The preceding chapters have extensively explained the state-of-the-art technical features that significantly improve the system performance of LTE compared to legacy systems. The main difference in both downlink and uplink between LTE Release 8 and UMTS Release 6 (HSDPA/HSUPA) is that the LTE system provides orthogonal resource allocation in the frequency domain, which enables frequency-domain multi-user diversity gain to be exploited.
In addition, the LTE downlink supports transmission with up to two or four spatial layers via
1Universal Mobile Telecommunications System.
2High Speed Downlink Packet Access.
3High Speed Uplink Packet Access.
Stefania Sesia, Issam Toufik and Matthew Baker.
LTE – The UMTS Long Term Evolution: From Theory to Practice,Second Edition.
©2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd.
multiple antennas, which enhances both the peak data rate and the cell average and cell edge spectral efficiencies. The key features are discussed in more detail in the following sections.
26.2.1 Multiple Access Techniques
Downlink OFDMA. As explained in Chapter 5, the LTE downlink is based on Orthogonal Frequency Division Multiple Access (OFDMA) which enables flexible channel-dependent multi-user resource allocation in both the frequency and time domains as illustrated in Figure 5.12. This leads to improved multi-user diversity gain. Inter-Symbol Interference (ISI) reduction by means of the Cyclic Prefix (CP) leads to a simplified receiver structure, which is well suited to Multiple-Input Multiple-Output (MIMO) transmission.
Uplink SC-FDMA. The Single-Carrier Frequency Division Multiple Access (SC-FDMA) scheme used for the LTE uplink (as explained in Chapter 14) achieves frequency-domain intra-cell orthogonality among User Equipment (UEs) while also maintaining a low Peak- to-Average Power Ratio (PAPR) which is important for maximizing data rates at the cell edge. In addition, the Sounding Reference Signals (SRSs) supported by the LTE uplink (see Section 15.6) facilitate multi-user scheduling and rate adaptation strategies to enhance spectral efficiency.
26.2.2 Frequency Reuse and Interference Management
Similarly to WCDMA,4 LTE is designed to operate with a frequency reuse factor of one to maximize the spectral efficiency. In such a system, however, data and control channels can experience a significant level of interference from neighbour cells, which reduces the achievable spectral efficiency, especially at the cell edge. LTE therefore supports various techniques to manage and mitigate inter-cell interference.
In the downlink, these include:
• A cell-specific frequency-shift is applied to the mapping of cell-specific Reference Sig- nals (RSs) to subcarriers to avoid inter-cell RS collisions, as described in Section 8.2.1.
• With respect to the downlink control channels, a cell-specific frequency offset is applied to the PCFICH and PHICH5positions, as described in Sections 9.3.3 and 9.3.4;
for the Physical Downlink Control CHannel (PDCCH), interleaving provides fre- quency diversity and enhances the robustness against inter-cell interference (see Section 9.3.5.1).
• For the data, Inter-Cell Interference Coordination (ICIC) techniques can be applied by utilizing the Relative Narrowband Transmit Power (RNTP) messages that can be exchanged among eNodeBs over the X2 interface as explained in Section 12.5.
Uplink interference mitigation techniques in LTE include the following:
• Fractional power control (see Section 18.3.2.1) is supported to improve the throughput near the eNodeB and mitigate inter-cell interference at the cell edge. Power control
4Wideband Code Division Multiple Access.
5Physical Control Format Indicator CHannel and Physical HARQ Indicator CHannel.
can be performed jointly with frequency-domain resource allocation, whereby cell- centre UEs are allocated more Resource Blocks (RBs) to enhance the data rate, while cell-edge UEs are allocated fewer RBs for coverage extension. When devising power control strategies for interference management, it is important to control the ratio of the Interference over Thermal noise (IoT) below a target level.
• Various means are provided to avoid inter-cell RS collisions, including cyclic shift hopping, sequence-group hopping and planning, as explained in Sections 15.3 and 15.4.
• For the Physical Uplink Control CHannel (PUCCH), cell-specific symbol-level cyclic shift hopping is applied for inter-cell interference randomization, as explained in Section 16.3.3. Each PUCCH RB pair is mapped to both edges of the system bandwidth to achieve frequency diversity. In addition, the fact that the PUCCH is mapped to different RBs in the frequency domain from those of the Physical Uplink Shared CHannel (PUSCH, see Section 16.3.1) means that independent interference management techniques (such as power control) can be applied for the control and data channels.
• Uplink ICIC techniques can be applied using the Overload Indicator (OI) and High Interference Indicator (HII) messages that can be exchanged between eNodeBs over the X2 interface as explained in Section 12.5.
Scrambling is applied to the data and control channels, and to the downlink RSs, to randomize inter-cell interference.
26.2.3 Multiple Antenna Techniques
Downlink Spatial Multiplexing and Diversity. The multiple antenna schemes supported by LTE contribute much to the overall spectral efficiency gain with respect to Release 6 of UMTS. The performance of the open-loop and closed-loop spatial multiplexing modes supported in LTE is shown in Section 11.2.4.
Uplink Multi-User MIMO. In the LTE uplink, as explained in Section 16.6.2, orthogonal demodulation RSs can be assigned to multiple UEs, thus enabling the eNodeB receiver to estimate the channel of multiple UEs scheduled simultaneously to transmit in the same set of RBs.
26.2.4 Semi-Persistent Scheduling
The Semi-Persistent Scheduling (SPS) provided in LTE (see Section 4.4.2.1) can alleviate pressure on the limited downlink control channel capacity by replacing dynamic scheduling signalling with semi-static signalling. This allows a larger number of UEs to be scheduled, which is especially beneficial for services such as Voice over IP (VoIP).
26.2.5 Short Subframe Duration and Low HARQ Round Trip Time
LTE has a subframe duration of 1 ms for both uplink and downlink – shorter than the 2 ms subframe duration of UMTS. This leads to reduced latency (with a shorter HARQ6 Round Trip Time (RTT)) and more flexible multi-user scheduling in the time domain.
26.2.6 Advanced Receivers
Advanced receivers provide an implementation method to enhance further the capacity of the LTE system. A typical example is the Linear Minimum Mean Squared Error (LMMSE) receiver with Interference Rejection Combining (IRC) [4]. Suitable for both uplink and downlink, such receivers compute the signal combining weights by exploiting statistical knowledge, such as the covariance matrix, of the inter-cell interference (unlike Maximum Ratio Combining (MRC) receivers which do not take the spatial characteristics of the interference into account). The ability of an IRC receiver to suppress interference is a function of many factors including the number and strength of the interfering signals and the number of receive antennas. For a single dominant source of interference, the mean Signal-to- Interference Ratio (SIR) gain that IRC is able to offer relative to an MRC receiver improves with the interference-to-thermal-noise ratio and not with the wanted Signal-to-Noise Ratio (SNR). However, if significant interference arrives from more sources or directions, an IRC receiver with only a small number of antennas is limited in the amount of interference suppression it can provide, especially if the multiple interference sources are received with similar powers.
Other more advanced receiver structures may also be considered, such as MMSE receivers with Successive Interference Cancellation (SIC) (particularly for downlink Single-User MIMO (SU-MIMO)) and Maximum Likelihood Detection (MLD) (see Section 11.1.3.3).
26.2.7 Layer 1 and Layer 2 Overhead
Any part of the time-frequency transmission resources that are not used directly for data transmission constitutes an overhead when considering the overall spectral efficiency. One design criterion for LTE was to minimize these overheads while achieving high system performance and flexibility. Table 26.1 summarizes the major sources of Layer 1 and Layer 2 overhead in the LTE downlink transmissions in a 10 MHz Frequency Division Duplex (FDD) deployment, as a percentage of the total transmission resources over the duration of a 10 ms radio frame.7The main contributors are guard bands, the OFDM CP, RSs, and control channels. It can be seen that the percentage overhead increases with the number of transmit antennas, due to the higher RS overhead for the larger number of antenna ports. The gain from MIMO therefore has to more than offset this increased overhead if it is to be worthwhile.
6Hybrid Automatic Retransmission reQuest.
7Note that transmission mode 7 (see Section 9.2.2.1) is not considered in these examples, and therefore the overhead due to UE-specific RSs is not taken into account. A worst-case downlink control channel duration of three OFDM symbols per subframe is used here, although in practice LTE supports dynamic resource allocation for the downlink control channels, so the average control channel duration would depend on the deployment scenario.
Table 26.1: Examples of percentage overhead in the LTE FDD downlink (calculated over a 10 ms radio frame for a 10 MHz system bandwidth).
1 antenna 2 antenna 4 antenna Illustration
Source of overhead port ports ports
Guard bands (1 MHz) 10.0 10.0 10.0 Figure 21.1
OFDM CP 6.0 6.0 6.0 Figure 5.13
Cell-specific RSs 4.0 8.0 12.0 Figures 8.2 & 8.3 Control channels (3 symbols) 17.0 16.0 14.0 Figure 9.5 Synchronization signals 0.29 0.29 0.29 Figure 7.4
PBCH 0.28 0.26 0.24 Figure 9.1
Total (%) 37.6 40.6 42.6
Table 26.2 similarly summarizes the major sources of Layer 1 and Layer 2 overhead in the LTE FDD uplink. Guard bands, CP, demodulation RSs and PUCCH constitute the main sources of uplink overhead.8
Table 26.2: Examples of percentage overhead in the LTE FDD uplink (calculated over a 10 ms radio frame for a 10 MHz system bandwidth).
Source of overhead Overhead (%) Illustration
Guard bands (1 MHz) 10.0
SC-FDMA CP 5.9 Table 14.1
PUSCH demodulation RSs (2 symbols per subframe) 6.0 Figure 15.7
PUCCH (4 RBs) 6.7 Figure 16.2
RACH (6 RBs, 10 ms period) 1.1 Figure 17.5
SRS (48 RBs bandwidth, 10 ms period) 0.55 Table 15.1
Total (%) 30.3