Part 2 of this CDE begins with a brief overview

Transcription

1 CDE #47923 ANALOG VS. DIGITAL TRANSMISSION The Migration to IP-Based Transport By Guy Ball Part two of a two-part series, the first of which ran in the January/February issue of this magazine. ISTOCK.COM/GNK82 INTRODUCTION Part 2 of this CDE begins with a brief overview of the basic performance characteristics of analog and digital signals as we compare the differences between the two in a transmission network. Next, we analyze imperfections of the digital Vocoder, the electronic device used to convert analog speech signals to digital bit streams for transmission over digital radio. The vocoder has been an ongoing issue for the Public Safety Project 25 (P25) committee as a result of a detailed report released by a federal group of engineers in 2008 comparing the audio quality of analog and digital networks. The report specifically dealt with problems firefighters were having using their digital P25 two-way radios in high noise environments. Furthermore, we will describe scenarios in which analog radio may have the advantage compared to digital radio. We will examine the importance of synchronization in digital radio and transmission networks as well as review the advantages of digital IP-based transmission networks. Part 1 of the CDE began with an overview of the history of transmission networks and the subsequent migration of the networks from analog to digital-based systems. Analog and digital signals were defined and the basic principles of transmitting data over analog radio were described. In conclusion, the basic principles of transmitting voice signals over digital radio networks were outlined. The principles outlined in Part 1 are expanded upon in Part 2. TRANSMISSION CHARACTERISTICS OF ANALOG AND DIGITAL SIGNALS The transmission of analog and digital signals over telecommunications networks presents very different performance characteristics for the design engineer. In order to get an idea of how different the performance characteristics are between analog and digital transmission, consider a coaxial cable transmitting an analog video signal over a long distance. In our example, repeater (amplifier) stations have been installed at regular intervals in order to compensate for the signal loss of the cable. A comparison can then be made between the performance characteristics of the analog signal and the same video signal converted to digital data for transport over a digital-transmission network. In our first example, (Figure 1) the analog video signal starts at the transmitter location. Let s say we have designed our system so that the signal has lost half of the power when it arrives at the first repeater/amplifier station. We have also designed the amplifiers to double the power in order to restore the signal to the original power level we started out with at the transmitter. However, 32

2 FIGURE 1: DIGITAL BIT RECONSTRUCTION USING REPEATERS background noise in an analog cable system remains constant along the distance of the cable. We have now restored the signal to its original level. Compensating for the signal loss. However, the background noise is now double what it was before degrading the performance. In this manner, noise in an analog system is cumulative as it passes through the many amplifying stations and at some point the signal becomes unusable. Now let s discuss the same scenario with the cable system transmitting a digital representation of the same video signal. With digital transmission, clean pulses are reconstructed at each repeater location and sent on to the next repeater where another cleansing process takes place. This reconstruction process of the digital data bits at each repeater allows digital data to travel through a noisy transmission medium without becoming more distorted and less recognizable (See Figure 1). Modern communications networks also use forward error correction techniques based on computer algorithms which are very effective at correcting most of the minor bit errors occurring over the communication link (i.e. an error occurs when a bit sent over a transmission system as a binary 1 is received as a binary 0 at the receiver end of the link or vice versa). Of course, exceptionally large noise impulses may destroy one or more of the pulses so that they cannot be reconstructed by the repeater. 3 In summary, it can be stated that digital systems using repeaters and error correction techniques to reconstruct data bits along the transmission path result in a major improvement of transmission quality over analog. However, bit errors (not compensated for by error correction) tend to accumulate in a digital transmission system over a longer distance. In an analog system background noise is cumulative as the signal traverses the transmission network. IMPROVED PERFORMANCE CHARACTERISTICS OF DIGITAL RADIO As discussed in the previous section, digital radio receivers use (over-the-air) error correction techniques based on mathematically based computer algorithms similar to those used by digital multiplexers transmitting signals long distances over cable. FIGURE 2: ANALOG VS DIGITAL RADIO AUDIO QUALITY Error correction detects bit errors created due to receiver channel noise and reconstructs the original bit stream from the transmitter effectively canceling out the noise. The result is usually a clear audio signal out to the point of the threshold of the receiver. The threshold of a hypothetical receiver can be seen by viewing the graphical representation in Figure 2 comparing the audio quality of signals traversing analog and digital radio. The digital receiver threshold is roughly the point on the graph where the blue line representing digital audio quality meets the horizontal x-axis on the graph. This is the point where bit errors become so numerous that they can no longer be compensated for by using error correction techniques, causing the receiver to lose synchronization and creating an outage event. The graph in Figure 2 also PSC March/April

3 FIGURE 3: MULTIPATH RECEPTION high levels of noise depending on the quality of the vocoder software. Analog and digital signals are both subject to dead spots and interference. However, digital modulation reduces human communication by eliminating the gray area afforded by analog technology. State and local government agencies deploying multi-mode analog/ digital radio systems can benefit from the ability of analog technology to operate in the so-called gray areas such as high-noise environments and especially where significant multipath is present (See Figure 3). shows that as the radio receiver input level transitions from a strong signal to a weak signal, the audio quality of the digital signal remains good (shown as a flat blue line) up to and almost to the point of the receiver threshold, even as the quality of the analog signal degrades in a linear fashion. This is the result of the error correction taking place in the receiver correcting bit errors and reconstructing the original bit stream. Note that the graph in Figure 10 assumes an ideal environment for radio reception representing a decrease in signal level only and does not take into account multipath reception received in the radio path (see Figure 3) or interference coming from external sources. In the real world, a slightly weaker (or faded) signal combined with multipath reception or interference from external sources can also cause bit errors in the receiver. Bit errors caused by interference may cause the receiver to lose synchronization, even with a faded signal level that would be strong enough under normal circumstances, in an interference-free environment, to allow for error-free operation. By contrast, the audio quality of the analog signal degrades as the signal level is reduced. This is due to the action of the automatic gain control (AGC) of the receiver compensating for the reduced signal level (See Figure 4). A radio receiver does this by using a detector to sample the incoming signal strength at the output of the amplifier shown next to the antenna. In the event the receive signal level fades, the AGC control signal compensates for reduced signal strength by adding more amplification to the amplifier. The action of the AGC feedback loop works to keep the wanted signal strength constant. However, increasing the amplification of the wanted signal also results in increased amounts of unwanted noise degrading the audio quality as shown in Figure 2. FIGURE 4: RECEIVER BLOCK DIAGRAM OF AUTOMATIC GAIN CONTROL (AGC) Digital receivers use an AGC circuit as well. However, digital receivers use sophisticated error-correction techniques to dynamically restore any bits received in error due to increased levels of noise. Error correction effectively cancels the degrading effect of receiver noise maintaining a good audio quality within the entire range of signal strength that the digital receiver is designed to operate (See Figure 2). SCENARIOS IN WHICH ANALOG MAY HAVE THE ADVANTAGE There are scenarios in which an analog system may continue to produce perceptible audio to the human ear beyond the range of a digital system. Consider an analog mobile radio unit travelling away from the transmitter and approaching the edge of the coverage area. As the wanted signal becomes weaker and weaker, the automatic gain control (AGC) attempts to compensate for the signal loss by adding more amplification so that it remains somewhat at a constant level. However, as the amplification of the wanted signal is increased, there is a corresponding increase of the analog receiver noise degrading the audio quality (See figure 2). In this case, the human ear has the ability to decipher audio which has been obscured by noise and interference. By contrast, a digital system may or may not be able to operate in an area with SYNCHRONIZATION OF DIGITAL DATA NETWORKS Large communications networks today require extremely accurate timing and synchronization systems for reliable transmission of high-speed data. The core network facilities for all large telecommunications networks such as AT&T, T-Mobile, Verizon, etc. typically derive the timing for their networks from redundant cesium atomic clocks with a stability of better than 1x The redundant cesium clocks are stabilized using coordinated universal time (UTC) time. The typical source of UTC time comes from global positioning system (GPS) receivers. 6 Digital transmission networks require extremely accurate and stable timing systems in order to maintain and distribute synchronization throughout the entire network. Synchronization is particularly critical for digital wireless systems such as time division multiple access (i.e. TDMA) technology. For example, the second generation wireless TDMA Cellular Network standard IS-136 (or digital amps) was widely deployed in the 1990s in North America. IS-136 made a 30 KHz-wide channel available for communication between a base station and three mobile units. Using TDMA-based technology, the repeater can communicate with each mobile unit onethird of the time using the entire 30 KHz bandwidth, increasing the number of users in this case 3:1. Three conversations can now take place within the same 30 KHz channel by allowing each mobile unit to receive for a few milliseconds at a time and in rotation (See Figure 5). Note that in order for successful communication to occur, the mobile unit must be perfectly synchronized with the transmission so that it only decodes the desired signal from the transmitter. 34

4 One of the most important advantages for public safety is secure communications. Digital signals are easier to encrypt than analog signals and, in the case of digital radio, encryption can be done without degradation of the audio signal throughout the coverage area. In addition to secure communications, both digital radio and transmission systems offer improved quality of service over analog using error correction techniques and reconstruction of clean bit pulses at each repeater station. FIGURE 5: TIME DIVISION MULTIPLE ACCESS (TDMA) CONCLUSION In summary, the transition to IP-based Ethernet transmission offers several advantages over analog and legacy TDM-based transmission systems. Digital IP-based transmission technologies make it possible to transport much greater bandwidths as well as offering more advanced data and voice capabilities to new and existing customers. The reduction of network elements, such as the elimination of the BSC/RNC from the 4G LTE access network, results in reduced capital expenditure as well as improved network reliability. Additionally, fewer network elements reduce the transport delay (i.e. latency) of packets through the transmission network. Low latency is mandatory for real-time data applications such as video. 1 Statewide networks such as the California Public Safety Microwave network (CAPSNET) providing connectivity between remote mountaintop radio transceivers can particularly benefit from the ability of IP-based transmission systems to transport data packets to their destination over independent, geographically separate transmission paths. Figure 6 shows planned migration of the existing hybrid analog/legacy TDMbased network on the left to the IP-based Ethernet capable microwave network on the right. All sites in the diagram on the right are connected in a ring configuration so that data packets can be re-routed around the ring in the event of a link outage. The Network Operations Center (NOC) in an IP-based transmission network no longer needs to be co-located with a major transmission center, giving the operator more flexibility on determining the actual location. Many of these sites are very remote and inaccessible during the winter months. IP-based transmission enables enhance remote configuration of base station and repeater radios at remote mountaintop sites from centralized dispatch centers. This includes the ability to program new radio channels (i.e. in support of critical events such as a major fire), or the switching between main and redundant hardware in the case of equipment failure. Furthermore, long travel times and the cost of dispatching technicians to remote sites can be reduced. FIGURE 6: CALIFORNIA PUBLIC SAFETY MICROWAVE NETWORK CAPSNET IN RING CONFIGURATION 2 Guy Ball is a Telecommunications Services Engineer specializing in VHF, UHF, and 800MHz frequency coordination and microwave transport systems at APCO with 35 years of experience in the cellular industry as a microwave/rf transmission planner for international and domestic corporations such as Hutchison Whampoa Ltd of Hong Kong, Telus Communications of Canada, and the AT&T Corp. in the United States. References: 1 LTE: An Overview of the Technology and the Benefits to Public Safety, Guy Ball, PSC Magazine, November/ December 2016, Vol 82, No.6 2 Public Safety Microwave Network Strategic Plan, California Technology Agency Public Safety Communications Office, March Telecommunications and the Computer, 2nd edition, James Martin, Published Selected Articles from, The Lenkurt Demodulator, PCM, Published March Understanding Wireless Communications in Public Safety, Kathy J. Imel and James W. Hart, P.E., March 2000, Revised: August Mobile Network Synchronization Plan, Viag Interkom (Company renamed to O2), Guy Ball and Thomas Angerer, Published September PSC March/April

5 1. In what type of system is noise cumulative as it passes through the many amplifying stations and at some point becomes unusable. a. Synchronization network b. Digital network c. Phase shift modulator+ d. Analog network 2. The audio quality of which type of radio signal remains good even when faded down almost to the level of the receiver threshold? a. Digital radio receiver signal b. Analog radio receiver signal c. Receiver AGC feedback signal d. GPS satellite receiver signal 3. The Network Operations Center (NOC) in an IP-based transmission network no longer needs to be co-located with a major transmission center giving the operator more flexibility when determining the location? a. True b. False 4. Which circuit in a radio receiver compensates for fluctuations of the incoming receive signals? a. ATPC CDE EXAM # b. Time division multiplexing c. Long range navigation technology d. Automatic gain control (AGC) 5. What type of transmission configuration is required for an all-ip network to be able to dynamically re-route IP packets around a hardware failure? a. Space diversity configuration b. Ring configuration c. Synchronized network configuration d. Error detection configuration 6. What type of radio transmission system may have the advantage when mobiles are operating at the edge of the coverage area or in a high noise environment such as in a city with high levels of multipath? a. Digital land mobile radio b. Analog land mobile radio 7. What transmission characteristics of digital transmission support improved quality of service over analog transmission? a. Error correction and bit reconstruction b. Phase jitter and unipolar line coding c. Error reconstruction d. Three level partial response 8. In an analog radio receiver, the quality of the received signal degrades simultaneously with the reduction of signal level. a. True b. False 9. What method of transmission requires an extremely accurate and stable timing source to maintain and distribute synchronization throughout the entire network? c. Analog transmission b. Digital transmission 10. What type of transmission technology makes it possible to transport much greater bandwidths as well as offer more advanced data and voice capabilities to new and existing customers? a. Analog transmission b. TDMA: Time division multiple access c. TDM: Time division multiplexing d. Digital IP-based transmission FOR CREDIT TOWARD APCO RECERTIFICATION(S) Each CDE article is equal to one credit hour of continuing education 1. Study the CDE article in this issue. 2. Answer the test questions online (see below for online exam instructions) or on the exam page from the magazine article (photocopies are not required). 3. Add/upload your CDE article information and certificate of achievement in the My Classes Taken section of APCO s Training Central at trainingcentral. Questions? Call us at (386) You can access the CDE exam online! To receive a complimentary certificate of completion, you may take the CDE exam online. Go to net/login/index.php to create your username and password. Enter the CDE article in the search box, and click on the 2018 Public Safety Communications Magazine Article Exams, then click on enroll me and choose Analog vs. Digital Transmission (47923) to begin the exam. Upon successful completion of the quiz, a certificate of achievement will be available for download/printing. IF YOU WOULD LIKE TO USE THE CDE ARTICLES FOR ANYTHING OTHER THAN APCO RECERTIFICATIONS AND NEED A PRINTED COPY OF THE CERTIFICATE: Complete the written exam and submit the following: 1. Answer the exam questions online, and fill out the form below. Photocopies are acceptable, but please don t enlarge them. 2. Mail to: APCO Institute 351 N. Williamson Blvd. Daytona Beach, FL Payment of $15 Name: Organization: Address: Phone: Method of Payment (US funds only) Check Purchase Order (attach copy) New Jersey Original Purchase Order Only Credit Card (circle one) VISA MASTERCARD DISCOVER AMEX Card #: Expiration Date: Name on Card: Cardholder s Address: Cardholder s Address: 36