This paper proposes a low-complexity physical (PHY) layer design to introduce cooperation in the downlink of an infrastructure-based multicell multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) network, aimed at supporting future high-throughput broadband wireless Internet access with large-scale coverage. In such a system, several multiantenna base stations (BSs) are organized in a cellular architecture to serve multiantenna mobile stations (MSs) and are connected to a central service unit via a high-speed wired backbone. To improve the network performance, a novel PHY layer design is proposed that allows cooperation among an arbitrary and unknown number of BSs by suitably randomizing the MIMO-OFDM block codes used by the BSs. Such a randomized MIMO-OFDM code renders the encoding/decoding rule independent of the number of actual BSs cooperating and works without any channel feedback, which greatly simplifies the protocol as well as the MS design. To provide performance insights and develop PHY layer designs, this paper provides analytical upper bounds on the symbol error probability for linear receivers, which allow to accurately evaluate the diversity order and the coding gain achievable through the proposed scheme. Lastly, we present numerical results that validate the theory, and highlight the performance gain and the coverage expansion attainable with our cooperative transceiver.