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LTE-Advanced: An introduction

Posted: 04 Jul 2011 ?? ?Print Version ?Bookmark and Share

Keywords:LTE-Advanced? 3GPP? 4G?

Enhanced uplink multiple access: Today's LTE uplink is based on SC-FDMA, a powerful technology that combines many of the flexible aspects of OFDM with the low peak to average power ratio (PAPR) of a single carrier system. However, SC-FDMA requires carrier allocation across a contiguous block of spectrum and this prevents some of the scheduling flexibility inherent in pure OFDM.

LTE-Advanced enhances the uplink multiple access scheme by adopting clustered SC-FDMA, also known as discrete Fourier transform spread OFDM (DFT-S-OFDM). This scheme is similar to SC-FDMA but has the advantage that it allows non- contiguous (clustered) groups of subcarriers to be allocated for transmission by a single UE, thus enabling uplink frequency-selective scheduling and better link performance. Clustered SC-FDMA was chosen in preference to pure OFDM to avoid a significant increase in PAPR. It will help satisfy the requirement for increased uplink spectral efficiency while maintaining backward-compatibility with LTE.

The figure 2 shows a block diagram for the enhanced uplink multiple access (clustered SC-FDMA) process. There is only one transport block and one hybrid ARQ entity per scheduled component carrier. Each transport block is mapped to a single component carrier, and a UE may be scheduled over multiple component carriers simultaneously using carrier aggregation, as described in the previous section.

Examples of different Release 8 and Release 10 uplink configurations are given in figure 3. The key point is that all Release 8 configurations are single carrier, which means that the PAPR is no greater than the underlying QPSK or 16QAM modulation format, whereas in Release 10 it is possible to transmit more than one carrier, which makes the PAPR higher than the Release 8 cases. Note that the multiple carriers referred to here as part of clustered SC-FDMA and simultaneous PUCCH/PUSCH are contained within one component carrier and should not be confused with the multiple component carriers of CA.

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Figure 2: Enhanced uplink multiple access block diagram.

The initial specifications are likely to limit the number of SC-FDMA clusters to two, which will provide some improved spectral efficiency over single cluster when transmitting through a frequency-selective channel with more than one distinct peak.

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Figure 3: Comparison of Release 8 and proposed Release 10 uplink configurations.

Enhanced multiple antenna transmission
The downlink supports in the the Release-8 LTE limits for antenna ports and spatial multiplexing layers shows a maximum of four spatial layers of transmission (4x4, assuming four UE receivers) and the uplink a maximum of one per UE (1x2, assuming an eNB diversity receiver). In Release 8, multiple antenna transmission is not supported in order to simplify the baseline UE, although multiple user spatial multiplexing (MU-MIMO) is supported. In the case of MU-MIMO, two UEs transmit on the same frequency and time, and the eNB has to differentiate between them based on their spatial properties. With this multiuser approach to spatial multiplexing, gains in uplink capacity are available but single user peak data rates are not improved.

To improve single user peak data rates and to meet the ITU-R requirement for spectrum efficiency, LTE-Advanced specifies up to eight layers in the downlink which, with the requisite eight receivers in the UE, allows the possibility in the downlink of 8x8 spatial multiplexing. The UE will be specified to support up to four transmitters allowing the possibility of up to 4x4 transmission in the uplink when combined with four eNB receivers.

The work to define the enhanced downlink is about 80% complete. There will be changes to the UE-specific demodulation reference signal (DMRS) patterns to support up to eight antennas. Channel state information reference signals (CSI-RS) and associated modifications to UE feedback in the CSI codebook design will be introduced. There also will be equivalent changes for downlink control signaling.

- Moray Rumney
??Agilent Technologies

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