correlation between sites (equivalent to BS in Omni antenna system) is of 0.5 and correlation between sectors of the same site is consequently of 1.

Propagation Model

This tab contains settings for the choice of propagation model to be applied when evaluating path losses in the internal links of the modeled OFDMA system. Please see the guidance regarding separate information the choice and settings of the propagation models.

Positioning

This interface is the same as for the CDMA network. Have a look at the CDMA positioning's page for more detailed information.

Transmitter to Victim Link Receiver Path

Similarly to CDMA, when OFDMA is an interferer an extra tab is accessible to set the characteristic of the path between the interferig transmitter (UE for UL or BS for DL) and the victim system. Note that this panel is the same either a CDMA (UL/DL) or OFDMA (UL/DL) is simulated.

Calculation of the UE frequencies in UL

The frequency of the UE in UL is calculated as follow

FUE = Fsystem - (BWsystem / 2) + ((((NRB_UE * BWRB) + (diff/ NUE)) / 2) * ((Indexlink * 2) + 1))


With

diff = BWsystem - (NRB_BS * BWRB)


Where

  • FUE: Centre frequency of the UE
  • Fsystem: Frequency of the system (i.e. the network) (input to SEAMCAT)
  • BWsystem: Bandwidth of the system (input to SEAMCAT)
  • NRB_UE: Number of Resource Blocks (RB) per mobile (input to SEAMCAT)
  • NRB_BS: Number of RB for the BS (input to SEAMCAT)
  • BWRB: Bandwidth of the RB (input to SEAMCAT)
  • NUE: Number of UEs in the system (calculated as NRB_BS/NRB_UE)
  • Indexlink: Index of the specific link UE to serving BS (input to SEAMCAT). Index = [0, NUE-1]


Note Diff takes into account for any difference between the BWsystem and the effective bandwidth (NRB_BS * BWRB)


OFDMA UL power control

In OFDMA UL, the power control is applied to the active users (i.e. the mobile users with specific RBs) so that the UE Tx power is adjusted with respect to the effective path loss (i.e. based on the MCL) to the BS it is connected to. In 3GPP [TR36.942], the UL power control is defined so that the UE transmit power is set such as: where Pt is the UE Tx power in dBm, Pmax is the maximum transmit power in dBm, Rmin is the minimum power reduction ratio to prevent UEs with good channels to transmit at very low power level. Rmin is set by Pmin / Pmax. PL is the effective path loss in dB for the UE from its serving BS and PLx-ile is the x-percentile effective path loss (plus shadowing) value. PLx-ile is defined here as the value in the CDF, which is greater than the effective path loss of x percent of the MSs in the cell from the BS (i.e. it corresponds to the parameter “power Scale Threshold”. It is set by default to 0.9, but you can change it. With this power control scheme, the 1-x percent of UEs that have a path-loss greater than PLx-ile will transmit at Pmax, i.e. are not power controlled. In SEAMCAT, gamma is assumed to equal 1. More detailed information on the algorithm is available here.

Depending on the direction of the interfering OFDMA link to be simulated, you should pay attention to the emission bandwidth of the unwanted emission mask and the system bandwidth.

  • When a DL simulation is considered, the unwanted emission mask corresponds to the BS transmitting over all the RBs (i.e. the emission bandwidth is the same as the System Bandwidth)
  • When a UL is considered, the emission bandwidth (i.e. in-band part of the unwanted emission mask) corresponds to the UE transmitting over a number of RBs (i.e. the emission bandwidth is equal to the RB bandwidth x Number of RBs requested per user) which is different from the DL where the system bandwidth is used as illustrated in Figure below. Note that the system bandwidth is input to SEAMCAT and approximately RB bandwidth x the total Number of RBs (i.e. Max. subcarriers per BS input).



Illustration of the emission spectrum mask in UL for a LTE transmission

OFDMA DL as interferer OFDMA UL as interferer
When OFDMA is a DL interferer, the OFDMA is not simulated as it is assumed that the BSs are transmitting at full power and in order to decrease the simulation time a full OFDMA simulation is not required. In OFDMA DL interferer, the position of the BSs will be calculated only.The fgure below presents the set-up of the OFDMA DL as an interferer. Note that only the system bandwidth is needed in this configuration, therefore the rest is shaded(not active).When OFDMA UL is the interferer, it is important to simulate the whole interfering network (i.e. power control) so that the interfering emission power from the UE is optimized. In this case, the GUI interface is similar to the victim one.


Illustration of the GUI for OFDMA DL

Methodology

Assumptions

The OFDMA LTE algorithm is only valid for a 100% loaded system and each user is allocated with a fixed number of resource blocks. This is equivalent to modelling a Round Robin scheduler with full buffer traffic model and a frequency reuse of 1/1 (i.e. Single Frequency Network is assumed). Moreover, E-UTRA system is assumed to be a fully orthogonal system, which indicates that in the UL case only UEs allocated with the same sub-carriers (frequency resource block) could introduce other-cell, intra-system interference.

The number of active users per serving BS simulated in SEAMCAT is the ratio between the Max subcarriers per Base Station and the Number of subcarriers per mobile. (both of these parameters are input). For instance, with 24 RBs at the BS and 8 RBs at the UE, the number of active users is 3 and the system is 100% loaded. In the case where there are 24 RB per BS and 7 RB, SEAMCAT generates 3 users per BS - but only 21 out of 24 RBs will be in use. Therefore the system load is equal to (21/24)*100 = 87.5%



Note that if the OFDMA is a DL interferer, the OFDMA is simulated as in “traditional” simulation with the BSs transmitting at full power. This decreases the simulation time of a full OFDMA simulation. In OFDMA DL interferer, only the position of the BSs will be calculated because full transmit power is assumed. For all other simulations (including UL) scenarios full OFDMA network simulation is required. Consequently, some of the input parameter of the GUI interface have been grey-out when the OFDMA DL interferer case is selected.

Since it is arguable that some simulation assuming a rural environment would not need to assume full power transmission (i.e. full loaded network) when the system is DL and interferer, you may need to manipulate either the input power or the spectrum mask (or both) in order to simulate the DL interferer case for rural deployment.

DL SINR calculation

The relationship between the contributors of the interference in a OFDMA network is illustrated below.


In this SEAMCAT OFDMA implementation, the term “BS” and “cell” have the same meaning. The SINR or C/I calculation in DL is calculated as


where C(j,k) is the received power at the k-th user from the serving BS, i.e., the j-thBS


and where is the power of resource block. Note that the effective path loss includes shadowing.

I(j,k) is the sum of the interference power (power of resource block * effective pathloss including shadowing)


which consists of adjacent cell interference Iinter(j,k) (from the same victim system, i.e. denoted inter-system interference)


the interference from external interfering system(s) in adjacent channel Iext(j,k) (interference power into this resource block including ACIR). The ACIR (Adjacent Channel Interference Ratio) is implicitly taken into account when both unwanted and blocking mechanism are summed in the computation


where


for each of the UE’s frequency where the DL information is received and


at the victim system frequency.

where N is the number of RBs (i.e. subcarriers) requested per UE, and M is the maximum number of RBs per BS and where N_external_cell is the number of external interfering BSs.

and the thermal noise Nt


where N is the number of RBs scheduled to a UE.

UL SINR calculation

The SINR or C/I calculation in UL is calculated so that C(j,k) is the received power from the UEj,k at the j-th BS.

where Pt is the transmit power of the UE in dBm (Note that UL Power control is applied).

Similarly to DL, the interference is derived from

where Iinter is the interference coming from UEs of the same system but from adjacent cells (i.e. the inter-system interference from other cells). Since a fully orthogonal system is assumed, only UEs which transmit in the same frequency subcarriers will introduce interference to each other, hence only UEs in other cells with the same k index are considered.

where Iext is the interference from external interfering UEs.

where K is the number of UEs in the external interfering cells and the number of external cells is limited to NExternal cell and the thermal noise Nt.
Note: In UL, it is important to remember that for LTE technology, each user will be transmitting its own RB. In SEAMCAT, it is assumed that each UE transmit the same amount of RBs therefore they have the same emission spectrum mask.

Note: when the OFDMA UL is the victim system, one has to remember that the interferer will impair each of the signals transmitted by the UEs serving its own BS (i.e. the victim BS). Therefore, for a specifc link (UE1 to BS1) the interference caused by an external interferer will only affect the spectrum occupied by the RBs allocated to UE1 for that link and not the whole system bandwidth at BS1.

A look up table is used to map throughput in terms of spectral efficiency (bps per Hz) with respect to calculated SINR (= C/(I+N)) (dB) level. This link level data (bitrate mapping) is user selectable and can be modified depending on the simulation to perform. The default values are extracted from 3GPP TR 36.942.


Note on the ACLR calculation

Note: The ACLR calculation is similar to the unwanted calculation BUT note that in 3GPP it is the integration of the interfering power in the adjacent channel where the bandwidth equal to the interfering emission bandwidth while the unwanted uses the victim bandwidth (See illustration from 3GPP TR36.942).


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