Output RF spectrum emissions

UMTS specificities

In order to quantify the interferences and pinpoint their origins, the 3GPP has introduced the terms: adjacent channel leakage power ratio (ACLR), adjacent channel selectivity (ACS), and adjacent channel interference ratio (ACIR) [58].

ACLR: as said, measures the transmitter’s performance, and for UMTS FDD signals is specified as follows:

First theAdjacent-CLR is defined as the ratio between the power within 3.84MHz of a carrier filtered by a RRC-filter of 3.84MHz bandwidth having a roll-off factor of 0.22, and the power again within 3.84MHz of an adjacent channel located at +/-5MHz from the central frequency of the carrier of interest, and also filtered by a 3.84MHz wide RRC filter with a oll-off factor of 0.22.

TheAlternate-CLR is defined likeAdjacent-CLR yet the adjacent channels are called alternate and their central frequencies are located at +/-10MHz from the central frequency of the carrier of interest.

In the case of multiple carriers the previous definition applies to a chosen carrier (reference), and the relative measurements are done at +/-5MHz and +/-10MHz outside of the multiplex and apply to the lowest and highest carriers’ central frequencies of the multiplex.

Whether for a single or for multiple carriers, the 3GPP specifies downlink [59] Adjacent- and Alternate-CLRs of -45dBc and -50dBc maximum respectively (Fig. 3.23a).

(a) (b)

Figure 3.23.: Definition of ACLR for: (a) UMTS signals, (b) LTE signals

Henceforth the term ACLR describes Adj-CLR and Alt-CLR, unless otherwise stated.

Concerning the fourth mobile generation (4G), the ACLR specification (Fig. 3.23b) have similar values, but are expressed as a function of the signal’s bandwidth given the tun ability of the channel’s bandwidth in such systems (1.5, 3.5, 5, 10, 15, and 20MHz).

ACS: measures the receiver performance’s ability of receiving the desired signal while completely rejecting the adjacent carrier signal. Therefore it is defined as the ratio of the receiver filter attenuation on the assigned channel frequency to the receiver filter attenuation on the adjacent channel frequency.

Performances and practical implementations issues can be found in §3.2.1, p. 54.

ACIR: measures the overall system performance. It is defined as the ratio of the total power transmitted from a source (BS or UE) to the total interference power resulting from both transmitter and receiver imperfections affecting a victim receiver.

Thus the ACIR takes into account the two previously defined metrics, and for the downlink is expressed as :

𝐴𝐶𝐼𝑅_𝐷𝐿 = 1

1/𝐴𝐶𝐿𝑅_𝐵𝑆+ 1/𝐴𝐶𝑆_{𝑈 𝐸}

According to the 3GPP specifications, in the downlink the limiting parameter is set to be the Adjacent Channel Selectivity (ACS) at the UE, since the specifications foresee the BS to bear the main effort for limiting the level of the interferences (see §3.2.1). Thus typically:

𝐴𝐶𝑆_{𝑈 𝐸} =−33𝑑𝐵𝑐 and 𝐴𝐶𝐿𝑅_𝐵𝑆 =−45𝑑𝐵𝑐

Finally ACIR is mainly of interest in network simulation where the total amount of interference, rather than the source of the interference, is the primary concern [58].

Network impact & importance of the ACLR for an operator

According to [72], the performance of the Third Generation (3G) W-CDMA system is limited by interference. The main sources are thermal noise, intra-cell interference from traffic in the same cell, inter-cell interference from traffic in adjacent cells, and inter-operator interference generated from operators using adjacent frequencies.

While a network operator should be able to control the interferences generated by its RAN, it has no effective control over interferences generated by third parties.

If the ACLR is sufficiently bad, it can cause the so callednear-far problem, where a BS simply cannot communicate with a far away UE because of a high ACLR from a nearby adjacent channel BS.

In case of mobility, as the terminal (UE) moves away from its own operator’s BS and towards that of a different operator (on an adjacent carrier) the received signal’s strength generally diminishes and the interferences signal from the other operator will rise.

Inter-cell interferences from the own operator may also rise under these conditions. The inter-cell interference is typically rather strong on the cell edge and considerably weaker closer to the cell center while the opposite is true for the received energy of the intended transmission [63].

Figure 3.24.: Cell edge located interferences degrade the ACIR

Also the fact that a neighboring cell can use the same carrier frequency makes it difficult to apply higher-order modulation near the cell edge. Thus the service range of high-speed connections (using higher order modulation schemes, see §3.2.2) may be very limited [62].

This affects the region around the other operator’s BS creating a potential dead zone where, for the worst case, terminals suffer dropped calls due to insufficient link budget to satisfy the required Quality of Service (QoS) (schematically show in Fig. 3.24). Prior to link interruption, if the radio standards uses adaptive modulation schemes, the BS uses simpler and more robust modulation formats, resulting in a lowered bit rate.

Furthermore dead-zones and unwanted call drops in the cell, make the concerned part of the network less profitable [72].

Finally the needs are conflicting: from an equipment design perspective a relaxed ACLR specification is attractive, whereas from a network planning perspective, low ACLR is very desirable.

Spurious emission level examples for UMTS Taking into account the channel band- width of UMTS carriers (3.84MHz) we have for Category A and B upper limits of respectively -7.16dBm/3.84MHz and -9.16dBm/3.84MHz.