Supplementary Download Link

In a normal mobile connection the channel used is a pair of channels, one uplink and one downlink. The model is that the traffic is two way, and there is equal capacity for both the uplink and downlink. However much of the use of mobile broadband is to engage with one way traffic such as entertainment services. Here there is significantly more download than upload. With a normal paired setup, the uplink is not being used much whereas the downlink is running near capacity.

The Suplementary Down Link adds more capacity to the downlink. SDL allows the bonding of the unpaired spectrum with a pair of FDD chanels to significantly enhance the download capacity. In the LTE release 10, up to 2 SDL downlinks can be associated with one pair of standard uplink and downlink channels. To achieve the successful use of SDL the UE (mobile phone) must support carrier aggregation. The downlink data will be sent in parallel from the Base Station to the UE, and the UE must be capable of putting the received data together in the correct order; this is carrier aggregation. Most modern high spec smart hones can manage carrier aggregation.

In the UK the frequencies specifically defined as SDL is between 1452MHz and 1492MHz. This 40MHz block is split in two and allocated to Vodafone (1452-1472) and Three (1472-1492). EE own 20Mhz between 738MHz and 758MHz as SDL frequency.