NEXEDGE digital was developed by Kenwood in 2008 based upon NXDN technology which was jointly developed by Kenwood and ICOM. Both companies worked together to develop the NXDN digital technology which has been set up as an open standard that is available to any radio company who desired to manufacture equipment to that standard. Therefore, all conventional NXDN radios are compatible with each other with the air interface, so any NXDN radio from one manufacturer can talk to any other NXDN radio from any other manufacturer. However, once you go beyond the conventional operation of the radios into trunking or networking, each manufacturer is free to pursue their own method of achieving the desired result since the NXDN standard does not define the parameters that are used to create these more advanced radio systems.
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Conventional Radio Operation
The NXDN digital standard defines the signaling used in conventional operation. For this reason, any manufacturer must be compatible with each other. Audio is converted to digital using the AMBE+2 vocoder (designed by Digital Voice Systems, Inc.) that has become an industry standard for two-way radio and other telecommunication fields. Embedded in the data stream of audio components are some extra data bits which are used for the RAN code (radio access number) in NXDN which allows one fleet of radios to distinguish themselves from another fleet of radios, just like CTCSS (continuous tone coded subaudible squelch aka PL, DPL, Channel Guard, Quiet Chanel, Private Call etc.) in analog radios. Since the RAN code is part of the basic signaling, any conventional radio can talk to any other conventional radio since they all use the same signaling scheme. In a conventional environment, each talk group is typically on a different radio channel, however, it is possible to have multiple talk groups on the same channel using a different RAN code. The problem with this last scenario is that the talk groups will interfere with each other if their radio system coverage has significant overlap such as a large office building that has radios for security and maintenance. Whenever security talks, it interferes with maintenance use of the radio and vice versa, so typically the radios will have each talk group on a different radio channel so that the two talk groups do not interfere with each other. However, this requires one repeater for each radio channel if the building is using a repeater for radio coverage. If the coverage from one repeater is not good enough, then a 2nd repeater location may be used to extend coverage. However, in conventional radio, one must manually switch to the other tower site and all the talk group users must also switch their radio to the other repeater. This requires significant user training or there will be significant communication issues.
CONVENTIONAL SYSTEM TYPES
Just like analog radio, there are many different types of radio systems that are designed to accomplish different purposes. Let’s take a short time to discuss the different types of conventional radio systems:
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- Simplex systems which are used for on site communications for short range. There is no infrastructure that is needed to be set up because the radios talk directly from one radio to another which limits their range to “line-of-sight” applications.
- Wireline base systems which utilize a base station at a high location and is controlled typically by a phone line with a remote control so that one can achieve better range from base to mobile and mobile to base. Mobile to mobile range is still “line-of-sight” communications.
- Wireline base 2 frequency half duplex systems that allow the mobiles to talk to the base and the base to talk to the mobiles, but the mobiles cannot talk to each other. This is typical for many types of transportation such as taxi, paratransit, bus lines.
- Repeater systems which allows all radios to talk to each other with the extended range of the repeater which is typically located at a high location such as a tall building or the top of a mountain. All radios will get the same range, however, if the repeater fails, there is no communications from one radio to another because all transmissions go through the repeater.
- Repeater systems with wireline control which is similar to item #4 above except that it has a wireline interface. The difference is the dispatch is connected via wireline to give dispatch absolute priority over any mobile attempting to speak. This is typical for police and fire departments. All radios will to get the same range, however, if the repeater fails, there is no communications from one radio to another because all transmissions go through the repeater.
- Repeater system with talkaround which is the same as the repeater system except that the radios have the ability to switch to a simplex frequency (that is the same as the repeater transmit frequency) that allows the radios to communicate directly from one radio to another with the limited range of simplex.
- Repeater system with wireline and talkaround which is the same as the repeater system with talkaround, but it has a wireline interface to give dispatch absolute priority over the mobile radios like item #5 above.
- Voting receiver system which has multiple receiver locations and the best received signal is automatically selected to be heard. There are 3 options with voting receiver systems which include:
- A single high-powered transmitter to cover the entire area served by the 3 receivers.
- Multiple transmitters where the primary purpose is to talk from mobile to base and base to mobile. The transmitter that is collocated with the selected receiver that is selected by the voter is used to talk to the mobile.
- Multiple transmitters where the primary purpose is to talk from mobile to mobile. The transmitter that is collocated with the previously selected receiver that is selected by the voter is used to talk to the mobile. This causes the system to use the transmitter that heard the best signal from the previous transmission to be used to talk to the mobile that said something and needed a response.
- Simulcast transmitter system which has multiple transmitter locations that transmit at the same time on the same frequency.
- Voted simulcast system which has multiple receiver sites like item #8 above and multiple transmitter sites like item #9 above. The best receive audio quality is routed to all the transmitter sites which transmit simultaneously.
- IP Site Connect which ties 2 or more transmitter sites together with an IP link. The two sites must be on different frequencies if they have overlapping coverage (unless converted to simulcast) and can be on the same frequency if they do not have overlapping coverage. The IP Site Connect will cause anything said over one repeater to be heard on all of the repeaters. The mobile and portable radios must change frequency when they move from one area to another (if on different frequencies) which can be done manually by the user or can be accomplished via use of a scanning feature.
A complete explanation of conventional system types is discussed in the article titled “Conventional Radio” located on this website. The discussion of CTCSS and DCSS in the Conventional Radio document that applies to analog conventional does not apply to digital conventional since the digital conventional uses the RAN codes which were discussed in the previous paragraph of this article. -
NXDN Advantage
Radios that utilize NXDN to transmit and receive have several advantages, some of which are due to being digital and some due to being NXDN. The NXDN standard allows radio to operate on a 6.25KHz channel with 4KHz bandwidth on the transmission. The very narrow radio transmission allows one to get licensed on channels that fit in-between other radio channels, thus making it possible to get licensed for exclusive channels that do not have radio interference that could not be licensed with a normal 12.5KHz radio. The very narrow transmission also provides for the ability to fit two NEXEDGE channels in the same space that would normally fit one analog transmission. The digital radio will also talk about 20% further than an analog radio if all other parameters are the same such as frequency, power, receiver sensitivity, antenna type & location, etc. The digital radio can transmit a caller ID or other data at the same time as voice, without interrupting the voice channel. Each of these advantages by themselves are significant; together then make the radio a major advancement in radio technology.
Trunking Theory
Trunking is a method of automatically assigning a talk group of radios to a specific radio channel utilizing a repeater on that channel. By having multiple repeaters at the same location, each on a different radio channel, multiple conversations can be handled simultaneously. This allows multiple talk groups to operate on the repeaters that are all located at the same site. Since each talk group only uses the radios for a small percentage of time, it is possible to have many more talk groups than repeaters while each talk group has access to one of the repeaters. Even though they share the airwaves, no two talk groups can be assigned to the same repeater channel at the same time, so no talk group hears another talk group. Also, only when more talk groups than repeater channels attempt to speak at the same time will there be an issue, in which case there will be a busy signal for the talk group that attempts to access the system after all channels are occupied.
Please see the discussion of “LTR Trunking” for a more complete discussion on trunking theory, its advantages and how it works. The technical portion of the discussion is specifically on how LTR trunking works which is different than how NEXEDGE trunking works. This article discusses how NXDN trunking operates.
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Trunked Radio Operation
When operating a trunked radio system, there are several repeaters at the same location. There must be some method of automatically assigning all the radios that are part of a talk group to a specific repeater channel at the same time. (Also, no other talk group can be assigned to that channel while it is in use by another talk group.) This requires more information to be transmitted than just the RAN code. More information is needed for automatic talk group assignment to different radio channels. Therefore, since the additional information is not part of the NXDN standard, each manufacturer of NXDN radios are free to design their own trunked radio standard. ICOM developed their trunking format known as Type-D trunking. Kenwood developed their trunking format which is known as Type-C trunking. Each format works on a completely different principal so that a trunking radio from ICOM is not compatible with a trunking radio from Kenwood.
Type-D Trunking
The ICOM trunking scheme utilizes a system that is very similar to LTR trunking in the analog world. Each talk group monitors a channel in the system to determine if a conversation is occurring from its talk group. If the repeater is not talking, then there is no conversation to hear. If the repeater is transmitting, it is handling a talk group and telling all other talk groups where to find a unused repeater. The problem with this type of system is that it works well for a single site trunked radio system, but it does not work well for a multiple site network. If you leave the coverage area of a site in the network, you will not know about it unless some talk group is using the repeater channel your radio is monitoring on the system, otherwise there is no radio signal to hear on the channel to determine that the signal is too weak and for the radio to look for another tower site on the network. Therefore, the system transmits a “heartbeat” signal every 5 seconds to let the mobiles know how strong of a signal is received by the tower site and then the radio can evaluate whether it should look for another tower site. This makes the site selection process slow and unreliable because once the radio looks for another site, it may or may not be able to receive the “heartbeat” signal from another site in the network due to the timing of heartbeat signal the time that the radio looks at the repeater channel.
Type-C Trunking
Kenwood trunking scheme utilizes a central control channel at each tower site to give the mobile an easy method of finding a tower site for monitoring. Since the control channel transmits all the time, it is a beacon that is always present which makes finding the tower site significantly easier and faster. The radio locks onto a tower site and stays on that tower site until the signal gets weak enough to make the radio decide it needs to find another tower site. Each site in the NEXEDGE network transmits information about adjacent tower sites so that the radio knows where to look to find the nearest site. If the radio fails to find another site from the adjacent site list, then it looks at all the sites in the network until it finds a site to work. When it is time for the radio to talk, it communicates with the site by sending a request to use the system on the control channel. The repeater responds with a channel assignment at which time all radios from the talk group change channel to the repeater channel designated by the control channel or it sends a message that no channel is available for use and you receive a “busy” signal. If you are on the same tower site as the radio that initiates the call, you are switched to the same channel as the initiating radio. If you are using another tower site, all the radios from your talk group that are using the same site as your radio are switched to the same repeater channel at that site. If your fleet of radios is spread out over 5 different tower sites, then the network connects a radio channel at each of the 5 tower sites together for the duration of the call so that all the radios from the talk group can communicate with each other.
The Seamless Roaming Advantage
Seamless roaming is a technology that was developed for cellular radio. Cellular utilizes many tower sites which they call “cells”. Each of the cells cover a small geographic area to provide reliable communications within that small area. However, since the cell is small, people will move around in vehicles or by other means and exhaust the coverage of a cell in a short period of time. This requires the cellphone to automatically find another cell to use and switch the call from the initial cell to the new cell. This seamless roaming technology is used with the NEXEDGE system to find another tower site in the network so that your conversations continue as you run out of coverage from one site and your radio has to find another site to use for its communications. Since this all happens by itself behind the scenes without user intervention, it provides a significantly enhanced user experience.
The NEXEDGE system is complex and robust. It has the complexity necessary to deliver what the subscribers want in features and has the flexibility to deliver coverage over a large geographic area for many different needs of the various subscribers using the network. It has a multitude of features that make possible almost any radio configuration to meet the needs of the subscribers, while delivering reliable communications. NEXEDGE is the superior solution for an unequaled user experience.