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ISO 2969 (1987). International Standard. Cinematography—B-Chain Electroacoustic Response of Motion-Picture Control Rooms and Indoor Theaters—Specifications and Measurements.”

5-1879677-5_TE Connectivity

ISO 2969 (1987). International Standard. Cinematography—B-Chain Electroacoustic Response of Motion-Picture Control Rooms and Indoor Theaters—Specifications and Measurements.”

Previous generation cellular systems have used multiple antenna techniques such as transmit and receive diversity and beamsteering to improve the link budget. In each of these cases, a single stream of data is sent between the base station and user equipment (UE). Release 8 of the 3GPP specifications, which defines the Long Term Evolution (LTE) towards 4th generation systems, includes new requirements for spatial multiplexing — also referred to as Multiple Input Multiple Output ( MIMO) — wherein the base station and UE communicate using two or more spatial streams. The goal is to increase both the overall capacity of a cell and the data rate that a single user can expect from the system.

As a result of the increasing data rates and flexibility, the design and test of LTE systems differs in many ways from previous generations of cellular technology. In particular, LTE receiver design and test present new challenges for which test equipment and measurement methods must be adapted.

5-1879677-5_TE Connectivity

Requirements for LTE receivers The 3GPP specifications define the LTE requirements that impact receiver design. For example, LTE must support the following changes:

Additionally, LTE can use transmit diversity (MISO) and receive diversity (SIMO) as well as beamsteering, either alone or in combination with MIMO. LTE specifies seven different downlink transmission modes, each of which is suited to different channel and noise conditions:

The terms codeword, layer, precoding, and beamforming have been adopted specifically for LTE to refer to signals and their processing. The terms are used in the following ways:

5-1879677-5_TE Connectivity

Figure 1 shows how two codewords are used for a single user in the downlink. It is also possible for the codewords to be allocated to different users to create multi-user MIMO (MU-MIMO).

The 3GPP receiver conformance tests for LTE require performance measurements, including MIMO performance measurements, on the entire receiver. However, before performance can be measured, the basic receiver sub-blocks must first be verified and the source of specific distortions quantified and reduced. These basic measurements are made earlier during the receiver design phase. If multiple receivers are used in a system, it is necessary to make the basic measurements on each receive chain separately before attempting to verify MIMO performance. A simplified block diagram of a typical LTE transceiver is shown in figure 2.

5-1879677-5_TE Connectivity

Modern receivers utilize the same building blocks as classic designs; however, today there is a higher degree of integration with single components performing multiple functions. Testing these components can be more difficult, particularly in handsets, because space is at a premium and there will be fewer places where signals can be injected or observed for testing.

FIGURE 18.10 (a) Loudspeaker I.” Top: the full anechoic data. Bottom: in-room measurements with the loudspeaker in the front-left location averaged over several head locations within a typical listening area in a typical rectangular room. As a means of understanding which sounds from the loudspeaker contribute to this measurement, the early-reflections” curve from the anechoic data set at the top has been superimposed. It is not a perfect match for the room curve, but it is not far off. Obviously, it is the off-axis performance of the loudspeaker that is the dominant factor in determining the sound energy at the listening location, and, in the several comparisons that have been done, the early-reflections curve seems to be a better fit than the sound power curve. As noted in Figure 18.5, the on-axis curve is the dominant factor at high frequencies. If some of the high-frequency portion of the on-axis” data were added to the early-reflections” data, the resulting curve would be an even better fit to the measured room curve. The inverted directivity index (DI) curve is included just to add support to the concept that the directivity of the loudspeaker is a factor not to be ignored, although in common audio discourse, it routinely is. As is expected, the standing waves in the room take control at low frequencies and the prediction fails. (b) Loudspeaker B.” This loudspeaker has directivity problems. In the DI curve, one can see the directivity of the woofer rising with frequency and then falling in the crossover around 350 Hz to a rather large (6-in., 150 mm) midrange driver that exhibits increasing directivity up to around 2 kHz before crossing over to an unbaffled tweeter with wide dispersion, which by 5 kHz has taken over. This behavior is clearly seen in the family of frequency responses, even at small-angles. There is a significant difference between the on-axis and the listening-window curves. In the lower box of curves, the pronounced mid- to upper-frequency undulations seen in the room curve are clearly associated with the off-axis behavior of this loudspeaker system. The shape of the room curve is clearly signaled in the shapes of both the early-reflections” curve and the inverted DI. As in (a), the addition of some on-axis” to the early-reflections” curve will improve the match at very high frequencies. However, the on-axis frequency response by itself is not a useful indicator of how this loudspeaker will perform in a room.

The next step is to try to understand where these metrics stand in the hierarchy of measurements that are usefully revealing of perceived sound quality in rooms. But first, a short digression into an important part of the industry and how its standardization practices compare with what has just been discussed.

Sound reproduction in homes began with the belief that it is necessary to understand what the loudspeaker is doing. It hasn't been very difficult – the loudspeakers are smallish, the far field is not very far away, and anechoic chambers can be built for, admittedly large, but affordable sums of money. We have just seen that with enough anechoic measurements, it is possible to get impressively close to predicting the shape of room curves, even in nonspecific, typically furnished, rooms – except, of course, at low frequencies.

In professional audio, things started off quite differently. Sound reinforcement loudspeakers for auditoriums are large, heavy, and used in arrays, aimed in different directions to cover a widely distributed audience. Measuring them is a physical and acoustical challenge; the far field of an array is a long distance away. Consequently, room curves (or house curves” as they are known in pro audio) were really all that could be measured once a system was assembled, usually at several locations throughout the audience. Early instruments permitted only steady-state measurements, using warbled tones and, later, bandpassfiltered pink noise.


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