Testing TD-LTE with real world test technologies: Page 6 of 7

July 24, 2012 // By Erik Org, Azimuth Systems
Many wireless data networks were initially designed to offer symmetrical data capacities as the original killer wireless application, voice, required equal capacity for both uplink and downlink. Spectrum blocks were licensed and auctioned as paired spectrum suitable for Frequency Division Duplexing (FDD) protocols, which served carriers and subscribers well when voice was the primary application.
2: Beamforming. Click image to enlarge.

This is because beamforming algorithms depend on uplink phase and amplitude information to steer the downlink antenna field pattern. Channel estimates and other signaling information are exchanged continuously between the BS and the MS and hence a bi-directional connection needs to be provided in the lab. Reciprocity means that transfer functions for each path in a MIMO system look exactly the same in both directions and the impulse responses h ij(t) must identical for both directions (figure 3).

Figure 3: Transfer functions of a beamforming MIMO system. Note that the channels must be reciprocal from end to end and not just internally to the emulator. Click image to enlarge.

In practical terms, the test equipment used for beamforming must ensure that the phase of the downlink channel should be calibrated to be equal to the phase of the uplink channel i.e. ΦDL needs to be equal to ΦUL, the balance should be end to end i.e. right from the point of the connection of the antenna port of the eNodeB to the point of the connection of the antenna port of the MS and similar amplitude balancing may be required for UL and DL paths (figure 4).

Impact of network loading on cell edge performance.  Click image to enlarge.

At the edge of a cell, a terminal device typically raises it’s transmit power to the maximum permissible in order to ensure continued robust communications. However, this quickly leads to the situation where the terminal device is only able to transmit one resource block in any given time period. Any attempt to transmit more than one resource block would necessarily spread all available transmit power across more resource blocks and reduce max range.
However, TD-LTE will in reality deliver cell edge performance comparable to FDD performance, since almost all cells will typically have more than one user and thus each device would be limited

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