WO2007004893A1

WO2007004893A1 - Channel adaptive packet transmission

Info

Publication number: WO2007004893A1
Application number: PCT/NO2006/000253
Authority: WO
Inventors: Odd GANGÅS; Jens Andenaes
Original assignee: Telenor Satellite Services As
Priority date: 2005-07-01
Filing date: 2006-06-30
Publication date: 2007-01-11
Also published as: NO20053241L; NO20053241D0

Abstract

Predicting the properties of a radio communication channel between a seagoing vessel and a low elevation radio station based on regularity of motion of the seagoing vessel, may be utilized to transmit a radio signal in time slots that are most likely to result in low degradation of the received signal.

Description

CHANNEL ADAPTIVE PACKET TRANSMISSION

1. Field of the Invention

The present invention relates to a method and a system for expanding the coverage area for naval communication. In particular the invention deals with an exploitation of regularity or periodicity of channel properties resulting from movement of a vessel caused by ocean waves.

2. Description of the Related Art

Mobile communication systems often suffer severe degradations caused by reflected signals interfering with the direct signal. Depending on the relative phase of the two signals the resultant signal may be enhanced or be severely attenuated to the point where communications are impossible. In a mobile satellite system one or more mobile satellite terminals communicate through a Gateway station via a geostationary satellite and, when the satellite is seen at a low elevation angle at the mobile terminal, reflections from the earth surface can interfere with the direct signal. For mobile terminal operating at sea, strong reflections from the sea surface can cause such interference as to make communications impossible at elevation angles.

The difference in phase of the direct signal and the reflected signal will, for a terminal in a vessel at sea, depend on the elevation of the antenna above sea level, and consequently on vertical movement of the vessel. Since the ocean surface rarely is calm, this results in a regular, or periodical, degradation of the communication channel between the terminal and a satellite, or some other radio station. This results in a variation in received signal strength and thereby also in bit error rate.

In a packet switching system, data will be sent in the first available time slot in the transmission channel, independent of any other properties of the channel. If a vessel is receiving communication from a station at a low elevation angle, which will be the case if the vessel is in the periphery of the coverage area of a satellite, there is a large likelihood of transmission error. Error correcting communication protocols involving retransmission will cause degradation of channel capacity, which may approach zero since the channel will be occupied by retransmitted data packets. Adaptive modulation and coding may to a certain extent overcome the problems described above, but will have too low dynamics and require too much power. Another alternative is interleaving, which will result in too much delay for this type of channels.

Summary of the Invention

A method for exploiting periodic changes in the channel properties for a radio transmission channel between a low elevation radio station and a seagoing vessel, may comprise creating a model of the relative motion and location of an antenna on said vessel; and predicting channel properties based on the created model.

The method may further comprise measuring received signal levels; correlating the predicted channel properties with the measured signal levels; and optimizing the model for best fit between modeled and observed signal levels.

The method may also include using the optimized model to predict future channel properties.

According to one embodiment consistent with principles of the invention, the prediction of transmit channel properties are further based on different transmit frequencies.

A further aspect of the present invention involves identifying time slots in the predicted channel properties with a likelihood of low distortion or destructive interference in a transmitted or received signal and using said time slots to transmit or receive a signal.

The invention may also be implemented as a device capable of operating substantially in accordance with the enclosed specification and drawings.

The invention may also be implemented as a computer program product on a computer readable medium, comprising computer code instructions enabling a computerized communications controller to operate substantially in accordance with the enclosed specification and drawings.

Brief Description of the Drawings

The invention will now be described in further details bay way of non-limiting examples and with reference to the enclosed drawings where: Figure 1 illustrates a reflection model;

Figure 2 shows signal strength vs antenna height assuming perfect reflection;

Figure 3 illustrates a simulation of observed and reconstructed heave motion;

Figure 4 illustrates simulated observed and reconstructed receive signal variations;

Figure 5 illustrates simulation of observed received signal level variations with 0.5 degree error in elevation value;

Figure 6 illustrates normalized correlation of receive signal and reconstructed signal as a function of model elevation angle;

Figure 7 illustrates simulation of observed and predicted received signal level variations after elevation optimization;

Figure 8 illustrates simulation of predicted return link (transmit) signal variation after optimization;

Figure 9 illustrates receive channel "green" slots detection;

Figure 10 illustrates transmit channel "green" slots detection;

Figure 11 illustrates a satellite system;

Figure 12 illustrates Channel Fading Predictor; and

Figure 13 is a Timing Qualifier flow chart.

Description of Exemplary Embodiments

Fig. 1 illustrates the problems associated with simple reflection of received radio signals. The magnitude of the combined signal entering the antenna can be expressed as:

C = D + Rcos(φ)

where D and R are the amplitudes of the direct and reflected signals respectively. For a carrier signal with wavelength λ the phase difference between the two signals is given by.

φ - — 2π + π ^Ψ λ

assuming a flat surface and perfect reflection. The difference in travel distance of the reflected and the direct signal is:

where h is the antenna height above sea level and Gf is the incident angle of the carrier signal (electrical elevation angle).

As the antenna height varies, the resultant signals go through successive maxima and minima as illustrated in Fig. 2. The example of Fig. 2 illustrates signal strength vs antenna height assuming perfect reflection, with an elevation angle = 2.3 degrees and frequency = 1540 MHz.

The effect is dependent on the antenna beam width since the reflected signal strikes the antenna with an offset angle 2α, such that large antennas having narrow beam width will be more resilient to the effect than smaller antennas. However, the trend for mobile terminals is towards smaller antennas thus increasing the problem, and as a consequence reducing the effective coverage area of the satellite system. Simply increasing the satellite power has limited effect since the interference is caused by the signal itself and increases in proportion to the signal level.

When a ship is subjected to ocean waves the vertical heave motion causes the antenna to move through the interference pattern and gives rise to fading of a periodic nature. This effect can be detected, and by modeling the mechanism it is possible to predict when the signal will suffer degradation in the near future and avoid transmission in these periods. Reference is made to Fig. 11, which illustrates how a satellite terminal Communications Controller has been enhanced with a Channel Fading Predictor operating in accordance with the present invention. Information about the fading predictions are transmitted to the Gateway and used by the Gateway Resource and Timing Mgr to optimize transmission timing to and from the terminal. This remedy cannot be used for all types of communications as it adds delay to the Communications links but packet transmissions carrying communications such as file transfer, message switched services and email can continue to work well as long as transmissions can be synchronized to time periods with high probability of successful transmission.

According to principles consistent with some embodiments of the invention, an accurate model of the ship (antenna) relative motion and location is created, and that model is used to predict the channel behavior. The modeled receive channel performance may be correlated with the measured signal levels and the model may then be optimized for best fit between modeled and observed signal levels. The future channel behavior is predicted using the optimized model. The transmit channel characteristics is also predicted by taking into account the different transmit frequency.

Some embodiments consistent with the principles of the invention may require accurate input data for ship latitude and longitude, satellite longitude (and latitude if the orbit has any inclination) as well as receive/transmit frequencies for the communications.

Ship latitude/longitude and heave motion may be obtained through appropriate processing of GPS supplied position data. The satellite data can be made available system wide by transmitting them regularly on a system (satellite) bulletin board or be sent to the ship regularly from a Gateway providing the multipath enhancement.

Receive signal strength, receive and transmit frequencies may be provided by the satellite terminal.

Reference is made to Fig. 12 for an illustration of the various units that may be part of one embodiment of the invention. The device may include a. Satellite Communications Terminal connected to an antenna and operating at a receive frequency F_rx and a transmit frequency F_tx. The device may further include a GPS Receiver or some other positioning device providing sufficiently accurate positioning information.

Accurate latitude (Lam) and longitude (Lorn) values of the ship may be obtained by low pass filtering the sampled latitude (JLa) and longitude (Lo) values from the GPS receiver through low pass filter LPl and LP2.

Lamoi) = (1 - ki)Lam(n - 1) + kiLaw

Lom(n) = (1 - kι)Lom(n - n + kιLθ(n) where Lam_(n) and Lom_(n) are the new latitude and longitude estimates, Lam_(n.i₎ and Lom_(n-i₎ are the previous estimates one sampling instant earlier, Lci_(n) and La^ are new latitude and longitude samples from the GPS receiver, kj is a coefficient in the range 0-1 chosen to give a filter time constant of several minutes to give good accuracy.

Similarly the antenna mean height above sea level (Ham) may be determined by low- pass filtering the height (Ha) output from the GPS receiver through low pass filter LP3,

Ham{n) = (1 - ki)Ham(n - n +

where fø is a coefficient in the range 0-1 chosen to give a time constant of several minutes to give the desired accuracy.

The instantaneous heave value (dH_(n)) will then be

.

The successively computed heave values may be stored in a vector for further analysis in a Heave Motion Analysis unit. The time span of the data is in the range 20-60 seconds to yield sufficient accuracy.

The frequency (Fh) of the heave motion can be found via FFT (Fast Fourier Transform).

y = ¥¥T[Pad,dH,Pad]

where y is the spectral components from the FFT, dH is heave samples and Pad is a string of zeros to increase the frequency resolution of the FFT.

The frequency estimate of the heave motion is given by:

[α,.] = maxO0

where i is the index of the bin with the highest amplitude, a is the complex magnitude of the frequency component, m is the number of samples in the input sequence to the FFT and T_s is the sampling interval. The phase angle of the fundamental component referenced to the beginning of the measurement period is given by:

ΦΛ = angle(α) - 2πFi,(Tmaas + Ts)

where T_meas is the time span of the actual measurement period.

The peak amplitude of the heave is given by:

where N_m is the number of samples in the measurement period.

In a Heave Model unit the heave motion may be reconstructed using the extracted parameters.

Hm - Ham + 2Ah cos(2πFht + Φή)

where t is a time-vector with resolution 71.

Fig. 3 shows plots of simulated original and reconstructed heave motion.

Returning to Fig. 12, the elevation angle to the satellite may be computed in a Satellite Elevation Computation unit. The geometrical elevation angle to the satellite as seen from the ship terminal can be found as:

where Las and Los are satellite latitude and longitude, RE is earth radius and Hs is the satellite orbital height.

Atmospheric refraction may be corrected for with the following approximation:

El =

^E1 _s +r₂ where El is the incident elevation angle, γi and γ∑ are parameters in the range 1.13-1.37 and 2.77-2.63 respectively as ship location changes from equatorial to arctic regions, the appropriate set can be selected by table lookup based on the ships latitude.

DE is a correction value for the elevation angle which is drawn off from Maximum Detector.

The expected received signal power P_rx is estimated in a Received Signal Model unit. In its simples form this estimate may be expressed as:

where P_1101n is the nominal level, k is a coefficient in the range 0-1 where 0 is no reflection from the sea surface and 1 is full reflection, and:

, ΔZ _ 3xlO⁸ φ = — Lπ + % Λ = λ F .

cos(2E7) J

AL = HJ — ^ffl l sin(£/) ^' sm(El) J

Fig. 4 shows a simulation of this model. The simulations are based on a receive frequency of 1540 MHz and the ship motion illustrated in Fig. 3 and depicts simulated and reconstructed receive channel signal variation during the observation (measurement) period.

However, the modeling is sensitive to errors in the elevation angle. This is illustrated in Fig. 5, which shows the effect on the previous simulation if the elevation angle applied in the model is 0.5 degrees in error. This simulation is based on a correct value = 2.3°, and applied value=2.8°.

Returning again to Fig. 12, in order to improve the channel prediction it may be beneficial to ensure that the elevation angle is optimized to a high degree of accuracy. This may be done by a Correlator unit correlating the recorded received signal level samples S_rx with the estimated channel variations for the same time period. This may be repeated over a range of elevation angles around the nominal value: -mean(&.))

where Cor _E0 is the correlator result and Eo is the corresponding elevation angle.

Fig. 6 shows a normalized correlation of receive signal and reconstructed signal as a function of model elevation angle.

The output of the Correlator unit is used as input to a Maximum Detector unit.

Optimum elevation angle Eopt is found by the Maximum Detector as the value of Eo resulting in maximum Cor Έ₀-

The final channel prediction is then performed by a Channel Model unit. The Channel Model unit may perform similar computations as the Received Signal Model unit described previously:

Var_lx - 1 + k cos(φiχ)

where Var_rx and Var_tx are the predicted channel gain variations, £ is a coefficient in the range 0-1 where 0 is no reflection from the sea surface and 1 is full reflection, and:

φ J,_r ^{()8 ■}

2 Δ£ o « „ 3x10⁸ f _te = — 2ττ + ;Γ and Λ_x ⁼ '

A. K

Hm = Ham + 2Ai, cos(2π Fht_f + ΦΛ)

Fig. 7 illustrates the resultant channel variation prediction after elevation optimization. It is based on the same set of parameters as before but the channel behavior is in this case the time vector t/is spanning the prediction period, in this case the next 30 seconds following the observation period. Fig. 8 shows the corresponding return link channel variation.

Returning to Fig. 12, a Timing Qualifier unit receives the predicted channel models A_rx and A_tx and identifies time periods with positive variations (tl-t2, t3-t4 etc) by searching the channel predictions over the entire prediction interval. The Timing Qualifier unit may be duplicated in order to perform parallel operations for identifying time periods for reception T_{q rx} and for transmission T_qjx, respectively.

Timing uncertainties (δ_t) are accounted for by reducing the width of the window a predetermined amount:

T₁₁ = T^S₁ and T₁₁ = T₂ -S₁

Reference is made to Fig. 13, which shows a flow chart outlining the Timing Qualifier process. At a first step 101 a counter i is set equal to i = 0, the start time t₀ is set equal to a clock time t₀ = t representing the beginning of the prediction interval, and a flag St is set to St = 0. Determination is then made in a following step 102 regarding the value of St. As long as the flag St is St = 0, a determination is made in a determination step 103 regarding the value Aj, where A, represents the i-th sample value of the predicted channel model. If Aj > 1 (or positive if given in dB as in Fig. 7 - Fig. 10) the flag St is set to a value St = 1 in a step 104 and a start time T₁ representing the start of a "green" period is set equal to T₁ = to + i*T_s in a computing step 105. T₅ is the sampling interval for the predicted channel model. Following the detection of the beginning of a "green" period, the counter i is incremented by one in a step 106, and unless a maximum value for the counter i is reached in determination step 107, the process returns to determination step 102.

If the determination at step 103 indicates that A; < 1 the flag remains at St = 0, counter i is incremented in step 106, and unless i has reached a maximum value, the process returns to step 102.

Upon return to determination step 102, the value of flag St is again determined. If this value was set equal to St = 1 in step 104, indicating that a "green" period has been detected, prosecution moves on to determination step 108. In determination step 108 the value of A; for the next sample of the predicted channel model is determined. If A, > 1, indicating that the "green" period still continues, prosecution again moves to step 106 and the counter i is incremented. If, in determination step 108, the value of A; is determined to be A; < 1, indicating that the "green" period has ended, the flag St is reset to a value St = 0. The finishing time for the "green" period is set equal to T₂ = t₀ + i*T_s in a computing step 110. In a following step 111, timing uncertainties are accounted for by reducing the width of the window a predetermined amount, as already described. This reduction of the window width results in a beginning time T_b and an ending time T_e for the detected window.

Finally, in a determination step 112 the width of the "green" period, as represented by Te - Tb, is determined. If the detected "green" period is larger than a defined minimum Tmin the detected beginning T_b and end T_e of the "green" period are stored.

The process then again returns to step 106 where counter i again is incremented. When the counter has reached a maximum value i_max as determined in determination step 107, all stored values T_b and T_e are sent to the gateway.

It will be understood that the purpose is to detect a plurality of "green" windows inside the prediction interval. This means that a plurality of begin and end times T_b, T_e will be stored as tl-t2, t3-t4 etc. as already described with respect to Fig. 9 and Fig. 10. In other words, if the process reaches step 113 several times during execution of the process for one prediction interval, previously recorded values for T_b and T_e are retained, they are not replaced by the newly detected values.

Fig. 9 shows receive channel "green" slots detection and Fig. 10 shows transmit channel "green" slots detection.

It will be realized that it will not be necessary to operate in accordance with the invention under all conditions. If signals are received from or transmitted to a station with a high elevation angle possibly also combined with favorable conditions involving little or no significant heave, operating in accordance with the invention, which may be referred to as CAPT Mode may not be preferable. As shown in Fig. 12, a device operating in accordance with the invention may further include a CAPT Mode Detector that receives as its input the received signal level power S_rx and the output from the Correlator. If the CAPT Mode Detector detects conditions requiring CAPT mode, the CAPT Mode Detector may instruct the Sattelite Communications Terminal to transmit a CAPT Mode Request to the Gateway station through which it communicates.

The CAPT Mode Detector may make a positive detection if the following two conditions are fulfilled simultaneously: (1) ^- ≥ η meets

(2) Cor_msκ ≥ P

where Ti₀ is the elapsed time the received signal is below the required minimum level in a measurement period T_meas and η is a predetermined threshold value les than 1. Cor_max is the maximum value of the correlator output and P is a threshold value less than 1.

The Gateway Resource Manager (Fig. 11) will, upon receipt of the CAPT Mode Request, prioritize time slot allocation to the ship earth station sending the request, so that the ship earth station receives and transmits packets during favorable fading conditions. When the terminal is engaged in communications, new predictions are computed and sent to the Gateway at twice the prediction interval to accommodate variations in traffic patterns and queuing at the Gateway.

When the CAPT Mode Detector no longer makes a positive detection, the terminal signals this to the Gateway which will then allow transmission on any timeslot.

According to one embodiment consistent with principles of the invention, the functionality is implemented as one or more software modules in the ship earth station communications controller. Although the computations required are quite complex, the update intervals may be of the order of 10 seconds, making modest demands on the control processor. The GPS receiver may be integral to the equipment and in this case no additional hardware is required, if not an inexpensive GPS receiver interfacing the ship earth station would be required. According to some embodiments, the GPS receiver may preferably be placed close to the terminal antenna and at the same height.

Claims

1. A method for exploiting deterministic changes in the channel properties for a radio transmission channel between a low elevation radio station and a seagoing vessel, comprising:

5 - creating a model of the relative motion and location of an antenna on said vessel; and predicting future channel properties based on the created model.

2. The method according to claim 1, further comprising:

measuring received signal levels; io - correlating the predicted channel properties with the measured signal levels; and

- • optimizing the model for best fit between modeled and observed signal levels.

3. The method according to claim 2, further comprising:

using the optimized model to predict future channel properties.

4. The method according to claim 1, wherein the prediction of transmit channel I₅ properties are further based on different transmit frequencies.

5. The method according to one of the previous claims, comprising:

identifying time slots in the predicted channel properties with a likelihood of low distortion or destructive interference in a transmitted or received signal; and using said time slots to transmit and/or receive a signal.

20 6. A method substantially in accordance with the enclosed specification and drawings.

7. A device capable of operating substantially in accordance with the enclosed specification and drawings.

8. A computer program product on a computer readable medium, comprising ₂₅ computer code instructions enabling a computerized communications controller to operate substantially in accordance with the enclosed specification and drawings.