Part II Physical Layer for Downlink 121
8.2 Design of Reference Signals in the LTE Downlink
8.2.1 Cell-Specific Reference Signals
The cell-specific RSs enable the UE to determine the phase reference for demodulating the downlink control channels (see Section 9.3) and the downlink data in most transmission modes1 of the Physical Downlink Shared Channel (PDSCH – see Section 9.2.2). If UE- specific precoding is applied to the PDSCH data symbols before transmission,2 downlink control signalling is provided to inform the UE of the corresponding phase adjustment it should apply relative to the phase reference provided by the cell-specific RSs. The cell- specific RSs are also used by the UEs to generate Channel State Information (CSI) feedback (see Sections 10.2.1 and 11.2.2.4).
References [7, 8] show that in an OFDM-based system an equidistant arrangement of reference symbols in the lattice structure achieves the Minimum Mean-Squared Error (MMSE) estimate of the channel. Moreover, in the case of a uniform reference symbol grid, a ‘diamond shape’ in the time-frequency plane can be shown to be optimal.
In LTE, the arrangement of the REs on which the cell-specific RSs are transmitted follows these principles. Figure 8.2 illustrates the RS arrangement for the normal CP length.3
1Transmission modes 1 to 6.
2In PDSCH transmission modes 3 to 6.
3In the case of the extended CP, the arrangement of the reference symbols changes slightly, but the explanations in the rest of this chapter are no less valid. The detailed arrangement of reference symbols for the extended CP can be found in [6].
0 0
0
0
0 0
0
0
R R
R R
R R
R R
Time
Frequency
The LTE system is designed to work under high-mobility assumptions, in contrast to WLAN systems which are generally optimized for pedestrian-level mobility. WLAN systems typically use a preamble-based training sequence, and the degree of mobility such systems can support depends on how often the preamble is transmitted.
The required spacing in time between the reference symbols can be determined by considering the maximum Doppler spread (highest speed) to be supported, which for LTE corresponds to 500 km/h [9]. The Doppler shift is fd=(fcv/c) where fc is the carrier frequency, v is the UE speed in metres per second, and c is the speed of light (3ã108 m/s). Considering fc=2 GHz and v=500 km/h, then the Doppler shift is fd≃ 950 Hz. According to Nyquist’s sampling theorem, the minimum sampling frequency needed in order to reconstruct the channel is therefore given byTc=1/(2fd)≃0.5 ms under the above assumptions. This implies that two reference symbols per slot are needed in the time domain in order to estimate the channel correctly.
In the frequency direction, there is one reference symbol every six subcarriers on each OFDM symbol that includes reference symbols, but the reference symbols are staggered so that within each Resource Block (RB) there is one reference symbol every threesubcarriers, as shown in Figure 8.2. This spacing is related to the expected coherence bandwidth of the channel, which is in turn related to the channel delay spread. In particular the 90% and 50%
coherence bandwidths4are given byBc,90%=1/50στandBc,50%=1/5στrespectively, where στis the r.m.s delay spread. In [10], the maximum r.m.s channel delay spread considered is 991 ns, corresponding toBc,90%=20 kHz andBc,50%=200 kHz. In LTE, the spacing between two reference symbols in frequency, in one RB, is 45 kHz, thus allowing the expected frequency-domain variations of the channel to be resolved.
Up to four cell-specific antenna ports, numbered 0 to 3, may be used by an LTE eNodeB, thus requiring the UE to derive up to four separate channel estimates.5For each antenna port, a different RS pattern has been designed, with particular attention having been given to the minimization of the intra-cell interference between the multiple transmit antenna ports. In Figure 8.3,Rp indicates that the RE is used for the transmission of an RS on antenna port p. When an RE is used to transmit an RS on one antenna port, the corresponding RE on the other antenna ports is set to zero to limit the interference.
From Figure 8.3, it can be noticed that the density of reference symbols for the third and fourth antenna ports is half that of the first two; this is to reduce the overhead in the system.
Frequent reference symbols are useful for high-speed conditions as explained above. In cells with a high prevalence of high-speed users, the use of four antenna ports is unlikely, hence for these conditions reference symbols with lower density can provide sufficient channel estimation accuracy.
All the RSs (cell-specific, UE-specific or MBSFN-specific) are QPSK modulated – a constant modulus modulation. This property ensures that the Peak-to-Average Power Ratio (PAPR) of the transmitted waveform is kept low. The signal can be written as
rl,ns(m)= 1
√2[1−2c(2m)]+j 1
√2[1−2c(2m+1)] (8.1)
4Bc,x%is the bandwidth where the autocorrelation of the channel in the frequency domain is equal tox% of the peak.
5Any MBSFN and UE-specific RSs, if transmitted, constitute additional independent antenna ports in the LTE specifications.
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Figure 8.3: Cell-specific RS arrangement in the case of normal CP length for (a) two antenna ports, (b) four antenna ports. Reproduced by permission of©3GPP.
wheremis the index of the RS,nsis the slot number within the radio frame and ‘l’ is the symbol number within the time slot. The pseudo-random sequence c(i) is comprised of a length-31 Gold sequence, already introduced in Chapter 6, with different initialization values depending on the type of RSs. For the cell-specific RSs, the sequence is reinitialized at the
start of each OFDM symbol, with a value that depends on the cell identity,NIDcell. The cell- specific RS sequence therefore carries unambiguously one of the 504 different cell identities.
A cell-specific frequency shift is applied to the patterns of reference symbols shown in Figures 8.2 and 8.3, given byNIDcellmod6.6This shift helps to avoid time-frequency collisions between cell-specific RSs from up to six adjacent cells. Avoidance of collisions is particularly relevant in cases when the transmission power of the RS is boosted, as is possible in LTE up to a maximum of 6 dB relative to the surrounding data symbols. RS power-boosting is designed to improve channel estimation in the cell, but if adjacent cells transmit high-power RSs on the same REs, the resulting inter-cell interference will prevent the benefit from being realized.