Modulation and Coding (OFDMA)

Modulation and Coding Schemes (MCS) for OFDM / SOFDMA Systems

To view a list with the currently defined MCS, click Project > Edit Project Parameter and click the Air Interface tab. Further transmission modes can be defined, existing ones can be adapted during the planning process. The defined transmission modes can be sorted considering various criteria. The order of the MCS does not influence the simulations but the sequential arrangement in the tree view and on the dialogs.

Figure 1. The Transmission Modes (MCS) dialog.

To define the transmission mode parameters of a OFDM / SOFDMA system, double-click on an item in Figure 1. The settings can be specified for the downlink (BS to MS) and the uplink (MS to BS) direction separately.

Figure 2. The Transmission Mode dialog.

Arbitrary name for transmission mode.
Direction to be analyzed
Downlink and uplink directions can be analyzed individually for each transmission mode. Accordingly it is possible to analyze transmission modes for Only Downlink or for Only Uplink without influence of the other direction.
Consider mode during simulation
If this option is enabled, the transmission mode will be considered during network planning simulation. Transmission modes which are specified to be not considered during simulation are display with red (see Figure 1).
Position of mode in simulation sequence
The position (priority) of the transmission modes affects the throughout results as the transmission modes are analyzed according to their position value. Typically, the networks are configured to maximize the throughput. For this purpose the transmission mode with the first position should be analyzed first. If there are some radio resources remaining (for example, resource blocks in LTE) then the transmission mode with the second position will be analyzed for checking if the throughput can be further increased and so on. In LTE with 5 MHz bandwidth 25 resource blocks are available for example. For a specific location in the network it might be possible to transmit 15 streams in parallel of the transmission mode with position 1 (leading to 710.2 kBit/s per stream). If for the remaining 10 resource blocks the SNIR of the first position is not fulfilled (as the interference for each sub-band depends on the frequency reuse in the neighboring cells), maybe the SNIR target for the second position (with 665.9 kBit/s per stream) is fulfilled. In this case the overall throughput would result in 15*710.2 kBit/s + 10*665.9 kBit/s = 17,312 kBit/s.So to maximize the throughput, the transmission modes should be assigned reasonable values for the position. Without this sorting it might happen that transmission modes with low data rates use the available radio resources and thus the maximum throughput is not exploited.
Note: It is not possible to have multiple transmission modes with the same position in simulation sequence. The individual positions of the transmission modes do not have to be consistent (gaps are possible).
Data Transmission Parameters (for downlink and uplink)
Modulation scheme used for data transmission.
Code rate
The code rate states the portion of the total amount of information that is useful (that is non redundant). If the code rate is k/n, k bits out of n totally generated bits are useful information, whereas (n-k) bits are redundant. The redundant bits are used for error detection and correction.
Nr of Resource blocks
Number of resource blocks, which are used for this transmission mode.
Overhead ratio
Ratio between gross and net data rate.
Data rate (read only)
Net data rate, which is automatically computed according to the parameters specified above using the following formula:

Data rate = NrResourceBlocks * NrSubcarriers (perRB) * NrSymbols (perSlot) * NrSlots (perSec) * NrBits (perSymbol) * CodeRate * (1 - OverheadRatio)

For example, for 5 MHz (25 RB) QPSK R=1/8 with 12% overhead:

Data rate = 25 * 12 * 7 * 2000 * 2 * 1/8 * 0.88 = 924 kBit/s.

Radio Parameters (for downlink and uplink)
Minimum required SNIR
Minimum signal-to-noise-and-interference-ratio, which is required to use this transmission mode. Receiver locations, where the predicted SNIR is above this threshold is assigned to this transmission mode and the maximum available data rate for the corresponding Rx locations is be set to the value specified for this MCS.
Minimum required Signal Level (optional)
Optional threshold of signal level, which is required to use this transmission mode.
Note: The specified minimum signal level for the uplink direction refers to the input of the base station receiver and not to the power received directly at the antenna. In case a cable loss is defined, the received power at the antenna is additionally reduced by the cable loss before it is compared to the specified minimum required signal level.
Figure 3. Antenna and Base Station Receiver scenario.

Tx power backoff
Power headroom which can be specified related to maximum available transmit power of the base station (for downlink direction) or the mobile station (for uplink direction). This headroom is used to reduce available transmission power and therefore crosstalk probability for high modulation and coding schemes.
Note: This back off value influences only the transmit power used for data transmission. The power assigned for pilot signals is not influenced.