Don Bishop 2018-03-05 02:16:58
Both technologies have advantages to offer. One need not replace the other. The key is to use each system’s strengths to deliver maximum reliability. At the Network Infrastructure Forum conducted during the International Wireless Communications Expo in Las Vegas on March 27, 2017, Fernando Sommariva emphasized the need for reliability in critical situations when selecting distributed antenna system (DAS) technology for public safety communications. He works at Fiplex Communications as director of engineering, and he manages the company’s research and development team. Comparing analog DAS with digital DAS, Sommariva explained that digital DAS holds promise because it works well with the limited bandwidth normally used for public safety communications. He said analog DAS can have problems with harmonic generation, near-far communications and uplink noise. A digital fiber DAS can use automatic gain control to overcome near-far communications problems, and it can use squelch, a circuit function that acts to suppress the audio output of a receiver in the absence of a sufficiently strong desired input signal, to limit uplink noise. Sommariva gave what he said is an old definition of DAS that applies to both the analog and digital versions: A distributed antenna system is a network of spatially separated antenna nodes connected to a common source via a transport medium that provides wireless service within a geographic area or structure. Analog DAS Architecture Figure 1 shows the basic architecture of an analog DAS. Green lines represent fiber-optic cable, and gray lines represent RF paths. An analog DAS basically consists of a headend with remotes. “The headend is where the base station or similar boosters are located,” Sommariva said. “Different vendors use a point of interface (POI) connection or a master optical unit (MOU) to connect the base station or boosters with the master distribution unit (MDU). Among vendors, you will find various architectures in use.” An analog system uses analog modulation of light in the fiber. It has analog RF to fiber transceivers, which Sommariva said offers the advantage of wide bandwidth. He said a standard bandwidth is 50 MHz to 2.7 GHz, but analog systems can be made with even more bandwidth. The broadband remote units amplify full bands, so wherever you connect on the MOU will appear at the output of the remote units. But, said Sommariva, the analog system needs Class A headend, such as a Class A channelized signal booster, if the objective is to pick the signal off the air. He said that most public safety DAS networks pick the signal off the air from a macro system, which requires using a channelized head end to feed the DAS. “In general, you can get up to 12 miles in fiber length,” he said. Transceiver Harmonics With public safety systems that use VHF, UHF, 700.MHz and 800.MHz frequency bands, the wide bandwidth of an analog DAS means everything will fit. But along with this advantage comes the need to deal with analog transceiver harmonics, Sommariva said (see Figure 2). For example, the third harmonic of VHF could place interference in the UHF band. The fifth harmonic of VHF could place interference in the 700.MHz band, the public safety 800.MHz band and the cellular 850. MHz band, if you use the same fiber for cellular. The UHF second harmonic could place interference in the public safety 800.MHz band and the cellular 850.MHz band. Sommariva said the harmonics do not mean analog DAS cannot be used, but it means the harmonics have to be taken into account. He said sometimes preventing harmonic interference requires separating the fibers and having VHF on one fiber and UHF in a different fiber. “Bandwidth shouldn’t be an issue when choosing digital DAS instead of analog DAS for a public safety application because 18 megahertz can be achieved easily.” — Fernando Sommariva, director of engineering and manager of research and development at Fiplex Communications Sommariva identified two aspects of a public safety DAS that often determine how well a system responds in a critical situation and whether the system would pass inspection for customer acceptance. The aspects are the far-end problem (sometimes called the near-far problem) and the noise problem. The far-end problem occurs when a remote unit captures a strong signal, making it difficult to receive a weaker signal. Sommariva said the extent of the far-end problem is one of the big differences between a public safety radio application and a cellular application, and it stems from the uplink power of the portable (see Figure 3). “A cellular system controls the uplink power of the portable. A cellular handset or other device has a maximum power limit much lower than that of a public safety two-way radio, which probably is 5 watts,” Sommariva said. “Also, the cellular system controls the handset uplink power. For example, when a cellphone is right below an indoor antenna, it receives a good downlink signal. In response, the cellphone won’t send much power in the uplink — the cellular system controls its power and reduces its output.” Most public safety radios don’t have power control, and those that do have a range of control that is only a few decibels, Sommariva said. When someone uses a portable public safety radio right below an indoor antenna, the DAS reduces its gain to protect the receiver. If someone is using a second portable near the edge of the coverage, the receiver may not hear it because of the reduced gain. The Noise Problem The noise problem stems from the use of analog fiber-optic transceivers that typically have a -40 dB noise floor. Sommariva said that noise from the transceivers adds together when the remote transceivers all feed a common master, further reducing noise performance (see Figure 4). “That’s something that may or may not bother you at the master, but if it doesn’t, the noise will make the far-end problem worse,” he said. “The more remotes you have, the more uplink noise will be received by the master. It doesn’t mean that you cannot use one of these systems and deal with the noise and with the far-end problem. That’s something that you can definitely do. But it’s something that you need to understand.” Turning to compare analog and digital fiber DAS, Sommariva explained the difference (see Figure 5). “With analog fiber DAS, RF from the analog RF transceivers modulates the light inside the fiber,” he said. “So what the figure shows are two fiber transceivers. The one that receives the RF converts it into light, so you have a light in the fiber modulated by RF. The fiber attenuates the light as it travels. At the other end, a fiber-optic-to-RF converter captures the light and produces the RF output. That’s how analog DAS works.” With digital fiber DAS, Sommariva said an analog-to-digital converter uses a field-programmable gate array (FPGA) to sample the RF and converts it into light that’s modulated by digital 1s and 0s in what amounts to a digital-to-fiber-optic transceiver. A transceiver on the other end with a different FPGA converts the digitally modulated light into analog RF. “Sommariva dispelled the myth that digital transmission in fiber means more delay.” “The optical link budget differs from the RF link budget such that fiber length does not affect the RF link budget,” Sommariva said. “The output has the same amplitude. The output RF signal has no difference compared with the input signal for intermodulation interference, gain and noise. This makes the digital fiber DAS easier to deploy because all you have to do is make sure you are within the needed loss. Once you establish the optical link, you already have the RF. You can extend the fiber-optic cable as far as 25 miles, instead of 12, as with the analog DAS.” Sommariva dispelled the myth that digital transmission in fiber means more delay. He said delay depends on the signal processing applied on the FPGA. With no signal processing, the delay will be too low to be considered. However, he specified that if, for example, the FPGA applies channel filtering to reduce interference, there will be more delay, but the delay will be because of the filtering and not the fiber cable. “In any case, if you are going to pick the signal off the air, you are indeed going to use a channelized signal booster to feed the DAS, so you are using filters, anyway,” Sommariva said. “When filters are used, the delay will happen. But it’s because of filter theory, not because of using 1s and 0s.” Bandwidth As for the bandwidth in a public safety DAS, Sommariva said the good thing is that as little as 18 megahertz of bandwidth will do the job. “That’s something simple to put into fiber and convert,” he said. “Bandwidth shouldn’t be an issue when choosing digital DAS instead of analog DAS for a public safety application because 18 megahertz can be achieved easily.” Another good thing Sommariva mentioned about digital DAS is the ability to program the FPGA to use an AGC per channel and an AGC per time slot. He said the use of AGC in a digital DAS means that if someone using a 5-watt portable pushes to talk right below an antenna, the AGC will shrink the gain and protect the uplink. The AGC only shrinks the gain in the particular channel and timeslot that the close-in portable is using, so it allows other portables at the end of the coverage zone to have full gain available to talk out (see Figure 6). “AGC is needed because it tells you how the system will react during a critical situation,” Sommariva said. “When testing an indoor DAS for satisfactory performance, a fire marshal may test several rooms. But the fire department isn’t going to put 10 first responders inside the building and have them try to talk over the radio simultaneously. That’s not going to happen. That’s why AGC is key parameter, because it will reveal how the DAS will perform.” Squelch Circuit Function Squelch, a circuit function that acts to suppress the audio output of a receiver in the absence of a sufficiently strong desired input signal, is another function Sommariva recommends, and one that is available with digital DAS. He said because public safety communications systems don’t always have a dedicated base station, it’s common to place a donor antenna for the DAS that points toward an antenna site that provides coverage. If the DAS sends noise to that site, its coverage will shrink. This creates problems for the macro system (see Figure 7). “A digital DAS has the technology to offer a squelch per channel and a squelch per for time slot,” Sommariva said. “Systems such as P25 Phase II, TETRA and digital mobile radio have more than one time slot. Having multiple time slots means that a single frequency can carry more than one call. The squelch prevents other remotes from sending noise on that channel and in that time slot, ensuring a cleaner spectrum at the output.” Sommariva’s presentation centered on several characteristics of analog and digital DAS that help to make sure the systems deliver reliable communications, especially during critical situations. He said reliability should be the key factor when considering either analog or digital DAS for public safety communications.
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