Cell Edge Rate in LTE Similar to other wireless communications systems, such as CDMA2000 EVDO, WiMAX and HSPA, the LTE features a rate layering feature. That is, the higher the required edge rate, the smaller the cell coverage radius. The lower the required edge rate, the larger the cell coverage radius.
This comes about due to the fixed power offered by UE (normally 23dBm) being spread evenly to the number of RBs involved in the modulation scheme assigned, assuming there is no power control (i.e. Downlink ICIC also disabled).
Some of the factors that affect the edge rate in the LTE system are as follows for Cell Edge Rate in LTE:
- Uplink/downlink TDD proportion
- MIMO schemes chosen
- eNodeB Power Amplifier power (affect downlink only)
- Number of RB used at the sector edge
- Modulation mode (1 of 29 coding methods)
- Repeated coding times
The formula for calculating the downlink cell edge rate is as follows:
Cell edge rate Phy = Number of Different data stream transmitted x Number of Resource Block assigned to user per frame x Number of available Traffic carrying Resource Element per Resource Block x Coding rate x Modulation model level / Duration of each frame
- Number of Resource Block Assigned in Cell Edge Rate in LTE (a single RB is the basic resource assignment level) reflects the number of resource blocks used by user at the edge of the sector. The smaller the number of resource blocks assigned, the lower the cell edge rate. In previous version of link budget tools, receive sensitivity of a base station is defined by the bandwidth of the RB which is 180 kHz. More recent version are using per subcarrier as basis of receiver sensitivity and the conversion value is simply 10log10. RB can be assigned down to a per TTI level (1 ms duration)
- Number of Different data stream transmitted in Cell Edge Rate in LTE is related to the number of data stream being simultaneously transmitted. Number can be ranging from 1 (SFBC) to 2 (MCW 2×2). In case of BF, the value should be 1 for single antenna port transmission mode 7 (port 7 or 8), and 2 streams for dual antenna port transmission mode 8 (port 7 and 8).
- Number of available Traffic carrying Resource Element per Resource Block in Cell Edge Rate in LTE indicates the number of RE available for each resource block. In FDD system, a maximum of 3 symbols (36 Res) can be consumed per frame (10ms) for control channel signaling purposes and there is at least 6 more extra RE can be used for Downlink Reference signaling per TTI (1ms). A minimum of 1 symbol (12 Res) will be required per RB for control signaling purposes. In TDD system, due to frequency sharing and time gap requirement for switching between uplink and downlink, 6 symbols equivalent (72 Res) will be the minimum overhead requirement per TTI.
- Coding rate indicates the volume coding rate of the channel code. For example, the volume coding rate of QPSK1/2 is 1/2, and the volume coding rate of 16QAM3/4 is 3/4.
- Modulation model level indicates the number of bits in the modulation mode. For example, the modulation mode levels of QPSK, 16QAM, and 64QAM are 2, 4, and 6 respectively.
- Duration of each frame indicates the frame size. As regulated by the protocols, the frame size in LTE networks is 10 ms.
In the link budget for Cell Edge Rate in LTE, the settings of the uplink/downlink cell edge rates (in particular the uplink cell edge rate) will determine the final cell coverage radius. Hence, an understanding of edge coverage requirement is very critical from a network planning perspective.
If Downlink ICIC is enabled, downlink power control must be enabled also (which is executed at 20ms intervals based on UE BER reported value) and edge rate calculation will be more complex and beyond the formula listed above. However, the cell edge data rate requirement will still be the single most important factor in any cell planning activities.