In any wireless communication system, two of the most important specifications are throughput and range. Throughput is a measure of the maximum amount of information (bits) that can be transferred from one radio to another in a unit time; range is a measure of the maximum distance between the two radios, with the communication link still operational.
However, in a typical communication system, the peak throughput cannot be achieved at the maximum range. Communication at peak throughput occurs only when the distance between the two devices is short; as distance increases up to the maximum range, the throughput decreases.
A communication system like PMP 450 supports multiple modulation modes. The higher the modulation mode, the more bits can be transferred between the two radios, but also the higher the required signal-to-noise ratio (SNR) is for successful communication.
PMP 450 is a 2x2 MIMO communication system (with two transmit chains and two receive chains), which supports the following modulations: QPSK, 16QAM, 64QAM and 256QAM. For each modulation mode, a MIMO-A option and a MIMO-B option can be selected.
With MIMO-A, the same information is transmitted by both transmitters. The communication channels between the transmitters and the receivers have different fading and multipath characteristics, and when a weighted average of the signals is calculated at the receiver, a combining gain increases the signal to noise ratio, possibly allowing the selection a higher modulation mode. MIMO-A is used to add robustness to the system and extend the range of communication.
With MIMO-B, different information is transmitted from the two transmitters. If the communication channel does not add excessive noise and interference, this system is equivalent to two parallel transmission systems. The throughput supported by MIMO-B is twice the throughput supported by MIMO-A at the same modulation level.
Area covered by each modulation
Figure 1 shows an example of an AP covering a circular area around it with an omnidirectional antenna, or a portion (sector) of the area with a directional antenna. In this figure the modulation levels are labelled from 1x to 8x and correspond to the following modulation modes:
1X -> QPSK MIMO-A
2X -> QPSK MIMO-B
4X -> 16QAM MIMO-B
6X -> 64QAM MIMO-B
8X -> 256QAM MIMO-B
The modulations are defined in this way because, for example, 256QAM MIMO-B (8X) provides a throughput which is eight times higher than the throughput of QPSK MIMO-A (1x) and four times higher than the throughput of QPSK MIMO-B (2X).
The figure does not show other MIMO-A modulations, because the rate adapt algorithm will select a MIMO-A modulation mode only when the received signal strength in the two paths is very different. When the two paths have similar received strength, a MIMO-B modulation mode is selected, because of the higher throughput that can be achieved. However, QPSK MIMO-A (1X) is also selected when the two paths have similar received signal strength, in order to extend the range of communication beyond what can be covered with QPSK MIMO-B (2X).
As Figure 1 shows, only SMs located closest to the AP (in the light blue circle) can achieve the peak throughput, which corresponds to 256QAM MIMO-B (8X) modulation. SMs located at the edge of the cell (in the dark blue ring, at the maximum range) will communicate using QPSK MIMO-A (1X) modulation. SMs located in intermediate rings will communicate at modulations between 6X and 2X, depending on their location.
The radius of each ring depends on the link budget parameters, namely transmit power, transmit and receive antenna gain, receive sensitivity of the devices, and also on the propagation environment (e.g. line-of-sight vs. non-line-of-sight propagation).
Because not all SMs communicate with the AP at maximum throughput, the average throughput experienced by the AP will also be lower than the maximum value. The average throughput of the AP (or capacity of the sector), depends on the number of SMs communicating at each modulation level, and the amount of traffic generated by each SM. If all SMs generate the same amount of traffic and the SMs are evenly distributed in the covered area, the expected sector capacity can be calculated using the ratios of the areas of the rings over the total covered area.
Limiting the range
When configuring an AP, the maximum range is defined for a sector. This parameter indicates the distance between the AP and the farthest SM served by that AP, and it can be shorter than the maximum range that could be achieved based on the link budget parameters and propagation environment.
This range value is used for example to configure the frame, in order to allow enough turnaround time between transmit and receive modes, for the signal to travel to the farthest SM and back to the AP.
If the range configured for the sector is smaller than the maximum range that can be supported given the link budget parameters and propagation environment, the sector capacity is calculated considering only the rings up to the configured range. This means that the capacity is higher than in the case in which the configured range matches the maximum range. However, if a shorter range is selected, care has to be taken when selecting channels during frequency planning, because the AP can interfere all the way to the maximum range.