Mobile broadband wireless communication systems employ several techniques for improving spectral efficiency. To achieve high data rates， yield optimal system capacity， and ensure reliable quality of service （QoS）， modern wireless communication systems use variable channel bandwidths （BW = 1.25MHz to 20MHz） with high-order modulation （16QAM to 64QAM） and code-division or orthogonal frequency-division multiple access （CDMA， OFDMA） as well as scalable， smart antenna technology （e.g.， multiple-input multiple-output or MIMO， spatial diversity）。

　　3GPP standards UMTS， TD-SCDMA， and long-term evolution （LTE） as well as others like IEEE? 802.16e， IEEE 802.11n， and IEEE 802.11ac are some common systems that use these techniques. As an example， a 4G LTE radio can achieve a peak data rate greater than 100Mbps with robust performance using 64QAM modulation， orthogonal frequency-division multiplexing （OFDM） with 2048 subcarriers， a 20MHz channel bandwidth， and a 2×2 MIMO architecture.