Software Implementation Of Automatic Link Establishment Capability ...

2006 INTERNATIONAL RF AND MICROWAVE CONFERENCE PROCEEDINGS, SEPTEMBER 12

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14, 2006, PUTRAJAYA, MALAYSIA

Software Implementation Of Automatic Link Establishment Capability For Hf Radio Communication Nurulfadzilah Bt Hasan' and Ahmad Zuri bin Sha'ameri2

IFaculty of Electrical and Electronic Engineering, Kolej Universiti Kejuruteraan dan Teknologi Malaysia, Karung Berkunci 12, 25000 Kuantan, Pahang, Malaysia

2Digital Signal Processing Lab, Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia

nurulfadzilahgkuktem.edu.my, ahmadzsgyahoo.com Abstract - HF radio spectrum, ranging from 3 to 30

system that is equipped with ALE capability is developed and tested.

multipath fading, interference and attenuation, communication using HF becomes very challenging. Besides, the availability of the channels varies depending on the time of day, seasons and the condition of the ionosphere. The purpose of this paper is to present the research work on the design and implementation of Automatic Link Establishment (ALE) capability, to enhance the reliability of HF radio communication. This paper also looked at the feasibility of implementing ALE as software, designed using Visual C++ programming language. Equipments used in this research are commercial HF radio and modem, which are both controlled by the software. Field testing is conducted between UTM Skudai and Chemor in the state of Perak to verify the performance of the system.

2. Automatic Link Establishment

MHz can be utilized as a low-cost voice and data communication medium. But due to the unpredictability and propagation problems such as

Keywords: calling, scanning, link quality analysis

1. Introduction To enhance the reliability of data transmission on the HF spectrum, adaptive radio technology [1] [2], specifically ALE (Automatic Link Establishment), and LQA (Link Quality Analysis) is used. Both technologies permit modem radio systems to adjust automatically to changing propagation conditions [3]. These new technologies in HF radio permit this medium to be used in computer-to-computer communication systems. At present, ALE is strictly for military applications and usually is in a form of an external equipment [3][7]. However, today, by using HF modem [4][5][6], transmissions of text, fax and images over HF radio can be done. Thus, in this research, in order to extend the technology to nonmilitary applications and implementing it as software, commercial HF radios and modems is used. In order to fulfill this objective, an HF radio messaging

0-7803-9745-2/06/$20.00 (©)2006 IEEE.

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ALE solves channel selection problems by continuously evaluating and testing the channel reliability using its' sounding and link quality analysis protocols. An ALE system, at fixed time intervals will perform channel evaluation on each channels assigned to the particular stations. The results are then saved so that it can be used each time the station wants to communicate to another station. An ALE system incorporates the basic operational rules listed in Table 1. The operational rules were based on US FED-STD-1045A [7]. However, some of these rules may not be applicable uncertain applications [11]. "Always listening" (Rule 2) for example is not required during temporary periods when not technically possible, such as during transmit with a transceiver, or when using separate transmitter and receiver with a common antenna. There are three states for an ALE system to be. The three states are available states when a station is not link to another station, linking states where a station is in the process of trying to connect with another station. If linking process fail, a station shall return to available state. However, if the process is successful, the station will enter the third state: the link state. A station enters this state when it has successfully link to another station. In this state a waitfor-activity timer will be running. Figure 1 shows these three states. The ALE procedures and protocols can be described into the following functions. Functions v and vi are optional:

i)

ii) iii)

iv) v) vi)

Calling Scanning Sounding Link Quality analysis (LQA) Automatic Channel selection Order wire Messages

Table 1: ALE Operational Rules (listed in order of decreasing precedence).

Independent ALE receiving capability in parallel with any other Always listening for ALE signals (critical) Always will respond (unless deliberately inhibited) Always scanning (if not otherwise in use) Will not interfere with active ALE channel (unless forced by operator Always will exchange LQA with other stations when requested (unless inhibited), and always measure the signal quality of others Will respond in the appropriate time slot to calls requiring slotted responses Always seek (unless inhibited) and maintain track of their connectivities with others Linking ALE stations employ highest mutual level of capability Minimize transmitting and receiving time on channel Automatically minimize power used (if capable)

1

2 3 4 5

6

7

8

9 10 11

Send or receivey ALE call /

vaa

( Linked)

)

iLnking succeed

implemented in this research can be described into the following functions: (i) Calling (ii) Sounding and Link Quality analysis (LQA) (iii) Scanning

3.1 Calling

The fundamental ALE operation of establishing a link between two stations proceeds as follows:

1) The calling station addresses and sends a call frame to the called station and starts its waitfor-response time (Twr). 2) If the destination station "hears" the call, it sends a response frame addressed to the calling station. 3) When the calling station receives the response frame, it now "knows" that the destination station accepted the call request. Then the calling station as a conclusion sends an acknowledgment frame. 4) After the destination station has received acknowledgment frame from calling station, a link has been established, and the stations may communicate with each other.

Timing is critical during calling process. If calling station does not receive response from destination station within Twr, the call is considered unsuccessful; and calling station may try again or terminates the call. The same goes for destination station, which wait for calling station's acknowledgment within Twr. If calling station's acknowledgment arrived later than Twr, destination station treats it as a new individual call and provides a new response for it. Figure 2 shows the flowchart for calling protocol and Figure 3 shows a basic call, response and acknowledgment frames.

Figure 1: ALE state diagram.

3. ALE Implementation Visual C++ is chosen as the programming language to develop this system including its graphical user interface (GUI). Then the database for this system was built using Microsoft Access. The data format chosen for this research is PACTOR mode that is used for messaging and ALE purposes. PACTOR or Packet Teleprinting Over Radio [10] is a modern radio Teletype data mode developed to improve inefficient modes such as AMTOR. PACTOR is a half-duplex synchronous ARQ system designed to operate in noisy channels. The ALE procedures and protocols

Figure 2: Flowchart for calling protocol.

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|20DEST

I20DEST

|2ODEST

|5OUTLL T

RESPONSE FRAME

20SCALL S2 C411 50SDEST

~~~~~~~~~~~~T.1 ACKNONIEDGMENTFRAME

Note:

DEST=- U spignvrdesti~ntion sllon

20DE :T 20DEST

5OC4Lq

Figure 3: Frames structure for calling protocol. 3.2 Sounding and Link Quality Analysis

(LQA) Sounding is a process where a station broadcast a

very brief, beacon-like short message, which may be utilized by other stations to evaluate connectivity propagation, and availability. At a predetermined time interval, the system transmits its sound frame on each of its preaffanged channels to the destination station. Sounding frame consists only conclusion cycle, repeated five times as shown in Figure 4.

50CALL

50CALL

I 50CALL

50CALL

I 50CALL

Note: CALL= cal sign for sounding station

Figure 4: Structure of a sound frame. The destination station, upon receiving the sounding frame, stops scanning and then performs LQA on the sounding frame. During sounding, the modem divided sounding frames into several packets before transmitting them. The messaging system uses PACTOR data format to transfer data, thus the time it takes for a data packet to arrive to destination is 0.96 second (assumed equal to 1 second). However, if the packet contains error; it is rejected and the destination modem will request for retransmission. The sender modem will then retransmit the packet which will arrive at destination after another second. This procedure is repeated until the packet received by the destination station contains no error. So, if the time taken for a complete error-free packet to arrive at destination is more than 1 second, it indicates that retransmission has occurred. The longer the time it takes for an error-free packet to arrive indicates that many retransmissions have occurred. This procedure is repeated until a complete error-free sounding frame is received. Then the total number of seconds needed for the frame to arrive becomes the LQA score of that channel. After completing LQA, destination station sends acknowledgment to the sounding station which is a

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short frame consisting of a flag character and LQA result for the current channel. The flag is set to either "1" to tell the sounding station to proceed to the next sounding channel or "0" which means that the current channel is the last sounding channel. Sounding station saves the LQA results and if not on the last channel, proceeds sounding on the next channel. If sounding process reached the last channel, both stations will then perform the process of ranking the channels based on LQA scores. The channel with the lowest LQA score is ranked as the best channel. The results are then saved in ALE database located in the hard disk. After that, both stations change their transceiver frequency to the highest-ranking channel according to the latest result and stay there until the next sounding process occurs. So by doing this, the stations can now communicate on the best channel for that hour.

3.3 Scanning

The purpose of scanning is to enable the station to detect sounding frame sent by the sounding station. At every hour, the system automatically starts scanning one minute before the expected sounding time of the other station. The dwell time, which is the time a station stays and listens on each channel, is 10 seconds, making the total scan cycle is 50 seconds. The destination station keeps on scanning until a request for sounding is detected. Then the scanning process will stop and the destination station will process the sounding frame. After sounding process is finished, scanning is disabled for an hour, until it is time for the other station to do sounding again.

3.4 ALE Database

The name of the database for this research is is "Ale.mdb" and it consists of three main tables, which are the Info table, the LQA table and finally the Test table. Each table contains results and information from soundings of all five channels for every hour. Among the information includes, the call signs for both stations, the channel used, the LQA scores and channel ranking. The purpose of saving the results in a database is because the past sounding results are used in LQA scoring process to determine the present LQA result. Besides that, graphs can be plotted based on the data kept in the database to view and summarize the performance of each channel for a given period of time. In addition, the results are also accessed by the system during initialization process.

4. Results Figure 5 shows the whole system structure. The equipments needed are Kenwood TS-570D HF radio transceiver, Kantronics Kam'98 HF modem, personal computers and dipole antenna.

COWUTER

9.400 9.200

WDEP

TRNSCEIVER LOCAL ST1ATION(SKUDAI

HF

9 200

CHANNELL 8.190

CQMPLJTER REMOTE STATION

Figure 5: Whole system structure. The results shown in this paper is from fieldtesting conducted between between Skudai and Chemor, Perak. The purpose of field-testing is to verify the system by comparing the best usable frequency obtained from the testing and predicted optimum working frequency (OWF). Frequency prediction software used in this research is Advanced Stand-Alone Prediction System (ASAPS) created by the Ionospheric Prediction Service (IPS), Radio and Space Services of the Australian Department of Industry, Tourism and Resources [10]. For testing purposes, the time of day is divided into four timeslots which represents hours in the morning, afternoon, evening and midnight/early morning. Every hour, sounding is conducted to determine the best usable channel. After that, results within the same timeslot are averaged to determine the best channel for each timeslots. Table 2 shows the details of this circuit. Then the comparisons between the best usable channels and predicted OWF are shown in Figure 6.

Pr1opeties

Table 2: Skudai-Chemor Circuit.

Descriptions

Distance

452 km (straight distance)

Duration Antenna type

7th June 2005 to 21st June 2005 Half wavelength dipole, with 1/4 wavelengths high above the ground. Channel 1:8.190 MHz, Channel 2:7.100 MHz, Channel 3:8.710 MHz, Channel 4:8.002 MHz, Channel 5:9.200 MHz, Timeslot 1: From 9.00 a.m. tol200

Frequencies Used Timeslots

noon

Timeslot 2: From 2.00 p.m. to 4.00 p.m.

Predicted OWF Timeslot 1: 8.600 MHz Timeslot 2: 8.300 MHz

*OWF

Skudai

-

-

-

-

-

-

L.--

!.?

-

-

-

-

-

-

7--

7--

7--

-

-

7---

-

-

-

!.., -

Timeslot

Figure 6: Comparisons between highest-ranked channels and OWF for Skudai-Chemor Circuit. From Figure 6, it

can

be

seen

that for timeslot 1,

most of the highest-ranked channels (channel 1: 8.190

MHz, channel 2: 7.100 MHz and channel 4: 8.002 MHz) are located below the OWF. Only channel 3, which is 8.71 MHz, and channel 5, which is 9.200 MHz, are higher than OWF. The same condition can be seen for timeslot 2, where the OWF is 8.300 MHz and the MUF is 10 MHz. Overall, for both timeslots, channel 2 which is the best channel for is located below OWF. The second best channel, which is channel 5, however is located above OWF for this timeslot. 4.1 Comparison with Standard ALE Systems The protocols listed above are different from the protocols in FED-STD-1045A [7]. The reason for this is because the ALE implemented in this research is constraint to the type of transceiver and most importantly the type of modem used. Therefore, all the ALE protocols used in this research must be made accordingly with the transceiver and modem. The major difference is that in FED-STD-1045A, ALE is implemented as separate equipment while ALE in this research is developed as software. Other differences are as listed in Table 3. 5. Summary

By applying proper adaptive methods such as ALE, HF radio can overcome the propagation problems and become a reliable, low cost means of wireless communication. But instead of implementing ALE as separate equipment, it is more convenient to 128

implement it as software embedded in a HF messaging system. The results obtained from field-testing in this research verify that the most important feature in an ALE system is that its ability to select the best possible channel to use at any time of day. This is achieved by performing sounding and LQA at every hour. Sounding and LQA featured in this system is a form of real-time channel evaluation that gives actual and immediate results of channels condition. Results from any frequency prediction software on the other hand are predicted results from calculation based on empirical data. From the field-testing results, it can be seen that both results do not differ very much from each other.

Table 3: Comparisons of ALE.

uses z-aly r;31X

Coding

Multiple stations

Scanning

modulation with eight orthogonal tones, one tone (or symbol) at a time Extended (24,12,3) Golay code is use. Thus FEC is implemented. Multiple stations operation is enabled.

Scanning is done continuously, with scan rate of either 2 channels per second or 5 channels per second.

Uses PACTOR

data mode, which implement ARQ error correction.

No multiple stations operation available. Only

point-to point connection is available. Scanning is only done once every hour. The dwell time is 10 second for each channel; thus the total scan cycle is 50 seconds. Scan rate is 1/10 channel per second.

129

References [1] M.D. Street, M. Darnell., "Results of new automatic link establishment and maintenance techniques for HF radio systems", Proceedings of Milcom 1997, pp 1067-1071, 2-5 Nov 1997 [2] W. Blair, R. Taylor, "Improving the performance of a high frequency radio message network", Proceedings of Milcom 2000, pp 48-53, 22-25 October 2000, [3] High Frequency Radio Automatic Link Establishment (ALE) Application Handbook, U.S. National Telecommunications and Information administration (NTIA), U.S. Inst. For Telecommunication Sciences (ITS), September 1998. [4] Sailmail: Email Services for Yachts via Marine HF SSB Radio, http://www.sailmail.com/, 2004 [5] CruiseEmail.com, http://www.cruiseemail.com/, 2004. [6] Radio Communications In The Digital Age Volume One: HF Technology, Harris Corporation, May 1996. [7] Federal Standard 1045A:Telecommunications: HF Radio Automatic Link Establishment. U.S. National Communications System Office Of Technology & Standards (NCS), October 1993. [8] J.M. Goodman, HF Communications Science and Technology, Van Nostrand Reinhold 1992. [9] Australian Space Weather Agency, IPS Radio and Space Services in: http://www.ips.gov.au [10] KAM '98 Multi-Mode HFIVHF Digital Controller User's Guide, Kantronics Co, U.S.A. [11] E.E. Johnson, R. I Desourdis Jr., G.D. Earle, S.C. Cook, J.C. Ostergaard. Advanced HighFrequency Radio Communications, Artech House, Norwood 1997.