THE WIN-T MF-TDMA MESH NETWORK CENTRIC WAVEFORMJohn Wiss and Rohit Gupta

L-3 Communications Linkabit CorporationSan Diego, CA

ABSTRACT

The Warfighter Information Network-Tactical (WIN-T) community has selected the L-3 Linkabit multi frequency-time division multiple access (MF-TDMA) demand-assigned multiple access (DAMA) Network Centric Waveform (NCW) as the networking standard for full-mesh IP over SHF satellite communications. The NCW supports a heterogeneous network of terminals with varying antenna, power, and overall transceiver characteristics, ranging from large-aperture strategic terminals to small-aperture tactical/mobile terminals employing satellite communications on the move (S-COTM) , and, by employing an advanced network scheduler, maximizes network data throughput and terminal population by making near optimal use of satellite and terminal resources. This paper describes the key features of the waveform MAC and Physical Layers.

INTRODUCTION

The Win-T MF-TDMA Network Centric Mesh Networking Waveform

The Warfighter Information Network-Tactical (WIN-T) community has selected the L-3 Linkabit Multi Frequency-Time Division Multiple Access (MF-TDMA) Network Centric Wwaveform (NCW) as the mesh networking standard for tactical IP over SHF satellite communications with special provisions for satellite communications on-the-move (S-COTM) terminals at Ku and Ka band frequencies of operation. . The NCW supports a heterogeneous network of terminals with varying antenna, power, and overall transceiver characteristics, ranging from large-aperture strategic terminals to small-aperture tactical/mobile terminals, and, by employing an advanced network scheduler, maximizes network data throughput and terminal population by making near optimal use of satellite and terminal resources. An NCW network consists of a set of up to 255 earth terminals (ETs), with one ET operating as the Network Controller (NC) and the other ETs operating as Network Members (NMs). The NC serves as the source for network time synchronization and as the focal point for network control and network resource management and for the establishment of priority-driven bandwidth-on-demand (DAMA) traffic communications. Certain NMs, depending on terminal configuration and performance

capabilities, may be capable of assuming the NC function, via seamless planned and unplanned handover protocols, and can also serve as store and forward hubs to assist NMs that cannot close single-hop as in Figure 1.

An NCW network consists of a set of earth terminals

(ETs), with one ET operating as the Network Controller (NC) and the other ETs operating as Network Members (NMs). The NC serves as the source for network time synchronization and as the focal point for network control and network resource management and for the establishment of priority-driven bandwidth-on-demand (DAMA) traffic communications. Certain NMs, depending on terminal configuration and performance capabilities, may be capable of assuming the NC function, via planned and unplanned handover protocols. As part of data communications, each ET in the network functions as an Ethernet/802.3 bridge port, with IP protocols encapsulated within Ethernet/802.3 frame data-units. The network, as a whole, functions as a link-layer Ethernet switch. Each ET implements a network-layer quality-of-service (QoS) adaptation

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NM(star)

NM(mesh)

NM(mesh)

NM(mesh)

Signaling and control

Packet communications

NM(mesh)

NC

B

B

B

B

B

B

B

B

LANc

B = ET bridge

LANa

B

B

LANa

LANb

B

for IP, using the differentiated-services (DiffServ) model for priority, service classification, and traffic-shaping. Figure 1 NCW Mesh Networking Architecture

As part of data communications, each ET in the network functions as an Ethernet/802.3 bridge port, with IP protocols encapsulated within Ethernet/802.3 frame data-units. The network shown as a whole, functions as a link-layer Ethernet switch as in Figure 2. Each ET implements a network-layer quality-of-service (QoS) adaptation for IP, using the differentiated-services (DiffServ) model [1] for priority, service classification, and traffic-shaping. This switch architecture leads to a straightforward and stable SATCOM waveform that can be adapted to different routing protocols and network-layer technology improvements. For example, upgrading the network from IPv4 to IPv6 does not impact the SATCOM waveform In support of a heterogeneous network, the NCW supports simultaneous narrow-band and direct-sequence spread-spectrum (DSSS) operation to support small aperture tactical/mobile terminals in meeting the off-axis emission density limitations when operating over densely spaced satellites. The modulation employed by the waveform and the burst structure allows the same physical hardware to modulate and demodulate narrowband or direct sequence spread spectrum (DSSS) signals which may change on a burst-to-burst basis.This network centric waveform (NCW) was developed as an IP physical layer combined with a scheduler that attempts to optimize the usage of satellite bandwidth while simultaneously optimizing the service of DAMA requests in the peer-to-peer mesh network in order to provide the best QoS to each user. The modulation employed by the waveform and the packet structure allows the same physical hardware to modulate and demodulate narrowband or direct sequence spread spectrum (DSSS) signals which may change on a packet-by-packet basis. For small aperture dishes the DSSS mode allows communications with smaller dishes at Ku-band and minimizes the off-axis EIRP density as not to interfere significantly with adjacent satellites.

Figure 2 Switched Mesh WLAN Architecture

MF-TDMA and Mesh NetworkingThe access methodmodulation used for the NCW is MF-TDMA which is an excellent choice when a mixed network with differing terminal capabilities are to be supported. The main advantage of MF-TDMA is that carriers bursts may be assigned to groups of terminals with similar EIRP and G/T capability and thus increase the overall efficiency of the network. Thus a network with large fixed terminals may be assigned carriers bursts (in a TDMA-fashion on each carrier) that are higher symbol rate when communicating peer-to-peer to each other, while mobile users may be assigned carriers bursts that are of lower symbol rate when communicating amongst themselves. The NCW employs a Serially Concatenated Convolutional Coder (SCCC Turbo Coder) for high coding gains.

The mesh nature of the network makes it possible to communicate without routing through a large hub and thereby reduces the satellite resources required to relay the data to the destination terminal and also significantly reduces the delays associated with two-hop communications such as full-duplex voice usable between any two terminals in the network.. Figure 3 MeshCommunications shows a mesh SATCOM Figure 3 shows a heterogeneous mesh networknetwork of large and small aperture terminalsexample. The larger fixed site terminals have a link (shown in light blue) that indicate a higher symbol rate carrier where as the lower rate carriers are used for communications between mobile or stationary mobile terminals (shown in green, blue, and grey). The figure represents a snapshot of the communications network which may change dynamically on a per-packet basis; terminals may communicate hundreds of packets per frame period to and from different destination terminals over the a 400 msec frame each with the maximum symbol rate that link will support. Hub-assist is also supported when a peer-to-peer link lacks sufficient aperture to close whereby the data is routed through a hub prior to arriving at the destination terminal to benefit from the greater EIRP and G/T of the hub as in traditional hub-spoke communications.

The nature of the mesh network and the multi-frequency carriers require that at a minimum the terminal receiver must demodulate more than one carrier simultaneously. The larger terminals may also transmit on more than one carrier (simultaneously) to maximize capacity.

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1-4244-1513-06/07/$25.00 ©2007 IEEE

Figure 3 Mesh Communications Figure 4 Mesh SATCOM Communications

NCW MF-TDMA

The time-frequency plane can be visualized (in SlotViewer®) to show the mesh nature of the communications accesses in an illustratative example indicated in Figure 5: in Figure 5, Terminal 6 is communicating simultaneously on 6 separate carriers-- full duplex with terminal 1 , transmitting to terminal 5, and is receiving data from terminals 9, 2, and 4 in the 200-240 msec interval (the x-axis) over a network bandwidth of 36 MHz (the y-axis) . In this example all of the terminals are bursting at a fixed symbol rate for clarity. In actuality every inbound and outbound link from a member terminal may be optimized for the EIRP, G/T, and propagation conditions that that link may support.

For Ku-band operation where adjacent satellite interference is an issue with small aperture terminals, the BPSK DSSS spread factor is also computed based on terminal-satellite geometry where the modem can select spread factors of up to 16 providing 15 dB of spreading compared with narrowband (OQPSK) modulation.

Figure 5 Slot Viewer Activity Example

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Terminal 6 is Transmitting Two

Bursts and Receiving Four

Bursts Simultaneously

Terminal 6 Activity In One Frame

Terminal 6 is transmitting and

receiving a total of 18 bursts in this 400 ms frame

Terminal 6 is transmitting and receiving at an

Aggregate TX and RX rate of 9.3 M

bps

9 – 6

6 – 5

1 – 6

2 – 6 4 – 6

6 – 2

6 – 1

NCW NETWORK SCHEDULER OVERVIEW

The network scheduler runs at the NC and is responsible for generating the MF-TDMA network burst plan for each 400 ms frame. The burst plan is generated given terminal resource requests, the latest reported link-quality conditions, and terminal transceiver information and capabilities, which are reported by NMs at the time of network login and via configuration updates during operation. Each burst assignment in the burst plan allows the specification of an uplink center frequency, such that there is not concept of fixed carriers (i.e. fixed center frequency) operation.

The NC schedules within the constraints of the total available power and spectrum at the satellite bandwidth-segment of operation and the total available power and power-control dynamic range at each terminal (i.e. the NC accounts for all transmissions in the network and ascertains that, at any instance instant in time, the total power out-of-the-satellite does not exceed the satellite EIRP allocation, the occupied spectrum does not exceed the allocated bandwidth, total terminal power is not exceeded across multiple modulators, and that the assigned terminal EIRP can be implemented by the terminal)). . The NC is required to respect priority, jitter (for jitter-sensitive services such as voice), and the following key requirements: 1) the BER requirement – each burst must achieve the required C/No at the receiving terminal’s modem and 2) the spreading requirement – each burst must be spread enough to meet the power-density (dBW/Hz) requirement for given terminal aperture sizes when operating over densely densely spaced satellites..The network scheduler applies adaptive burst parameters (data-rate/modulation/FEC code-rate/interleaver block-size), along with adaptive power-control, to maintain the required BER for a link. A link-budget for each burst is performed by predicting the total C/No at the destination for each link (NC-to-NM, NM-to-NC, NM-to-NM), given the EIRP at the transmitting terminal, the uplink path loss, the transfer gain and G/T of the satellite bandwidth segment, the downlink path loss, and the G/T at the receiving terminal. Of these parameters, the uplink and downlink path loss values characterize the dynamics in the quality of each link introduced by fades. The adaptive burst parameters (burst-mode) are selected with respect to the predicted C/No at the destination and the uplink and downlink spread requirements and within the satellite and terminal resource constraints.Network data throughput is maximized as a result of maintaining a tight power-control loop and minimizing link budget margins and spectral usage. A large set of

adaptive data-rates (in approximately 1.5 dB steps), code-rates, modulation types, spread factors, and adaptive power-control (in 0.25 dBW increments), along with the ability to have multiple simultaneous transmitters and receivers at a terminal maximize usage of terminal and satellite resources.

Error: Reference source not found Figure 3 Figure 7 illustrates the components for the link budget computation:

EIRPu = Terminal power transmitted on uplink

EIRPT = Total available (maximum) terminal EIRPATX = Transmitting terminal antenna apertureR = Burst Mode

(data-rate/code- rate/modulation/interleaver- size/spread-factor)

Lu = Uplink path loss(G/T)S = Satellite uplink receiver sensitivity GS = Aggregate transfer gain

across satelliteEIRPS = Total satellite bandwidth segment EIRPLd = Downlink path loss(G/T)T = Terminal receiver sensitivity ARX = Receiving terminal antenna aperture(C/No)T = Total carrier to noise density ratio at terminal receiver

Figure 6 Link Budget ComponentsFigure 7 Link Budget Components

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The NC computes and maintains uplink and downlink path loss values for each terminal, including itself, in its database and is updated every frame period of 400 msec which allows for operation due to highly dynamic link losses typical of the on-the move environment. The NC computes its own path loss values by first computing the clear-sky path losses based on terminal and satellite position and the frequency of operation; the clear-sky path losses are used to compute a predicted clear-sky C/No, given a terminal EIRP value, the satellite parameters, and the transceiver characteristics; the actual C/No is compared with the predicted clear-sky C/No to compute the path loss values. Path loss values for NMs are computed using actual C/No collected on orderwire bursts between the NC and NMs, the satellite parameters, and the transceiver characteristics of the NMs, while using the NC’s path loss as a reference. Figure 8 shows a typical loaded network slot plan for a 36 MHz transponder. For this example a packing efficiency of 93.5% is obtained.

Figure 8 Typical Loaded Network Slot Plan

NCW NETWORK SCHEDULING STEPS:

Collect scheduling inputsCollect latest terminal transceiver parameters and capabilitiesCollect latest reported link quality measurement valuesCollect terminal packet resource requests

Compute terminal uplink and downlink path losses

Sort resource requests by strict priority identified in the resource requests

Convert resource requests to bursts

Assuming that the source can transmit at its maximum power, select adaptive burst parameters and power according to the BER and spreading requirementsSelect burst payload size based on resource request sizeInvoke hub-assist if necessary

Eliminate bursts, with respect to priority, that cannot be scheduled even with perfect packing

Bursts that cannot be accommodated within the source terminal’s transmit duty cycle (number of modulators * frame duration) are eliminatedBursts that cannot be accommodated within any destination terminal’s receive duty cycle (number of demodulators * frame duration) are eliminatedBursts that exceed the satellite spectrum resources given perfect packing are eliminatedBursts that exceed the satellite power resources given perfect packing are eliminated

Place bursts, with respect to priority and jitter sensitivity, in time and spectrum (select start-time and frequency for each burst). Jitter sensitive bursts are scheduled to match the packet-rate of a jitter sensitive service and are placed at approximately regular intervals (sub-framed burst placement)

For each burst, find a position in time where the following are available:

Source terminal transmit duty cycle

Source terminal transmit power Destination terminal receive

duty cycle Satellite bandwidth segment output power Satellite bandwidth segment

spectrum

If a burst could not be placed due to transmit/receive duty cycle, retry by reducing the size of the burst, with the same power and adaptive burst parameters.If a burst could not be placed due to power or spectrum limitation, retry by reducing power and symbol-rate (spectrum).If a burst is eventually placed, subtract terminal power and duty-cycle resources and satellite power and spectrum resources.

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SATCOM ON THE MOVE CHALLENGES

There are several conditions that make S-COTM challenging:

1. The links may suffer blockages and periodic outages due to terrain, tunnels, buildings, and vehicular orientation

2. The antenna positioner must point to a fixed absolute azimuth and elevation (and polarization angle) regardless of vehicular motion

3. The communications must be tolerant to absolute Doppler shift and Doppler accelerationrate

4. The modulation, coding, data rate, DSS spreading needs to change potentially in a 31 sec interval between bursts both on receive and transmit to make full use of the scheduler capabilities

The antenna used in the S-COTM system is 12” for Ka-band operation and 18” for Ku-band operation with a variant that is 3-axis and a low-profile 2-axis positioner. The antenna typically uses an inertial reference unit (IRU) such as a Talin® and GPS to allow open-loop tracking of the satellite and under rough terrain can maintain pointing accuracy to a typical value of less than ¼ degree. Variable spreading may be used on a burst-buy-burst basis to ensure off-axis radiation to and from adjacent satellites is acceptably small per FCC/ITU regulations.

The scheduler has mechanisms for devolution of control based on missing the reference forward order wire (FOW) in successive intervals from the NCT which serves as the heartbeat of the system and is transmitted stood up by the terminal acting as the Network Control Terminal (NCT). Also the packet/frame structure has inherent rapid re-synchronization properties. Figure 9 Figure 9 shows an event called an “unplanned handover” in which the NCT (A) reference FOW (RFOW) is missed by the member terminals several times in a row and is handed to the next NCT-capable terminal (B) which is blocked and finally to (C)). A slot assignment may also be defined as “persistent” which reserves a slot assignment for a high-priority S-COTM user to allow operation in a low-availability environment.

Figure 9 Unplanned Handover

The antenna used in the S-COTM system is 12” for Ka-band operation and 18” for Ku-band operation with a variant that is 3-axis and a low-profile 2-axis positioner. The antenna typically uses an IRU such as a Talin® to allow open-loop tracking of the satellite and under rough terrain can maintain pointing to a typical value of less than ¼ degree.

The modem design is based on a software defined radio concept and hosts in the basic configuration provides 4 receiver/demodulation chains and 2 transmit/modulation chains. Each receiver chain is designed that are designed to detect, acquire, track and demodulate narrowband BPSK, OQPSK (64 ksym/sec) and DSSS spread BPSK with chip rates from 64 kcps up to 8 Mcps per carrier providing 15 dB of spreading relative to the OQPSK mode.. The architecture of the modem is specifically designed to allow the same physical “gates” to be used for all possible modulations, code rates, and symbol rates supported by the hardware. Figure 10 shows a unified packet structure consisting of a Pseudo-Noise (PN) sequence that is used to settle the AGC and allow for signal energy detection. This PN sequence is chosen to not spoof the detection circuit in the preamble processor block. A series of Binary Phase-Shift Keyed (BPSK) symbols are impressed with a secondary modulation (Bi-Periodic Repeats) to allow a very large correlation peak when the entire preamble is in the detection window. Phase, frequency, symbol timing, and start of frame indications are made through parameter estimation from this very special preamble which uses identical hardware to process narrowband and DSSS (BPSK) spread signals with 15 dB of spreading gain relative to the narrowband OQPSK payload modulation. The high Doppler shift and Doppler acceleration rate is handled by robust phase, symbol timing, AGC, and higher-order tracking loops that will accommodate over 1 kHz/sec of Doppler rate and over 1 kHz/sec2 of Doppler acceleration at a data rate of 32 kbps at slightly over 2 dB

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FOWC FOWC FOWC FOWCFOWB FOWBFOWB FOWBFOWA FOWA FOWAFOWA

FOWDFOWC FOWC FOWC FOWCFOWB FOWBFOWB FOWBFOWA FOWA FOWAFOWA

0 1 2 3 4 5 6 7 8 9 10 11 12 13

141 2 3 4 5 6 7 8 9 10 11 12 13

14

TxFrame

RxFrame

where: Countdown = 4(k-1), k = sequence order = 1..number_of_nodes_in_precedence_list

FailureDecision

Point

CountdownNode

ABC

048

NOTE: Node_takeover_frame = Countdown + 2

ATakeover

BTakeover

CTakeover

258+20ms

400ms

FOWB FOWBFOWA FOWA FOWAFOWA

1 2 3 4 5 6 7Tx

Frame

258+20ms

400ms

FOWB FOWBFOWA FOWA FOWAFOWA

1 2 3 4 5 6 7Tx

Frame

258+20ms

400ms

FOWB FOWBFOWA FOWA FOWAFOWA

1 2 3 4 5 6 7

Rx Frame

258+20ms

400ms

0

FailureDetection Point

A Takeover

B Takeover

Loss of RFOW

Es/No. Figure 12 shows the 3rd order loop filter sawtooth tracking performance at 2.5 dB Es/No without any pilot symbol aiding for the OQPSK modulation payload.

Figure 10 Unified NB/DSSS Packet Structure

Figure 11 Doppler Tracking With 3rd-Order Loop

Figure 12 3rd Order Doppler Rate Loop

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Phaselock Loop Filter Output Under Doppler(Normalized To Hz)--1000 kHz/Sec

-3.0E+3

-2.0E+3

-1.0E+3

0.0E+0

1.0E+3

2.0E+3

3.0E+3

0 1 2 3 4 5 6 7 8

Time (Secs)

2.5 dB Eb/No @ 64 ksym/sec

Ideal

sss

sssH

23

2

Variable Length Payload Spread/NB Flush

28 Bi-Periodic RepeatsSacrificial

PN BPSK/OQPSK

PHYSICAL LAYER PROCESSING

The NCW physical layer processor modem is designed to support a wide range of data rates (about a factor of 100 from lowest to highest) and the modem currently supports four independent receivers that may operate simultaneously with two transmit carriers that cover similar data rates per carrier and is hosted in gate array technology. The same “gates” also process DSSS modulations up to over 8 Mcps with the modem being configurable in about 30 sec to switch between any of the narrowband or spread modes of operation. Figure 13 shows the signal processing chain for one NB or DSSS carrier. Considering the spread modes of operation the modem must perform upsampling in the transmit path of over several hundred and in fact can upsample by the thousands in a single step using a novel modulation and interpolation approach in a gate array device utilizing less than 3% per carrier of a Xilinx V2Pro P70 device. Figure14 shows the transmit spectral purity from the modem for an upsampling of 2000 using this proprietary approach. Figure 15 shows a zoom of the spectrum from Figure 14 and the highest artifact is approximately 70 dB below the carrier.

Figure 13 Key Digital Receiver Blocks

-1000 -800 -600 -400 -200 0 200 400 600 800 1000-140

-120

-100

-80

-60

-40

-20

0

Figure 14 Single Stage Tx Spectrum Upsampled By 2000

-60 -40 -20 0 20 40 60-140

-120

-100

-80

-60

-40

-20

Figure 15 Close-up Of Transmit Spectrum

CONCLUSION

The NCW has been demonstrated successfully at several exercises and demonstrations such as at Capstone Experiment 2007 and NCW hardware has been deployed in Iraq is with the MBCOTM Program, specifically the Stryker 3/2 Brigade and is in the process of deploying Triton II to Iraq, specifically the 525th Battlefield Surveillance Brigade.

REFERENCES

[1]   K. Nichols, S. Blake, F. Baker and D. Black, "Definition of Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998.

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MatchedFilter

PhaseRotate

NCO

FarrowResampler

NCO

E/LDespreader

PreambleProcessor

AGC

MetricGen

2 SPC

ToSlicer &

TC Decoder

-test

-fest

-est

MetricsFor Hyp. Testing

LF

-PLL

1 SPS

FromDig

Downconv

Saturate

Saturate

LF

-

[2]    B. Hawe, A. Kirby and B. Stewart, "Transparent Interconnection of Local Networks with Bridges", Journal of Telecommunications Networks, June 1984.Figure 16

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