icient resource assignment to multiple users along with spatial mode selection and adaptive modu- lation in real time on standard digital signal processing hardware. This represents a new medium access control (MAC) layer [12,13,15]. This MAC layer is steered over wireless feedback and control channels in the uplink in a closed-loop manner. The implementation uses parameters close to the forthcoming LTE of the 3G air interface (as in Table 11.1), thus illustrating the feasibility of the new functions in next-generation cellular radio systems. This section describes our real-time imple- mentation and reports several test results. These results were taken from a small-cell measurement scenario. 11.3.1 Fair and Efficient Scheduling The opportunism inherent in multiuser wireless communication with mobile users is that each user experiences a unique channel (see Figure 11.20). The channel is Channel UE 0 Channel UE 1 Frequency Figure 11.20 Examples of frequency responses within 20-MHz bandwidth. © 2009 by Taylor & Francis Group, LLC Early Real-Time Experiments and Field Trial Measurements Space Per antenna Resource block Nsubcarriers Nsymbols Figure 11.21 Resource allocation. time variant and frequency selective. In the case of a multiuser environment, the system throughput benefits from this multiuser diversity. An additional require- ment, however, is that in order to support terminals on their best resources, regular feedback links are required. To support each user on its best resources, OFDMA, in combination with MIMO, allows allocation of resource blocks flexible in time, fre- quency, and space domains (see Figure 11.21). There are clearly several possibilities to find a scheduling strategy that allocates resources in an efficient and fair manner over these domains. The basic idea of multiuser MIMO-OFDM is that there is a fundamental trade-off between spatial multiplexing and spatial diversity. In transmit mode selection, we consider spatial multiplexing using multistream transmission using per-antenna rate cont