  The identification capacity region of the compound broadcast channel is
determined under an average error criterion, where the sender has no channel
state information. We give single-letter identification capacity formulas for
discrete channels and multiple-input multiple-output Gaussian channels under an
average input constraint. The capacity theorems apply to general discrete
memoryless broadcast channels. This is in contrast to the transmission setting,
where the capacity is only known for special cases, notably the degraded
broadcast channel and the multiple-input multiple-output broadcast channel with
private messages. Furthermore, the identification capacity region of the
compound multiple-input multiple-output broadcast channel can be larger than
the transmission capacity region. This is a departure from the single-user
behavior of identification, since the identification capacity of a single-user
channel equals the transmission capacity.
