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 Technical Considerations For Concerting From Analog To Digital Microwave Radio By Bob Verlander Although digital microwave radio (DMR) has been around for over twenty-five years, there remains a surprisingly large amount of analog microwave radio (AMR) still in service today. This paper is intended to introduce those users of AMR who wish to convert to DMR but are not familiar with DMR concepts or terminology, to the differences between the two technologies and the effect that their differences will have on converting from one to the other, particularly in the areas of: Performance, Reliability, Cutover and, Cost. The priority of each area will vary depending on the primary use of the system. For example, a system used primarily for Public Safety applications would place more emphasis on cutover and reliability than cost. However, needless to say, the first three can always be improved to any degree that the last, i.e., the pocketbook, will withstand. This article
will address the different units used to measure system quality of
performance, the effect microwave radio signal fading has on AMR and DMR,
Link reliability calculations, as well as Cutover considerations and
Licensing considerations.
The major impairment to the quality and reliability of a working microwave radio link is the effect that atmospheric anomalies have on the signal as it propagates from one end of the link to the other, more commonly referred to as fading. Link fading is divided into two basic categories:
Although AMR and DMR signals are each susceptible to the same transmission anomalies when transmitted over the same link at the same frequency, their reaction to the effects of frequency selective and nonfrequency selective are manifested entirely differently. Simply put: frequency selective fading has little noticeable effect on AMR links and a large noticeable effect on DMR links. Conversely, non-frequency selective fading has a large noticeable effect on AMR links and a small noticeable effect on DMR links. For this reason, one should not assume that a DMR link will perform satisfactorily over a given link simply because the AMR link’s performance has been satisfactory (see Figure 1).
Figure 1 
Different Microwave Radio Fade Margins In the AMR world the term fade margin (FM) means the difference between the normal un-faded Received Signal Level (RSL) and the receiver threshold (RxTH) as defined by the RSL required to cause the worst 3kHz slot of the receiver’s baseband to have a 30 dB S/N and is equal to: Fade Margin (dB) = System Gain (dB) – Net Path Loss (dB) where: System Gain = Absolute Sum RxTH (dBm) + TxOUT (dBm) Net Path Loss (NPL) (dB) = See Figure 2
Figure 2
Shortly after DMR was introduced, other “fade margin” terms appeared to account for the different effect that frequency selective fading has on DMR. These include Composite Fade Margin (CFM), Dispersive Fade Margin (DFM) and Flat Fade Margin (FFM) also referred to as Thermal Fade Margin (TFM). where: CFM = -10 Log (10-FFM/10 + 10-DFM/10) FFM = Analogous to analog radio’s FM, is controlled by the system designer and is the difference between normal RSL and the RSL required to produce a 10-3 or 10-6 BER on a DMR DFM = Not controlled by system designer. A DMR equipment parameter that is unique to each manufacturer’s equipment and serves as a figure of merit defining the equipment’s ability to deal with the effects of frequency selective or dispersive type fading. Microwave Radio Link Reliability The generally accepted formula used to calculate the annual outage probability (U) resulting from frequency selective type fading of a non-diversity microwave radio path is the classic formula U (non-diversity) = 2.5 c*f*D*3x10-6 x T0 x 10-FM/10 (Vigants, 1974) Where: U = non-diversity annual outage probability due to frequency selective fading D = path length in miles f = frequency in GHz FM = Analog: Flat Fade Margin (FFM) Digital: Composite Fade Margin (CFM) c = climate and terrain factor 0.25 = mountainous and dry climate 1.0 = average terrain and climate 4.0 = over water and gulf coast climate T0 = duration of fading season 0.375 = southern Temperatures (75 F) 0.250 = average Temperatures (50 F) 0.175 = northern Temperatures (35 F) The difference in the use of the above formula for AMR and DMR link design is the value of FM. For AMR links, it is well agreed that the fade margin used for link reliability calculations is the difference, in dB, between the link’s equipment system gain and its net path loss (NPL). But for DMR links, there is not the same level of agreement. Although the use of the CFM term and its formula are widely accepted, there are differences in opinion over the value of RxTH used to derive the DMR link’s CFM. Some support using a 10-3 BER RxTH, others support using a 10-6 BER RxTH, while a third support using the former for links transporting mostly voice traffic and the latter for links transporting mostly data traffic. It is beyond the scope of this paper to expand on the merits of any of these positions but rather to make the reader aware that varying viewpoints exist. AMR vs. DMR Link Performance Characteristics Figure 3 is the classic figure used to compare the performance of AMR and DMR. Note that the S/N of an AMR is directly proportional to the RSL on a dB for dB basis. A loss of 30 dB RSL produces a 30 dB loss in S/N. On the other hand, the performance of a DMR link is not directly proportional to the RSL. The receiver does not care whether a “1” or “0” symbol is strong, weak, ugly or pretty; as long at it can determine its correct state. Thus, the BER of a DMR link is relatively constant or flat over a wide RSL range and does not start to degrade until the demodulator of the link’s receiver starts having trouble distinguishing between “1’s” and “0’s” which usually occurs as the RSL begins to approach the RxTH. The DMR link’s BER during non-faded conditions is referred to as the link’s residual BER.
Figure 3
An analysis of Figure 3 reveals another important characteristic difference between AMR and DMR. Since the quality or S/N of circuits transported over an AMR link is directly proportional to the link’s FM, the link must be designed for a high FM regardless of its length. For example, to achieve a 70 dB S/N, the link’s RSL must be designed to be 40 dB above the receiver’s 30 dB S/N RxTH. That is, the link’s FM must be 40 dB whether its one mile long or forty miles long (see Figure 4).
Figure 4
However, note this is not true for a DMR link. Its BER is unchanged over a wide RSL range, allowing its CFM to be just large enough to produce the desired link reliability; thus requiring smaller antennas, lower power or both. Microwave Radio Link Propagation and Capacity Considerations Propagation Protection Although numerous protection schemes have been developed to improve the propagation reliability of lineof-sight (LOS) point-to-point microwave radio links including, space diversity, frequency diversity, quad diversity and angle diversity, this paper will limit its discussion to space diversity since it is the one that, if required, is most likely to be used in converting from AMR to DMR Per Vigants’ formula above, the reliability of a microwave radio link is a function of its FM, which in turn is a function of the link RSL. However, often the RSL of a single antenna is reduced by the effects of multiple signals arriving at the antenna out-of-phase causing the link’s resultant or composite RSL to drop below the link’s RxTH resulting in an outage. Locating a second antenna, connected to a separate receiver, such that the resultant RSL is not below RxTH on both antennas simultaneously, greatly reduces the probability of such outages. Such an arrangement is called space diversity (S/D) (See Figure 5)
Figure 5
The improvement in link performance realized from such an arrangement is expressed by ISD = 7 x 10-5 f s2 v2 10 FM/10D Where: ISD = Space Diversity improvement factor f = frequency in GHz D = path length in miles s = vertical distance between antennas v = relative voltage gain factor for different size antennas v = 10 –[(Gl – Gs) / 20] where: Gl = Gain of larger antenna in dB Gs = Gain of smaller antenna in dB Thus U(space-diversity) = U(non-diversity) / ISD AMR will usually use S/D on links where excessively large antennas would otherwise be required to meet the link’s reliability objective. The reliability of an AMR link is more a function of signal amplitude than signal quality. However on DMR links, as seen in Figure 1, bigger is not necessarily better. It is not uncommon for a relatively strong RSL to be so severely distorted by multi-path fading, that the demodulator of a DMR receiver cannot demodulate the signal without producing a large number of errors. Under such conditions, the link’s performance can also be greatly improved by locating the S/D antenna such that the error causing distortion caused by the multi-path fading does not occur on both antennas simultaneously. This arrangement, together with errorless switching between the two receivers to insure that the receiver with the fewest errors is always on-line; greatly reduce the effects of frequency selective fading on DMR links. Capacity Considerations AMR and DMR systems both employ standard equipment protection schemes; i.e., monitored hot standby (MHSB) equipment and ring or loop system architecture, each with similar effectiveness. However, the type of equipment protection is a factor that must be considered when determining the capacity of the DMR required to replace the existing AMR. AMR systems transport intelligence via a single FDM baseband that connects all sites. Individual circuits are easily bridged on and off of the common baseband at each site as required. On the other hand, DMR systems transport intelligence between individual sites via dedicated 1.544 MB data signals, each representing twenty-four circuits, and called a T1 or DS1 signal. Normally, a DS1 connects only two sites and is not shared with other sites, as is the FDM baseband of an AMR system. This means that twenty-four channels of the system’s capacity is required to provide a single circuit between any two sites. Fortunately, on linear backbone DMR systems, this unnecessary use of system capacity can be avoided by using more expensive drip-and-insert (D/I) type digital channel banks. By doing so, the capacity of the DMR system can be made comparable to that of the AMR system it is replacing. Unfortunately, the channelizing advantage of D/I channel banks cannot be used on loop/ring protected DMR systems because such protection schemes involves the switching of individual T1/DS1 signals, which is not compatible with the D/I channel bank. Thus, a loop protected DMR system will usually require more capacity than the AMR system it is replacing. High capacity synchronous DMR system that occupy 30 MHz of RF spectrum and transport the equivalent of 2016 circuits, tend to favor SONET ring protection switching schemes for economic reasons. Cutover Considerations One of the most challenging aspects of converting an existing in-service AMR system to DMR is doing so with minimum interruption of service to the system users. Because no two systems are alike and each system will have it own unique channelizing and cutover requirements, the following discussion will simply present ideas that the reader may find applicable to their particular requirements. The simplest and easiest cutover plan is to turn the existing AMR system off, remove any unusable equipment, install and test the new DMR equipment, and return the system to service. Although the simplest, many users cannot tolerate the interruption of service or cost caused by such a plan. A less intrusive plan is to build the new DMR system, including a new antenna system, parallel to the existing AMR system. Once the installation and testing of the DMR is completed, individual circuits can be tested and transferred one at a time from the AMR to the DMR using cut blocks with minimum interruption of service. Once all traffic has been transferred to the DMR system, the AMR equipment, including its antenna system, can be physically removed. But here again, some links of an AMR system may not be able to operate in parallel with the new DMR even for short periods of time due to tower loading and/or tower and building space limitations. Although often more costly, a sometimes-used solution for this situation is to build the parallel DMR links where possible and use leased digital lines to replace the troublesome AMR link(s). Once the entire DMR system, including leased digital lines, has been installed, tested and carrying all traffic, the troublesome AMR links can be physically removed from the system and replaced with DMR. When the installation and testing of the troublesome link’s DMR is complete, traffic can be transferred from the leased digital lines and the lease ended. Ring protected systems tend to be easier to cutover by virtue of the fact that an entire link of AMR can be removed from service and replaced with DMR without interrupting service to system users. Once the DMR installation is complete it can be placed in service, allowing the next link of AMR to be removed from service and converted. This procedure continues on a link-by-link basis in a clockwise or counterclockwise direction until all links have been converted. Note that should an equipment failure occur in either the AMR or DMR while a link is being converted, a service outage will occur. What Can be Reused One of the first questions that is asked when discussing converting an existing AMR system to DMR is “how much is it going to cost”, which in turn, is directly related to how much of the existing plant can be reused. The following paragraphs will divide an existing AMR system into sub-systems. Each will be discussed in terms of its reusability. Equipment Shelter or building Existing equipment shelters can usually always be reused. If the shelter is already cramped for space, the existing AMR equipment rack can be temporarily slid into an isle, making room for the new DMR. Once the DMR is placed in-service the AMR rack can be removed from the shelter completely. Modern DMR equipment occupies a smaller footprint than equivalent capacity AMR equipment. Antenna supporting structure Existing antenna supporting structures and towers can almost always be reused but should be analyzed if its loading is changed by adding larger, additional antennas or relocating existing antennas. Antenna locations may change depending on the results of a detailed link analysis. If the AMR being replaced has been in-service for over five years, it is a good idea to resurvey the link before finalizing the DMR link design to insure that new obstructions have not been built or grown along the path. Antenna system Can the existing AMR antenna system be reused with the new DMR system? The answer to this question depends on the system gain of the DMR equipment. If the system gain of the DMR is enough to meet the desired link reliability objective then the existing antenna system can be reused in its entirety. This usually means that the system gain of the DMR needs to be equal to or greater than that of the AMR it is replacing. However as seen in Figure 4 above, this may not necessarily be the case. The height of the DMR antennas is another story. Rarely will the DMR antennas have to go higher if the AMR is in the same frequency range and operating satisfactorily. Because AMR links are fairly immune to the effects of multi-path fading but very susceptible to effects attenuation fades caused by insufficient microwave beam clearance, a general link design rule of thumb was, “if in doubt, move it up”. However, since DMR responds in a completely different manner in the presence of multi-path fading; without unnecessarily jeopardizing the microwave beam’s clearance, it is desirable to keep the antenna at one end of a non-diversity link as low as possible so that unwanted reflected or multi-path signals will be blocked by the link’s terrain, foliage or obstruction fade causing anomaly. (See Figure 6a and 6b) In determining the correct antenna heights for non-diversity DMR links the designer is faced with the dilemma; if he goes too high he risks exposing the link to the effects of frequency selective fading, if he goes too low he risks exposing the link to the effects of non-frequency selective fading. A solution to this dilemma on questionable links is the use of S/D. It is wise to have each link analyzed by an experienced microwave radio transmission professional prior to finalizing the DMR antenna system design.
Occasionally, larger antennas will be required for frequency coordination purposes even though the system gain of the DMR equipment is adequate to achieve the desired link reliability with smaller antennas. Existing waveguide should be swept and the waveguide pressurization system inspected for pressure leaks before it is reused. Power system If the existing AMR system operates from a -24VDC power source, thought should be given to replacing it with a –48VDC source. Much of today’s DMR equipment can be powered from +/- 20VDC to +/- 56VDC power sources. Unfortunately, most digital multiplex and channel bank equipment is not as versatile and operates only from –48VDC power sources. Although 24/48VDC voltage converters can be used to power the –48VDC equipment, their use not only reduces the cost advantage of reusing the existing power plant but also impact overall system reliability by adding additional equipment to the system. If the existing power plant is a –48VDC plant, has been properly maintained and is of sufficient capacity to power the new digital load for the desired amount of standby time, then there is no reason to discontinue its use. Microwave radio equipment In order to have the least impact on system reliability and antenna system upgrade cost; the most important equipment specification to consider when selecting a DMR product to replace an existing AMR product is its system gain. In many cases the system gain of the existing AMR system is going to be greater than the new DMR system. Therefore, the higher the DMR system gain the better. SYSTEM GAIN = $$$$$$ A DMR specification that is often over emphasized is its DFM, particularly on radios that occupy 10MHz of RF bandwidth or less. Today, the DFM of these radios is large enough to so that even a 10 dB increase in its value will result in virtually no improvement in link reliability; whereas, only a 2 dB improvement in system gain will result in a significant improvement in link reliability. To maximize the useful life of the new DMR equipment, the product selected to replace the AMR should utilize a scalable platform and have a wide range of input signal interfaces including SONET and IP. Multiplex and channel bank equipment The bad news is that the FDM (frequency division multiplex) used with the AMR cannot be reused on DMR systems. They will be replaced with TDM (time division multiplex) digital channel banks. However, the good news is that standard D4 type PCM channel banks are less expensive, have a smaller footprint, have a larger verity of interface options and consume less power than their analog counterpart. Drop and insert type digital channel banks are also available. Although more expensive, they provide much more efficient use of system capacity. Alarm & control system AMR alarm and control systems such as those manufactured by Badger and Lears Corporation can be reused on DMR system providing simple contact closure alarm and control functions. However, today’s DMR products make available a wide collection of performance monitoring (PM) information that allows system operators to monitor and more accurately analyze and diagnose problems from anywhere on or off of the microwave radio network. However, the interface to such PM data is usually via a proprietary or SNMP protocol thus requiring the use of a more expensive proprietary or open architecture SNMP Network Management System. Licensing Considerations The FCC requires a link of microwave radio be re-licensed if any major change is made to its operating parameters, including changes in frequency, power, emission designator, antenna size and height or otherwise would cause interference to other microwave radio users in the immediate area. Additionally, if the new newly licensed radio is going to occupy 10 MHz or more of RF spectrum in the 6 GHz band, the applicant must make a showing that 50% of the microwave radio’s capacity will be utilized within the first thirty months of operation. Existing AMR systems can be successfully converted to DMR provided the technical differences in their performance characteristics are taken into consideration. When selecting a DMR to replace an existing AMR, look for a product that has high system gain. The fact that an existing AMR link appears to be working satisfactorily is no guarantee that the DMR replacement will work just as satisfactorily without link modifications. Too much antenna height on DMR links can often cause more problems than not enough height. Space diversity using smaller antennas is more desirable than non-diversity using a single large antenna and can often be accomplished without increasing tower loading. Re-licensing of each hop will most always be required. The system capacity of linear hot-standby protected DMR systems can usually be made the same as the AMR system it is replacing. DMR loop protected systems will usually require higher capacity systems for equivalent channel loading. For more information contact 281-263-6500 or visit www.microwavenetworks.com.
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Microwave Radio Signal Quality Measurement Microwave Radio Link Fading Different Microwave Radio Fade Margins Microwave Radio Link Reliability AMR vs. DMR Link Performance Characteristics Microwave Radio Link Propagation and Capacity Considerations Cutover Considerations What Can be Reused Download Entire Document: |
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