The unique characteristics of small cells, and in particular the potential interference scenarios, give rise to specific Radio Frequency (RF) challenges and requirements. These are explained in this section and compared to those defined for macrocells (see Chapter 21). In Release 8 of the LTE specifications, RF requirements were defined only for the Wide Area eNodeB class which covers macrocell applications, while Release 9 saw the introduction of the Local Area eNodeB class (covering picocell applications) and the Home eNodeB class (covering femtocell applications).
A major factor influencing the RF requirements for pico and Home eNodeBs is the lower minimum separation distance (and hence lower coupling loss) between UEs and the pico or Home eNodeBs when compared to the Wide Area eNodeB case. As explained in
Section 24.1, picocells tend to be small cells planned by the network operator, with the pico eNodeBs mounted at low elevation (e.g. interior walls), in contrast to femtocells for which the Home eNodeBs are typically self-installed in a home environment by the end user.
Both picocells and femtocells will have lower Minimum Coupling Loss (MCL) to UEs than macrocells. Typically an MCL of 70 dB is assumed for macrocells compared to 45 dB for picocells and femtocells [6, 7].
Other factors influencing the differences from the Wide Area eNodeB requirements include lower transmit power, degraded receiver sensitivity (reflecting the lower cost), and for Home eNodeBs, the lack of a planned deployment. For these reasons in many cases the Home eNodeB class generally has the most stringent RF requirements, with the Local Area class requirements being intermediate between those of the Wide Area and the Home classes.
24.4.1 Transmitter Specifications
A comparison of transmission requirements between Home, Local Area and Wide Area eNodeBs is provided in Table 24.2.
Table 24.2: Summary of transmitter requirements.
Requirement Wide Area Local Area Home
Maximum Output Powera(MOP) (dBm) 46 24 20
Power adjustment for adjacent channel No No Yes
Adjacent Channel Leakage Ratio (ACLR) lower limit (dBm) −15 −32 −50
Spurious emissions (dBm/100 kHz)b −62 −62 −71
Frequency Error (ppm) 0.05 0.10 0.25
aSummed over transmit antennas.
b For co-existence with UTRA or E-UTRA downlinks in other bands.
Maximum Output Power (MOP). For the Local Area class, the MOP was selected to provide acceptable performance loss to overlaid Wide Area macrocellular systems on the same or adjacent carriers, and also to ensure safety from electromagnetic radiation [8, 9]. For the Home eNodeB class, the adjacent channel selectivity of UEs was also taken into account, such that UEs are still able to operate when a HeNB at 45 dB MCL is transmitting in a channel adjacent to that being received by the UE [7].
An HeNB is required to detect if the downlink adjacent channel is being used by another operator for either UTRA or E-UTRA, and to set its MOP according to the measured power of the adjacent channel base station and the total measured noise plus interference in its channel,Ioh[6]. This aims to reduce the impact of adjacent channel leakage on neighbouring operators. For a UTRA victim system the requirements are the same as for a UMTS Home NodeB (HNB) [10]. For an LTE victim system, different MOP levels can be set depending on the value of the cell-specific Reference Signal (CRS) Received Power (RSRP) per Resource Element (RE) of the adjacent-channel eNodeB, ˆEc,CRS[6]. If ˆEc,CRS<−127 dBm, then there are unlikely to be UEs connected to this adjacent channel eNodeB in the vicinity of the
HeNB, since the signal is weak, so no power reduction by the HeNB is required. If ˆEc,CRS is less than a threshold, then it is considered that the power measurement on the adjacent channel is unreliable due to adjacent channel interference from the uplink channel being used by the HeNB, and the MOP is set to an intermediate value of 10 dBm. Otherwise, the MOP is set between 8 and 20 dBm depending on the value of ˆEc,CRS[6].
Adjacent Channel Leakage Ratio. For all eNodeB classes the ACLR is defined by a relative value (45 dBc), subject to an absolute lower limit [6]. For the HeNB class this lower limit is lower than Wide Area eNodeB class due to the lower MCL [11].
Spurious emissions. Spurious emissions limits are defined for co-existence with HeNBs operating in other frequency bands. In deriving the HeNB requirement it was assumed that the HeNB is in an adjacent apartment to a neighbouring HNB or HeNB operating in a different band, and that an MCL of 47 dB applies in this case. Moreover, a 0.8 dB desensitization criterion was assumed [12]. Spurious emission requirements for the protection of the receiver band (Home and Local Area FDD classes), and for co-located base stations (Local area class) are all relaxed by 8 dB, corresponding to the relaxation in the sensitivity requirement relative to the Wide area eNodeB.
Frequency error. Table 24.2 also shows the requirements for frequency error. The total error seen by a UE is the sum of the errors due to frequency error at the eNodeB and Doppler shift due to mobility. For Home and Local Area eNodeBs the mobility is assumed to be restricted to 30 km/h and 50 km/h respectively, compared to a maximum speed of 350 km/h for Wide Area eNodeBs. Therefore for the same total error, the component arising from frequency error can be larger for the Home and Local Area eNodeB classes.
In [12] it is shown that 0.25 ppm is a sufficient accuracy considering handover measurement performance, demodulation performance and maintenance of timing synchronization for TDD HeNBs. For the Local Area eNodeB class the frequency error is the same as that already supported in GSM and UTRA Local Area base stations, i.e. 0.1 ppm [13].
Spectrum Emission Mask (SEM). The SEM for the Home and Local Area eNodeB classes differs from the Wide Area class in both the absolute value and the level relative to the carrier power. For the Home class, the mask at high frequency offsets is a function of the transmitted power (summed over all antennas), subject to a minimum of 2 dBm and a maximum of 20 dBm. This is shown in Figure 24.7 for different system bandwidths.
24.4.2 Receiver Specifications
This section introduces the receiver specifications for the Local Area and Home eNodeB classes. Table 24.3 summarizes the wanted and interfering signal levels for these classes for the case of a 10 MHz LTE system.
Reference sensitivity and noise figure. For Local Area and Home eNodeBs the maximum path-loss between a served UE and the eNodeB is considerably less than for a Wide Area eNodeB. In addition, lower implementation cost is important, especially for HeNBs. For
these reasons the assumed noise figure for Local Area and Home eNodeBs is 13 dB, 8 dB higher than for Wide Area eNodeBs.
Figure 24.7: Spectrum Emission Mask for Home eNodeB class.
Table 24.3: Wanted and interfering signal power level requirements for 10 MHz system bandwidth.
Wide Area Local Area Home Area
Wanted Inter- Wanted Inter- Wanted Inter- Signal ference Signal ference Signal ference
(dBm) (dBm) (dBm) (dBm) (dBm) (dBm)
Reference sensitivity −101.5 – −93.5 – −93.5 –
Dynamic range −70.2 −79.5 −62.2 −71.5 −25.7 −35.0
In-channel selectivity −98.5 −77.0 −90.5 −69.0 −90.5 −69.0 Narrowband blocking −95.5 −49.0 −87.5 −41.0 −79.5 −33.0 ACS −95.5 −52.0 −87.5 −44.0 −71.5 −28.0
Receiver IM −95.5 −52.0 −87.5 −44.0 −79.5 −36.0
Out-of-band blocking −95.5 −15.0 −87.5 −15.0 −79.5 −15.0
In-band blocking −95.5 −43.0 −87.5 −35.0 −79.5 −27.0
Co-located blocking∗ −95.5 +16.0 −87.5 −6.0 −− −−
∗For the Local Area class these requirements are adopted in the case of co-location with UTRA or E- UTRA (however, the interfering signal level is slightly different in the case of co-location with GSM/DCS – see [6, Table 7.6.2.1-2]).
This increase of the noise figure has an impact on several other receiver requirements.
Specifically, both the wanted signal level and interfering signal level are increased by 8 dB for the in-channel selectivity (for both Local Area and Home classes), in-band blocking (for the Local Area class), narrowband blocking (for the Local Area class) and Adjacent Channel Selectivity (ACS) (for the Local area class).
Dynamic range. The required dynamic range in an eNodeB depends not only on the ability to control the transmit power of the uplink transmissions from served UEs, but also on the interference levels seen from UEs in neighbouring cells, in particular from co-channel UEs served by the macrocellular network which could be transmitting with high power. For the Home eNodeB class operating in closed access mode, the fact that the deployment is unplanned means that such co-channel MUEs could come close to the HeNB. However, as discussed in Section 24.3.1, in such cases the HeNB will create a ‘deadzone’ around itself due to the interference caused to MUEs in the downlink. As a result of this deadzone, a coupling loss higher than the MCL of 45 dB was assumed when deriving the dynamic range requirement.
Blocking, Narrowband Blocking, Adjacent Channel Selectivity (ACS) and Receiver Intermodulation. The eNodeB blocking requirements consist of three components as shown in Figure 24.8. The first component is Out-Of-Band (OOB) blocking, which is defined over a wide frequency range,7excluding the operating band.
The second component is the in-band blocking, for which the interfering signal is an LTE signal at a specified frequency separation from the wanted signal. For the HeNB class, the interfering signal level is increased relative to the other classes to take account of the unplanned nature of HeNB deployments. This is also the case for the narrowband blocking and ACS requirements. For these cases, system simulations were used to define the interference powers [12].
The third component is the co-located blocking, which is defined for co-located (e)NodeBs operating in other frequency bands. No requirements are currently defined for the HeNB class. For the Local Area class, requirements are defined which assume the same 30 dB coupling loss as for the Wide Area class and a maximum transmit power of 24 dBm from interfering Local Area E-UTRA eNodeBs.
Due to the lower probability of two large interfering signals being present simultaneously, the power level of the interfering signals for the inter-modulation requirement is lower compared to the blocking requirement [12].
24.4.3 Demodulation Performance Requirements
For the Home eNodeB class, the maximum served UE speed is assumed to be 30 km/h which corresponds to a maximum Doppler of about 70 Hz at a 2.5 GHz carrier frequency. For the Local Area class, the maximum served UE speed is assumed to be 50 km/h. Furthermore, due to the small cell size in the indoor environment, for both these classes the multipath delays tend to be significantly smaller than for the Wide Area eNodeB class. Therefore only a subset of the Wide Area demodulation performance requirements are specified, consisting of the
7For most operating bands, it is defined for a frequency range of 20 MHz either side of the operating band.
Figure 24.8: Blocking requirements.
EPA and EVA propagation models8with a maximum Doppler frequency no larger than 70 Hz.
In these cases the performance requirements are the same as for the Wide Area eNodeB class. In addition for the Home class, new requirements are defined for the decoding of Hybrid Automatic Repeat reQest ACKnowledgements (HARQ-ACKs) and channel quality information feedback.
24.4.4 Time Synchronization for TDD Operation
Time synchronization in TDD systems is an important consideration for the avoidance of interference between uplink and downlink transmissions on neighbouring eNodeBs (see Section 23.3). For the HeNBs this is particularly important in view of the potential inter- ference scenarios and unplanned deployment. Therefore a mandatory accuracy requirement is specified for HeNBs, even though, in common with all eNodeBs, no specific method is mandated. The difference in radio frame start timing, measured at the transmit antenna connectors, between the HeNB and any other HeNB or eNodeB which has overlapping coverage [12] is required to be less than 3μs, except in cases where timing is obtained by monitoring another eNodeB which is more than 500 m away. In this case the requirement is relaxed in line with the additional one-way propagation delay beyond 500 m.
The requirement is designed to ensure that the combination of synchronization error, propagation delay to a victim, and multipath delay spread remains less than the smallest Cyclic Prefix (CP) length, taking into account that multipath delay spreads tend to be small in femtocells; if this condition is satisfied, interference at the uplink/downlink switching points within the TDD radio frame will be avoided.
8Extended Pedestrian A and Extended Vehicular A – see Chapter 20.
-15 all classes
1
+16 wide area
-6 local area
-27 home
Colocation band
Frequency (MHz) 12500
20 MHz
-27 home
Blocker Power (dBm)
-43 wide
Operating band -35 local