Forward Link Power Control

Abstract

A method of forward link power control in a communications system comprises: grouping a plurality of user terminals into a plurality of groups, at least one of which comprises more than one of the user terminals; for each said group, determining a corresponding forward link power level required to satisfy an aggregate demand of the group; and assigning one or more forward link carriers to the group, with the corresponding power level.

Description

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for power control in a forward link of a communication system, particularly but not exclusively a satellite communications system.

BACKGROUND OF THE INVENTION

Many satellite communications systems have Adaptive Coding and Modulation (ACM) that aim to maximize the throughput of a forward link (i.e. for transmission to a user terminal (UT)). An example system is disclosed in US2014/0056335 A1.

FIG. 1 depicts a satellite communication system comprising a ground station 1 that transmits uplink signals UL to a satellite 2, which transmits corresponding downlink signals DL in one or more beams. A plurality of UTs (User Terminals) 6 in a beam 5 are served by two forward carriers FC1 and FC2. The forward carriers FC1, FC2 are shared between multiple user terminals 6. In a typical satellite communication network both the forward carriers FC1, FC2 will operate with a fixed EIRP (Effective Isotropic Radiated Power).

Consider an example where a UT 6 is making a voice over internet-protocol (VOIP), and is in an area where there is excellent signal strength. In a conventional ACM method the UT 6 will report its link conditions and the network will adapt the code rate and modulation so that the user can achieve maximum data rate. However, the UT 6 only requires a sufficient data rate to make a VOIP call, whereas the maximum data rate may only be required if the UT 6 is streaming real time data. Hence, the conventional approach to ACM may not give the optimum overall system performance.

SUMMARY OF THE INVENTION

Aspects of the present invention are defined by the accompanying claims.

Embodiments of the invention include a method to optimise the system performance for given aggregate satellite power in the forward direction. This method may maximise total system throughput, rather than per user throughput in cases where the forward link is shared with a plurality of users.

Embodiments of the invention may use an algorithm that overcomes the conventional mismatch of requirements by adjusting the forward carrier EIRP.

The method may maintain equilibrium between ACM and optimisation of system capacity, for example by grouping of user terminals based on demand and/or geographic location, optimising the forward link power control such that the demand for each group is met, and balancing the total power available to optimise the link per user terminal group.

Optimising the forward link power may involve one or more of the following benefits. First, power distribution is no longer fixed so that user terminal groups with higher demand can be serviced with a higher power. Second, some user links may be operated with a lower power than in a conventional ACM method, which leads to reduction in interference, further improving system performance.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of a satellite communication system using a conventional ACM method;

FIG. 2 is a diagram of a satellite communication system using an ACM method according to an embodiment of the invention;

FIG. 3 is a flowchart of the ACM method in the embodiment; and

FIG. 4 is a diagram illustrating a specific use example of the embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

Apparatus for use in an embodiment of the invention is shown in FIG. 2, in which similar parts to those in FIG. 1 carry the same reference numerals.

In the following discussion, the term ‘carrier’ may refer to a frequency channel having a predetermined bandwidth. The carrier may be shared between different UTs 6, for example using time slots in a TDMA system or spreading codes in a CDMA system.

The ground station 1 is arranged to maintain a record of demand from the UTs 6, for example from forward data queue length and/or demand requirements received from the UTs 6. The ground station 1 and/or the satellite 2 are able to adjust independently the power (e.g. EIRP) of each forward link carrier, for example for the forward carriers FC1, FC2.

The ground station periodically runs the algorithm shown in FIG. 3 to achieve optimised system capacity. Step S1 is a trigger event to start the method. The trigger can be a periodic time to wake up the ground station 1 to run the algorithm or an event such as addition of a new UT to the pool. The next step S2 is to group the UTs 6. The ground station 1 monitors the forward data queue and any pre-negotiated demand parameters for a particular UT to calculate the user terminal demand in the forward link. The ground station 1 then sorts the UTs 6 based on their traffic demand. The grouping is then done based on segregating the UTs into groups 6.1 and 6.2. There may be more than 2 groups, but preferably the number of groups is limited to a maximum predefined value.

In Step S3 the ground station 1 calculates the maximum EIRP needed within a group 6.1, 6.2 to satisfy the demands of every UT within that group.

In step S4 the power control function of the satellite 2 and/or the ground station 1 is used to adjust the EIRP for each forward link FC1, FC2, preferably subject to the following constraints.

Constraints

Let there be a maximum of θ_max groups per beam 5 and let up_iθj be the EIRP required to achieve a data rate dr_iθj for user_iθj where i=i^th user and θj=j^th user group.

Constraint 1: Total Beam Power

${\mathrm{bp}}_n=\sum _{j=0}^{\theta \max }{\mathrm{up}}_{\theta \mathrm{jmin}}\le \mathrm{Max}EIRP\mathrm{per}\mathrm{Beam}5$

Where up_θjmin is the minimum power required in the j^th user group to attain max(dr_iθj).

Constraint 2: Aggregate EIRP

${\mathrm{power}}_{\mathrm{total}}=\sum _{j=0}^{\theta \max }{\mathrm{bp}}_n\le \mathrm{Aggregate}EIRP\mathrm{for}\mathrm{the}\mathrm{satellite}2$

Algorithm

An example of the method of FIG. 3 will now be described as an algorithm.

Start S1:

• • init_qos_i=Read QoS per User
  • cno_currentⁱ=Read CNo for User “i”
  • capacity_carrierⁱ=lookupdatarate(min(cno_currentⁱ), ue_type)
  • Look-up Table for Data-rate or carrier to Noise Ratio

For each User Equipment Type the network will store a look up table a mapping of the CNo and Datarate, for example as shown below.

• • LookupDataRate(Look up value, UT Type)

	Type	Sub Type	C/No	Data Rate
	T5X4	L8	51.5	110.7
	T5X4	L7	52.6	135
	T5X4	L6	53.75	162
	T5X4	L5	54.72	190.8
	T5X4	L4	55.77	220.5
	T5X4	L3	56.83	250.2
	T5X16	L2	57.35	270
	T5X16	L1	58.43	315
	T5X16	R	59.53	360
	T5X16	H1	60.50	400.5
	T5X16	H2	61.50	441
	T5X16	H3	62.48	486
	T5X16	H4	63.74	531
	T5X16	H5	64.80	559.8
	T5X16	H6	65.45	576
	T5X64	H2	66.25	634.4
	T5X64	H3	67.45	702
	T5X64	H4	68.64	767
	T5X64	H5	69.75	819
	T5X64	H6	70.95	858

Where Type indicates the modulation type (the number after ‘X’ indicating the number of possible modulation symbols), and Sub Type indicates the FEC coding rate.

GROUP Users S2:

cnorequiredi = lookupdatarate(init_qosi, uetype)
deficiti = cnocurrenti − cnorequiredi
sort_useri = sorthightolow(deficit)
Let qosi = init_qosi of sort_useri
cum                      i                                        =                                                                  ∑                                                  k                          =                          0                                                                          k                          =                          i                                                                                                                  qos                        i
n = 0
Loop1:
i = 0
loop2:
If(cumi/capacitycarriern > 0):
capacitygroupn = cumi
userni = sort_useri
Increment i
Else:
Exit loop1
End of If:
End of loop2:
total_carriers = n
Increment n
End of loop1:

Calculate EIRP S3:

groupcnorequiredn = lookupdatarate(capacitygroupn, uetype)
usernleast_capable = Read CNo for least capable UT in Group ′n′
cno_deficitn = usernleast_capable − groupcnorequiredn
For all Carriers
If cno_deficitn = 0 then don't do anything for this carrier
If cno_deficitn > 0:
Add carrier to the list ‘dec_power’ as the carier power has to be decreased
Else:
Add carrier to the list ‘inc_power’ as the carrier power has to be increased
Let D = number of carriers that have to be decreased in power, B be the
total number of carriers that have to be increased in power and C be the
total number of carriers that do not need any power change
Loop:
For each carrier (n = 1 to D) that needs power decrease
eirpn = Read EIRP of carrier ′n′
usernleast_capable = Read CNo for least capable UE in Group ′n′
delta_cnon = usernleast_capable − groupcnorequiredn
If delta_cnon = 0
D = D − 1 Remove carrier for list of acrriers to be reduced
Else:
power_adjustn = delta_cnon * α where α is the convergence factor < 1
Adjust_EIRP (carriern, − poweradjustn)
For each carrier (n = 1 to B) that need power increase
eirpn = Read EIRP of carrier ′n′
current_power=∑j=0j=D+B+Ceirpj
available_power = Max EIRP per Beam − current_power
power_adjustn = available_power/B
Adjust_EIRP ( carriern, poweradjustn)
Decrement D
If D = 0 Exit Loop:
End of Loop:

Adjust EIRP S4:

Adjust_EIRP ( carriern, poweradjustn)
If poweradjustn < Satellite_power_Adjust_Threshold
satellite_carrier_eirpn = satellite_carrier_eirpn + power_adjustn
Else poweradjustn > Satellite_power_Adjust_Threshold
satellite_carrier_eirpn
= satellite_carrier_eirpn
+ Satellite_power_Adjust_Threshold
power_adjustn
= power_adjustn − Satellite_power_Adjust_Threshold
SAS_carrier_eirpn = SAS_carrier_eirpn + power_adjustn

End S5:

End Algorithm

Specific Use Example

A specific use example will now be described with reference to FIG. 4, which shows a beam pattern of a satellite network utilising a four-colour re-use scheme. In other words, carriers are assigned into four groups, with the groups being re-used between different beams having a minimum separation.

In an example, there are 3 users in each of Beam A and Beam B, with symmetric demands. For ease of link analysis, the beam isolation at all points is assumed to be the same. The user demands are as shown in Table 1 below.

TABLE 1
User Demands
			Max QoS kbps
	User	Service Type	requested
	User 1A and 1B	Voice Call	24
	User 2A and 2B	Streaming	128
	User 3A and 3B	High Quality Video	858

The system will assign two full carriers F1 and F2 to users 3B and 3A respectively as they demand the maximum throughput that can be achieved by the system. Users 1A and 2A can be served on the same frequency as their maximum aggregate rate is 152 kbps which can be served with one carrier.

As the frequencies can be re-used, Users 1A and 1B are assigned frequency F1 and Users 2A and 2B are assigned frequency F2. In a conventional scenario the network will set both carriers with the same EIRP, for example 41.5 dBW. The conventional ACM allocates the best possible modulation and code rate. Table 2 details the link analysis for such a situation.

TABLE 2
Link analysis for all carriers with equal EIRP
			Max QoS	EIRP	CCI EIRP	C/No		Throughput
Beam	Frequency	Users	kbps	dBW	dBW	dBHz	Code Rate	kbps
A	F1	1A-F1	24	41.5	41.5	64.5	T5X16-H4	531
		2A-F1	128
	F2	3A-F2	856	41.5	41.5	64.5	T5X16-H4	531
B	F2	1B-F2	24	41.5	41.5	64.5	T5X16-H4	531
		2B-F2	128
	F1	3B-F1	856	41.5	41.5	64.5	T5X16-H4	531

The total power used by the two beams A and B is 47.5 dBW and the useful aggregate Data Rate is 1366 kbps.

In contrast, an implementation of the algorithm in an embodiment of the invention will now be described. The network has the information that Users 1A and 1B need a maximum of 24 kbps (IP voice) and Users 2A and 2B need a maximum of 128 kbps streaming rate i.e. an aggregate maximum of 152 kbps. Therefore the network can lower the power for those carriers. On the other hand users 3A and 3B need 858 kbps, which they cannot achieve. Therefore, the network will have to increase the power. The network optimises management of the co-channel interference and the required data rate, and adjusts the carrier powers as shown below, maintaining the same total power per beam.

TABLE 3
Link analysis for all carriers with optimised EIRP
			Max QoS	EIRP	CCI EIRP	C/No		Throughput
Beam	Frequency	Users	kbps	dBW	dBW	dBHz	Code Rate	kbps
A	F1	1A-F1	24	31.5	44	54.4	T5X4-L6	162
		2A-F1	128
	F2	3A-F2	856	44	31.5	67.6	T5X64-H3	702
B	F2	1B-F2	24	31.5	44	54.4	T5X4-L6	162
		2B-F2	128
	F1	3B-F1	856	44	31.5	67.6	T5X64-H3	702

The total power used in the total beams is almost the same as in the conventional case (the embodiment requires 0.25 dB less). The total data rate achieved is 1708 kbps i.e. 25% increase in total capacity for the two beams. In addition, the maximum per-user throughput goes up by 32% from 531 kbps to 702 kbps.

The application of the algorithm in the embodiment to the above example will now be described.

Start S1:

Each User signals their Max QoS (Quality of Service):

• • 1A, 1B Signals 24 kbps
  • 2A, 2B signals 128 kbps
  • 3A, 3B signals 858 kbps
  • The RAN records the current CNo for reach user. For simplicity in this example, they all have a CNo of 64.4 dBHz
  • Current max capacity of each current carrier=531 kps

Group Users S2:

The required CNo is obtained from a look up table which is specific to a given satellite network.

In this case for beam 1 the following are the required CNo

• • 1A, 1B require 51.5 dBHz
  • 2A, 2B require 52.6 dBHz
  • 3A, 3B require 71 dBHz

The above steps are shown for Beam A; the same applies to Beam B.

• • Calculate deficit
  • 1A=64.4−51.5=12.9
  • 2A=64.4−52.6=11.8
  • 3A=64.4−71.0=−6.6
  • Sort User from Highest to Lowest
  • 1A, 2A, 3A
  • Calculate Cumulative Max Qos
  • 24, 152, 1010
  • Group based on max capacity of current carrier=531
  • Group 1=1A, 1B with aggregate data requirement of 152
  • Group 2=3A with aggregate data requirement of 852

Calculate EIRP S3:

The required CNo for each group is obtained from a look up table which is specific to a given satellite network.

• • Group 1=53.75
  • Group 2=71
  • Calculate deficit CNo
  • Group 1=64.4−53.75=10.75
  • Group 2=64.4−71=−6.6
  • Let F1 be the frequency for Group 1 and F2 for Group 2

Therefore, power of F1 has to be reduced whereas the power of F2 has to be increased.

Applying the EIRP control algorithm of the embodiment:

TABLE 4
Example 1: alpha = 0.5 slower convergence
EIRP	EIRP	Current			Power	Total
F2	F1	CNo	DeltaCNo	Delta *	adjust	Power
dBW	dBW	dBHz	dB	0.5 dB	dB	dBW
41.5	41.5	64.5	10.75	5.4	5.0	44.5
44	36.5	59.13	5.38	2.7	3.0	44.5
44	33.5	56.54	2.79	1.4	1.0	44.5
44	32.5	55.39	1.64	0.8	1.0	44.5
44	31.5	54.43	0.68	0.3	0.0	44.5

TABLE 5
Example 2: alpha = 0.8 fast convergence
EIRP	EIRP	Current			Power	Total
F2	F1	CNo	DeltaCNo	Delta *	adjust	Power
dBW	dBW	dBHz	dB	0.8 dB	dB	dBW
41.5	41.5	64.5	10.75	8.6	9.0	44.5
44	32.5	55.39	1.64	1.1	1.0	44.5
44	31.5	54.43	0.68	0.5	0.0	44.5

Alternative Embodiments

Alternative embodiments of the invention may be envisaged, which may nevertheless fall within the scope of the accompanying claims.

Claims

1. A method of forward link power control in a wireless communication system, the method comprising: a. grouping a plurality of user terminals into a plurality of groups, at least one of which comprises more than one of the user terminals;b. for each said group, determining a corresponding forward link power level required to satisfy an aggregate demand of the group, and assigning one or more forward link carriers to the group, with the corresponding power level.

2. The method of claim 1, wherein the user terminals are grouped by forward link demand.

3. The method of claim 2, wherein the demand is determined from forward data queue length.

4. The method of claim 2, wherein the demand is determined from demand requirements received from the user terminals.

5. The method of claim 2, wherein at least one of the groups comprises a single said user terminal having a higher demand than other ones of the user terminals.

6. The method claim 1, wherein the users are grouped by geographic location.

7. The method of claim 1, wherein the wireless communication system is a satellite communications system.

8. The method of claim 7, wherein the user terminals are arranged in a plurality of beams in which the carriers may be re-used subject to a minimum re-use distance.

9. The method of claim 1, wherein the number of groups is limited to a predetermined maximum.

10. The method of claim 1, wherein the forward link power levels are constrained by a maximum aggregate power level.

11. The method of claim 1, wherein the power level comprises an EIRP.

12. The method of claim 1, wherein the carriers comprise frequency channels.

13. The method of claim 1, including assigning a corresponding coding and/or modulation rate to each of the forward link carriers.

14. The method of claim 1, including transmitting to the user terminals using the forward link carriers with the corresponding power levels.

15. A wireless communication system comprising: a plurality of user terminals; and a transmitter arranged to transmit wireless signals to the plurality of user terminals over a forward link;wherein the plurality of user terminals are grouped into a plurality of groups, at least one of which comprises more than one of the user terminals;for each said group, a corresponding forward link power level required to satisfy an aggregate demand of the group is determined, and one or more forward link carriers are assigned to the group, with the corresponding power level.

16. A non-transient computer readable medium containing program instructions for causing a computer to: a. group a plurality of wireless user terminals into a plurality of groups, at least one of which comprises more than one of the user terminals;b. for each said group, determine a corresponding forward link power level required to satisfy an aggregate demand of the group, andc. assign one or more forward link carriers to the group, with the corresponding power level.