def lookup_capacity(capacity_lut, env, ant_type, frequency,
    generation, ci):
    """
    Use lookup table to find the combination of spectrum bands
    which meets capacity by clutter environment geotype, frequency,
    bandwidth, technology generation and site density.

    Parameters
    ----------
    capacity_lut : dict
        A dictionary containing the lookup capacities.
    env : string
        The settlement type e.g. urban, suburban or rural.
    ant_type : string
        The antenna type, such as a macro cell or micro cell.
    frequency : string
        The frequency band in Megahertz.
    generation : string
        The cellular generation such as 4G or 5G.
    ci : int
        Confidence interval.

    Returns
    -------
    site_densities_to_capacities : list of tuples
        Returns a list of site density to capacity tuples.

    """
    if (env, ant_type, frequency, generation, ci) not in capacity_lut:
        raise KeyError("Combination %s not found in lookup table",
                       (env, ant_type, frequency, generation, ci))

    density_capacities = capacity_lut[
        (env, ant_type,  frequency, generation, ci)
    ]

    return density_capacities