The channel quality between a base station and a user may vary due to fading.
A channel can fade along:
- Time
- Frequency
- Position
In the following are different responses to compensate the effects of channel fading.
Link adaptation
Set transmission parameters, based on current channel condition:
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Transmission power control: adjust Tx power according to channel quality (high quality $\rightarrow$ low Tx power vs. low quality $\rightarrow$ high Tx power). Ensure constant data rate at receiver.
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Transmission rate control: adjust transmission data rate according to channel quality (high quality $\rightarrow$ high data rate vs. low quality $\rightarrow$ low data rate). The data rate is adjusted by the choice of the modulation scheme and the coding rate. Another name for transmission rate control is Adaptive Modulation and Coding (AMC).
Scheduling
Share radio resources (time, frequency, code, space, Tx power) among users. More commonly, decide to/from which user to transmit at a given time and/or frequency.
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Maximum C/I scheduling (maximum rate scheduling)
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Round-robin scheduling
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Proportional-fair scheduling
Packet retransmission
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Automatic Repeat Request (ARQ): receiver checks incoming packets for errors. If no errors, receiver confirms reception to transmitter with ACK. If the packet contains an error, the receiver discards the packet and requests retransmission of the packet from the transmitter by sending a NACK (negative ACK).
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Hybrid ARQ (HARQ): combination of Forward Error Correction (FEC) with ARQ. Receiver first corrects correctable errors with FEC. If an error cannot be corrected with FEC, the receiver discards the packet and requests retransmission of the packet.
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HARQ with soft combining: same as HARQ with the difference that the receiver stores packets with uncorrectable errors before requesting a retransmission, and, if the retransmission still contains an error, combines the stored erroneous packet with the retransmitted erroneous packet before applying FEC.