# Frequency division multiple access ![](attachments/Frequency%20division%20multiple%20access-attachment.png#invert) Imagine the [bandwidth](Bandwidth.md) is a cake. Try to slice the cake in different directions. Take a link (uplink/downlink). Try to separate it into n orthogonal channels (don't self-interfere), one per user (or device). Users sit on different channels ## Benefits Transmit the whole time - Narrow bandwidth channels - Flat fading means no complex receivers (equalisers) needed - No synchronisation overheads ## Drawbacks - Limited [bandwidth](Bandwidth.md) for transmission - Guard band channels may be needed to avoid adjacent-channel interference - If an FDMA channel is not in use, it sits idle ## Questions How to choose who gets what channel? WiFi - OFDMA Different channels are specified by the resource allocator. ## Guard Bands Non dead sharp filter - need some overlap. Number of channels supported by FDMA is $N=\frac{B_t-2B_{guard}}{B_c}$ where $B_t$ is the total spectrum allocation, $B_{guard}$ is the guard band at the edge of the spectrum and $B_c$ is the channel [bandwidth](Bandwidth.md). ![](attachments/Frequency%20division%20multiple%20access-attachment-1.png#invert) [^1] # References [^1]: [Map of TELE4652](../../Atlas/Map%20of%20TELE4652.md)