ROAMEO System Frequency & Timeslot Reuse

ROAMEO uses DECT (Digital Enhanced Cordless Telecommunications) as its RF protocol. DECT is a mature radio technology that allows multiple users to occupy RF carriers by means of TDMA (Time Division Multiple Access), which means each carrier is divided into multiple timeslots. In DECT, there are 24 timeslots in a 10 mS window, which makes a frame. There is an downlink path timeslot(s) for communication from an Access Point (AP) to a Beltpack (BP) and a uplink path (BP to AP) timeslot(s). The Intercom use of the DECT Protocol only allows a total of 20 timeslots to be used while the other 4 timeslots are used internally by the DECT protocol. Both transmission paths use either one-timeslot or two timeslots per BP, depending on the amount of audio data that must be passed (CODEC selection). To simplify the discussion of timeslots and BPs, we refer to one timeslot as a system that uses a single timeslot for the forward path and a single timeslot for the reverse path. The same methodology applies to the use of two timeslots. The ROAMEO system is unique in the industry as it has two settings for the use of these timeslots. In narrowband mode, one timeslot is used per Beltpack to carry audio and control information. In wideband mode, two timeslots are used per Beltpack to carry audio and control information.

In Europe and most parts of the world, there are 10 RF carriers available. In the US, Canada and most of North America there are 5 RF carriers available. Other variations existing, using between 5 and 10 RF carriers, depending on the radio frequency rules and laws in those countries. Negotiation of the DECT frequency being used at any point in time is handled by the DECT protocol.

In this example in Figure 1, there are 7 US Frequency narrowband Access Points, each circle indicating the coverage area of each Access Point, respectively. In a given system, if each AP supports 8 BPs, then the whole system is supporting 56 BPs. This is above even the theoretical limit of what the 5 carriers in the US can support, but this is possible due to frequency reuse. No one area exceeds the 40 maximum BPs limit that RTS recommends (See Table 1) and it is below the 50 BP theoretical limit for a narrowband US Frequency system.

This is best shown by example. At AP 5, there is the potential that the adjacent BPs for AP’s 2, 3, 4, and 6 could be detected by AP 5 if the BPs are all at or near the overlap coverage areas. AP 5 will then tag these BP timeslots as unavailable for use by BPs within AP’s 5’s coverage area. Therefore, the 32 timeslots taken by the BPs of APs 2, 3, 4 and 6 plus AP 5’s own 8 BPs timeslots is a total number of 40 timeslots. This number of timeslots is at the RTS recommend BP limit of 40 for a narrowband system, US Frequency system, so this area would be correctly balanced. However, if there were additional APs with a coverage area’s intersecting AP 5 such that AP 5 could detect those BPs when they are in the overlapped coverage areas, then the capacity of this area would be exceeded and not enough BP timeslots would be available for all of the potential BPs in the area to allow intra-AP timeslot negotiation. This would then result in unstable operation of the system with Beltpacks dropping out or not being able to fully boot due to the lack of available timeslots in the area. All DECT based systems operate with these same limitations.

Looking at another area with the same operating conditions (Figure 1), AP 6 could detect the BPs of APs 3, 5 and 7, in the worst case scenario, when all BPs are near or at the overlap area of AP 6’s coverage area. This would result in AP 6 detecting 24 BP timeslots as used. When added to AP 6’s BP timeslots, that is a total of 32 timeslots being used, so this area is correctly balanced too.

Simplifying this concept, consider that each AP has, at any point in time, negotiated it’s RF path frequencies and timeslots. The DECT Protocol, will avoid operating with frequencies and timeslots that are already used within the RF coverage area of an AP. The process of physically spreading out the APs and BPs to facilitate the reuse of frequencies can be used to make a RTS ROAMEO system of hundreds of BPs and many APs as shown in Figure 2. This can be done as long as the Access Points and Beltpacks are sufficiently spaced (RF spacing) from each other. The evaluation of an adjacent system’s Access Points has to be done with respect to a worst case scenario which is at or below the recommended number of BPs that can be supported.

¹See “ROAMEO Beltpack Capacity” White Paper to understand why a maximum number of Beltpacks per Access Point are recommended.

Through the proper layout of a system, taking advantage of the reuse of frequencies through physical spacing of APs and BPs, the number of Beltpacks in an installation can be increased well beyond the limit for a single physical area. Careful consideration should be taken in regard to the capacity of BPs that an area can support when designing systems that take advantage of frequency reuse. In Table 1, RTS recommends the BP limits that an AP can detect in any one physical area. The reuse considerations regarding RF spacing and DECT capacities apply to all DECT-based Systems.

Note that the examples herein are for systems that have the maximum number of Beltpacks per Access Point but still have ample timeslot availability for DECT intra-AP timeslot negotiation. This approach provides for the most stable systems. With respect to any DECT based Beltpack system, the maximum number of BPs that can be connected to an Access Point will vary greatly depending on the number of BPs that are desired to be roaming through a given area. Thoughtful planning should be performed to allow all Beltpacks to find timeslot availability, either while roaming or stationary, on one or more Access Points in a system.

_{Written by Jim Andersen and Kevin Moore, Bosch / RTS 2018}