Enhanced Position Location Reporting System (EPLRS)
ECEN 4242 Communication Theory
Table of Contents
7. Pulse Shape
The Enhanced Position Location Reporting System (EPLRS) is a computer controlled communication network designed for vehicular and dismounted platforms – primarily military. It provides near real-time position and location information and was introduced to the US Army in 1996. Utilizing GPS position information, EPLRS provides a secure and jam-resistant method for communicating information over an ad-hoc network beyond line-of-sight (LOS) coverage [Q1]. The radio does not employ Multiple-Input-Multiple-Output (MIMO) or other technologies that improve the spectral efficiency. It uses only a single antenna for both transmit and receive [Q10].
Currently, it is employed by all four branches of the US Armed Forces as well as allied nations. In its current stage, EPLRS uses an Army Data Distribution System version of two standard communication protocols, X.25 and MIL-STD 1553. These two standards are used to accommodate both legacy and evolving Army Tactical Command and Control System (ATCCS) networks. There are several companies that produce EPLRS radios including Raytheon, Harris, Northrop Grumman and Thales [Q11].
Established in the 1970s, X.25 is a protocol standard that defines connections for Wide Area Networks (WAN). Specifically, X.25 is primarily used in packet-switched networks where TCP/IP protocols are now used as the primary standard. However, X.25 is still implemented in several Data Terminal Equipment (DTE) devices today due to legacy networks that have long since been established such as telecommunication, ATMs, etc. Currently, this protocol is administered by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T).
Figure 1: The X.25 standard communication protocol defines how Data Terminal Equipment and Data Circuit-Terminating Equipment establish and maintain communication.
This standard protocol was also developed in the 1970s and serves as the standard for many legacy communication devices for power, sensor, and control systems. In the early 1990s the US Army released the oversight responsibilities to the Society of Automotive Engineers (SAE) where it is commercially maintained as AS15531.
Since the EPLRS radio uses two standard protocols, there are two places to find the definition of the physical layer [Q3].
Standard Body: International Telecommunication Union-Telecommunication (ITU-T)
Physical Layer is defined at: Recommendation X.25 (10/96) Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for terminals operating in the packet mode and connected to public data networks by dedicated circuit. ITU-T
MIL-STD 1553B (AS15531):
Standard Body: Society of Automotive Engineers (SAE)
Physical Layer is defined at: Digital Time Division Command/Response Multiplex Data Bus. SAE International
Physical Layer is defined at: MIL-STD-1553 Tutorial and Reference Alta-dt which explains the physical layer on Page 8.
EPLRS radio systems are designed for UHF communication to transmit unit position information over an ad-hoc network. This is done by a combination of Time-Division Multiple Access (TDMA) and Carrier-Sense Multiple Access (CSMA) over a 420MHz-450MHz band of operation [Q2] [Q9]. The radios are also configured to channel-hop across 4, 6, or 8 channels to increase multiple-access capability. However, once the number of channels is configured for a specific network, it cannot be changed until the entire network is reconfigured.
Figure 2: The EPLRS radio can be configured to hop across 5, 6, or 8 channels within the operation band.
· A collection of Radio System (RS) nodes are configured and managed by a single EPLRS Network Manager (ENM) node.
o RS units can simultaneously send and receive different information from different RSs
· The EPLRS network is organized in to a Time Division Multiple Access (TDMA) structure
o Each RS in a network of RSs is assigned small slices of time (called timeslots) in which the RS can transmit while other RSs can receive. To accomplish this, each RS has a clock that is synchronized with the RS network. The time division entities that are fundamental to EPLRS are the timeslot, frame, and epoch. Timeslot is the smallest time unit in EPLRS during which one unit of information is transferred between two RSs.
o Time slots can be configured for either 2msec or 4msec and remains fixed once configured for the Frame
o A Frame is a group of 128-consecutive timeslots and 1 epoch is 256 consecutive Frames
Figure 3: Data is encoded and packaged with error encoding and encryption then transmitted on a single time slot. A single epoch consists of a group of 256 Frames which contains 128 time slots.
· EPLRS radio also uses Carrier Sense Multiple Access (CSMA).
o A node verifies the absence of other traffic before transmitting on a shared transmission medium such as allocated frequency spectrum
o The radio system also hops on 5,6, or 8 channels
1. The host generates the data package and information (I) sequence using the X.25 protocol
2. Radio display and configuration (encoding, message type, desired data, etc.) is provided by a control panel on the radio
3. Data packet is transferred to a processor and error checked with ARQ where data encryption is applied and a pseudo noise code generated
4. Reed-Solomon error code is applied to new data sequence
5. Packet is passed through A/D converter and modulated with a combination of TDMA and CSMA
6. Data goes into up/down converter and put into/taken out of transmit frequency on the desired channel (420MHz-450MHz)
7. Signal is then amplified and then transmitted or received then amplified. [Q4]
· X.25 uses Automatic-Repeat-Request (ARQ) codes, specifically, Go-Back-N ARQ as default for all devices implementing the X.25 protocol [Q5]
o Error correction is implemented by retransmission of faulty frames.
o A timeout is used to determine if a retransmission is required.
o When the information I frame is transmitted a timer is started, if acknowledge frame is not received before timer expires then the frame is retransmitted.
Figure 4: The X.25 standard protocol uses ARQ as its default error control.
· EPLRS radios will generally employ Forward Error Correction (FEC) or Reed-Solomon error control codes depending on manufacturer of the radio.
Forward Error Correction:
This error control scheme requires the transmitter to send additional redundant data at the end of the transmitted message. These data serve as the error code that can be used to recover parts of the message, thereby avoiding a complete resend of the entire message. The redundant data message is encoded by a predetermined algorithm that uses a set number of bits within the information stream.
The constellation used by the EPLRS radio is dependent on the manufacturer of the unit. Defined by MIL-STD-1553B, EPRLS uses Pulse-code modulation (PCM) but Minimum-shift Keying (MSK) and BPSK have also been documented as an employed constellation, such as the radio built by Raytheon.
For PCM the signal is sampled and digitally quantized much like a analog to digital converter. After the signal is digitized, then additional encoding mechanisms can be applied.
Figure 5: The signal is digitized for pulse-code modulation and afterwards additional encoding may take place.
This signal can be encoded as digital data that fully represents the original signal.
MSK is type of continuous-phase frequency-shift keying. The EPLRS radios that use MSK implement a proprietary adaptation of this scheme defined by a Military spec derivative.
A MSK signal can be represented by the following signal:
Here, and are the encoded I and Q components of the baseband complex signal [Q7].
Figure 6: The IQ mapping is shown for minimum-shift keying.
Pulse shape for the EPLRS radio is a Raised-Cosine as specified by Raytheon Technical Workbook EPLRS System Technical Description with 40% excess bandwidth. The filter response for the raised-cosine pulse is given by [Q8]
This filter is characterized by two parameters
β: Roll-off factor – a measure of the excess bandwidth of the filer denoted by Δf . The excess bandwidth exists outside the Nyquist bandwidth of .
T: Period of the symbol
Figure 7: The time-domain (left) and frequency-domain (right) pulse of the EPLRS waveform is plotted for various values of β.
The true parameters for β are not made public by any EPLRS manufacturer. In Technical Workbook EPLRS System Technical Description a simulated waveform is considered for an EPLRS network for determining efficiency. For a raised-cosine, it is observed that the lower value of β, the greater the spectral efficiency.