interference are usually happening in one direction only : UL interference created by mobiles a little too far from their serving cell. DL interference created by a cell covering too far away from its own best server area.
Otherwise, bad quality in one direction can be the consequence of a faulty TRX (either UL or DL)
QoS – Quality of Service – is such a wide topic. For 4G, having a QCI – QoS Class Indicator – setting that will ensure a CIR will help. This will be a part of a higher (e.g. Gold, Premium, etc..) segmented Package. A combination of a PCEF_PCRF / 3GPP OCS / Sy is a good starting point for a segmented QoS.