Source: http://www.google.nl/patents/US20040166887
Timestamp: 2017-12-15 04:31:42
Document Index: 464134197

Matched Legal Cases: ['art 1500', 'art 1700', 'art 1750', 'art 1700', 'art 1900', 'art 1997', 'art 2000', 'art 1998', 'art 2001', 'art 2001', 'art 2005', 'art 2006', 'art 2006', 'art 2007', 'art 2007', 'art 2008', 'art 2010', 'art 2011', 'art 2011', 'art 2006', 'art 2014', 'art 2014', 'art 2013', 'art 2006', 'art 2015', 'art 2010', 'art 2005', 'art 2006', 'art 2013', 'art 2013', 'art 2005', 'art 2007', 'art 2007', 'art 2008', 'art 2008', 'art 2008', 'art 2008', 'art 2007', 'art 2009', 'art 2010', 'art 2011', 'art 2006', 'art 2006', 'art 2006', 'art 2006', 'art 2006', 'art 2010', 'art 2010']

Patent US20040166887 - Pilot signals for use in multi-sector cells - Google Patenten
Pilot signal transmission sequences and methods are described for use in a multi-sector cell. Pilots in different sectors are transmitted at different known power levels. In adjacent sectors a pilot is transmitted while no pilot is transmitted in the adjoining sector. This represents transmission of...http://www.google.nl/patents/US20040166887?utm_source=gb-gplus-sharePatent US20040166887 - Pilot signals for use in multi-sector cells
Publicatienummer US20040166887 A1
Aanvraagnummer US 10/648,767
Publicatiedatum 26 aug 2004
Aanvraagdatum 25 aug 2003
Prioriteitsdatum 24 feb 2003
Ook gepubliceerd als US9544860
Publicatienummer 10648767, 648767, US 2004/0166887 A1, US 2004/166887 A1, US 20040166887 A1, US 20040166887A1, US 2004166887 A1, US 2004166887A1, US-A1-20040166887, US-A1-2004166887, US2004/0166887A1, US2004/166887A1, US20040166887 A1, US20040166887A1, US2004166887 A1, US2004166887A1
Uitvinders Rajiv Laroia, John Fan, Junyi Li
Oorspronkelijke patenteigenaar Rajiv Laroia, Fan John L., Junyi Li
Patentcitaties (100), Verwijzingen naar dit patent (211), Classificaties (17), Juridische gebeurtenissen (3)
US 20040166887 A1
measuring at least one of an amplitude and a phase of a first pilot signal corresponding to a first pilot tone to produce a first measured signal value;
generating a first channel quality indicator value from said first measured signal value according to a first function which uses at least said first measured signal value as an input;
transmitting the first channel quality indicator value;
measuring at least one of an amplitude and a phase of a second pilot signal corresponding to a second pilot tone to produce a second measured signal value, the second pilot signal having a different transmission power than said first pilot signal;
generating a second channel quality indicator value from said second measured signal value according to a second function which uses at least said second measured signal value as an input; and
transmitting the second channel quality indicator value.
2. The method of claim 1, wherein one of the first and second pilot signals is a NULL signal transmitted with zero power.
3. The method of claim 1, wherein generating a first channel quality indicator value from said first signal measurement value according to a first function includes:
estimating the power included in at least one of the first and second received pilot signals.
4. The method of claim 3, wherein generating a second channel quality indicator value from said second signal measurement value according to a second function includes:
estimating the received power included in at least the second received pilot signal.
5. The method of claim 3, wherein generating a second channel quality indicator value from said second measured signal value according to a second function further includes:
estimating the signal to noise ratio of the second received pilot signal.
6. The method of claim 1, wherein generating a first channel quality indicator value from said first measured signal value according to a first function includes:
estimating the signal to noise ratio of the first received pilot signal.
7. The method of claim 6, wherein generating a second channel quality indicator value from said second measured signal value according to a second function includes:
8. The method of claim 1, wherein said first and second pilot tones are received during different non-overlapping time periods.
9. The method of claim 8, wherein said first and second pilot tones correspond to the same frequency.
10. The method of claim 1, wherein said first and second pilot tones are received during the same time period, the first and second pilot tones corresponding to different frequencies.
wherein transmitting the first channel quality indicator value includes:
incorporating said first channel quality indicator value into a first message; and
transmitting said first message over a wireless communications link.
wherein transmitting the second channel quality indicator value includes:
incorporating said second channel quality indicator value into said first message; and
transmitting said second channel quality indicator value with said first value in said first message over the wireless communications link.
repeatedly performing said steps of:
incorporating said first channel quality indicator value into a first message;
transmitting said first message over a wireless communications link;
incorporating said second channel quality indicator value into a second message which is different from said first message; and
transmitting said second message over said wireless communications link.
periodically repeating said steps of transmitting the first channel quality indicator value and the second channel quality indicator value to transmit the corresponding values generated by repeatedly performing said measuring and generating steps, the generated first and second channel quality values being transmitted in an interleaved manner over time.
15. The method of claim 14, wherein said interleaved manner includes alternating the transmission of said first and second messages.
16. The method of claim 13, wherein said first and second messages are transmitted using communications channel segments dedicated to carrying channel quality indicator values, said messages carrying no explicit message types to indicate said messages are to report channel quality values.
17. The method of claim 16, wherein said messages are transmitted during pre-selected dedicated time slots dedicated for use by said wireless terminal, said dedication of said dedicated time slots precluding other wireless terminals using said dedicated time slots.
18. The method of claim 1, wherein said wireless terminal is located in a first sector of a sectorized cell in which each sector uses the same set of tones, the step of measuring at least one of an amplitude and a phase of a first pilot signal to produce a first measured signal value including:
performing said first pilot signal measurement during a time period during which a sector located adjacent said first sector transmits another pilot signal on the same tone as the first pilot but using a different pre-selected transmission power from the pre-selected transmission power used to transmit the first pilot signal.
19. The method of 18, wherein said another pilot signal is a NULL pilot signal and wherein said different pre-selected transmission power used to transmit said another pilot signal during said time period is zero.
20. The method of claim 19, wherein said second step of measuring at least one of an amplitude and a phase of a second pilot signal to produce a second measured signal value, includes:
performing said second pilot signal measurement during a time period during which a sector located adjacent said first sector transmits an additional pilot signal on the same tone as the second pilot using the same pre-selected transmission power as the pre-selected transmission power used to transmit the second pilot signal.
21. The method of claim 20, wherein the first and second pilot signal measurements are performed at the same time.
measuring, at said same time, the power received on a third tone on which no signals are transmitted during said same time, said same time being a symbol period used to transmit one symbol.
determining relative position of the wireless terminal to at least two adjacent sectors to the sector in which the wireless terminal is located based on said first and second signal measurements; and
selecting channel information from to be transmitted to said base station as a function of the determined relative position to a sector boundary.
25. The method of claim 24, wherein different channel condition information is transmitted when said wireless terminal is near a first sector boundary than when it is near a second sector boundary.
26. The method of claim 18, wherein the first channel quality indicator value is a function of a ratio of channel gain of an interfering sector and the sector in which the wireless terminal is located.
27. The method of claim 18, wherein the second signal measurement is made during a time period where each of the sectors transmits a NULL on said second tone; and
wherein said second channel quality indicator value is a measurement of the noise on said second tone during the transmission of said NULL by each of the sectors of the cell on said second tone.
28. The method of claim 18, wherein said method is further directed to using channel quality information to control transmission power in a sector of a cell, the method comprising:
operating a base station to receive said first and second channel quality indicator values; and
operating the base station to calculate from the first and second channel quality indicator values, an amount of transmission power required to achieve a desired signal to noise ratio at said wireless terminal, said calculating requiring at least two different channel quality indicator values to determining said amount of transmission power.
periodically repeating said step of operating the base station to calculate said amount of transmission power using a different set of first and second channel quality indicator values received from said wireless terminal, each different set of first and second channel quality indicator values corresponding to a different symbol time during which said first and second pilot signal measurements were made.
30. A wireless terminal, said wireless terminal including:
measuring means for measuring at least one of an amplitude and a phase of a first pilot signal to produce a first measured signal value and at least one of an amplitude and a phase of a second pilot signal to produce a second measured signal value;
channel quality indicator value generation means for generating a first channel quality indicator value from said first measured signal value according to a first function which uses at least said first measured signal value as an input and generates a second channel quality indicator value from said second measured signal value according to a second function which uses at least said second measured signal value as an input; and
31. The wireless terminal of claim 30, wherein said channel quality indicator value generation means includes software instructions for controlling a processing device to:
estimate the received power included in at least one of the first and second received pilot signals.
32. The wireless terminal of claim 31, wherein said channel quality indicator value generation means further includes additional software instructions for controlling the processing device to:
estimate the received power included in at least the second received pilot signal.
33. The wireless terminal of claim 31, wherein said channel quality indicator value generation means further includes additional software instructions for controlling the processing device to:
estimate the signal to noise ratio of the second received pilot signal.
34. The wireless terminal of claim 31, wherein said means for transmitting includes:
a message generation module for generating a first message including said first channel quality indicator value.
35. The wireless terminal of claim 34, wherein said message generation module includes said second channel quality indicator value in said first message.
36. The wireless terminal of claim 34, wherein said message generation modules includes machine executable instructions for controlling a machine to generate a second message including said second channel quality indicator value.
37. The wireless terminal of claim 34, further comprising:
means for determining the position of the wireless terminal relative to a sector boundary from received signals.
38. The wireless terminal of claim 37, wherein said message generation module includes position information in said first message.
39. A base station, comprising:
means for determining from at least two different channel quality indicator values a transmission power required to achieve a desired signal to noise ratio at said wireless terminal.
40. The base station of claim 39, wherein said at least two different channel quality indicator values correspond to different power signal measurements made by said wireless terminal at the same time, said determined transmission power being a function of said at least two channel quality indicator values.
41. The base station of claim 40, further comprising:
means for transmitting a signal to said wireless terminal using a transmission power determined from said at least two channel quality indicator values.
42. The base station of claim 41, further comprising:
means for extracting said at least two different channel quality values from a single message received from said wireless terminal.
43. The base station of claim 41, further comprising:
means for extracting said at least two different channel quality values from two separate messages received from said wireless terminal.
44. The base station of claim 40, further comprising:
means for receiving channel quality indicator information indicating the position of the wireless terminal relative to a second boundary included in a multi-sector cell.
45. The base station of claim 40, further comprising:
a multi-sector transmit antenna for transmitting pilot signals into a plurality of sectors of a cell at the same time; and
a transmitter coupled to said multi-sector antenna for transmitting pilot signals into each sector in a synchronized manner such that transmission of the pilot tones into all sectors of the cell use the same set of tones and are transmitted at substantially the same time in each of the sectors, said wireless terminal being located in one of said multiple sectors.
The present application claims the benefit of U.S. Provisional Patent Application S.No. 60,449,729 filed Feb. 24, 2003.
The present invention is directed to wireless communications systems and more particularly to methods and apparatus for performing measurements of channel conditions.
The pilot sequences and signal measurements of the present invention provide mechanisms that enable a wireless terminal (WT), and a BS that receives channel condition feedback information from the WT, to predict downlink receive SNR for the WT as a function of the signal transmit power in the presence of signal dependent noise. Feedback from individual WTs, in accordance with the invention, normally includes at least two channel quality indicator values per WT, as opposed to a single SNR value, where each of the two channel quality indicator values is generated using a different function. One of the two channel quality indicator value generator functions has a first pilot signal measurement corresponding to a received pilot signal having a first known transmission power as an input. A second one of the two channel quality indicator value generator functions has as an input a second pilot signal measurement corresponding to another received pilot signal having a second known transmission power which is different from the first known transmission power. Each of the first and second channel quality indicator value generator functions, which may be implemented as software modules or as hardware circuits, may also have additional inputs to those just mentioned.
According to one embodiment of this invention, ‘cell null pilots’ are used in conjunction with regular pilots to characterize the dependence of total noise at a WT on the power of the signal transmitted by the BS to that WT. Cell null pilots are downlink resources (degrees of freedom) where none of the sectors of the cell transmit any power. Noise measured on these degrees of freedom provides an estimate of the signal-independent noise at the WT. Regular pilots (or simply pilots) are resources (degrees of freedom) where each sector of the cell transmits known symbols using fixed or predetermined powers. Noise measured on the pilots thus includes inter-sector interference and provides an estimate of the total noise, including signal-dependent noise.
In the embodiment of a sectorized deployment with directional sector antennas, a single cell is divided into multiple sectors, some or all of which may be sharing the same frequency band (degrees of freedom), corresponding to a frequency reuse of 1. In this situation, in addition to the cell null pilot, the invention describes the use of sector null pilots that are present in a subset of the sectors but not all sectors, and also gives a pattern for pilot tones such that a null pilot tone in one sector is time/frequency synchronized with a pilot tone in some or all of the other sectors. This allows the WT to measure two or more signal-to-noise ratios, which include interference from different combinations of sectors. On a reverse link, the WT reports a set of SNR-related statistics, which enables the BS to make an estimate of these received SNR levels at a WT as a function of the base station's transmit power. The BS uses the reported channel quality values to determine the power level at which to transmit to achieve a desired SNR at the WT.
[0025]FIG. 1 is a simplified diagram showing a transmitter and a receiver used for explaining the present invention.
[0026]FIG. 2 shows an exemplary wireless cellular system.
[0027]FIG. 3 shows an example where noise is dependent on transmitted signal power and is used for explaining the present invention.
[0028]FIG. 4 shows an example of an exemplary noise characteristic line, showing received power vs total noise, and is used for explaining the present invention.
[0029]FIG. 5 shows a graph of power vs frequency corresponding to an exemplary embodiment of the invention illustrating data tones, non-zero pilot tones, and a null pilot tone.
[0030]FIG. 6 is a graph illustrating the relationship between SNR1, a wireless terminal received SNR including signal dependent and signal independent noise, and SNR0, a wireless terminals received SNR including no signal dependent noise for 3 cases: where noise is independent of the signal, where the signal dependent noise is equal to the signal, and where the signal dependent noise is less than the signal.
[0031]FIG. 7 shows exemplary signaling for a three sector OFDM embodiment of the invention illustrating non-zero pilot tones, sector null pilot tones, and cell null pilot tones in accordance with the invention.
[0032]FIG. 8 illustrates an example of tone hopping of the non-zero pilots, sector null pilot, and cell null pilots in accordance with the invention.
[0033]FIG. 9 illustrates three situations for an exemplary wireless terminal in a 3 sector embodiment used to explain the present invention in regard to the sector boundary information aspects of the present invention.
[0034]FIG. 10 illustrates a scheme using 3 sector types, which are repeated for the cases with cells involving more than 3 sectors in accordance with the present invention.
[0035]FIG. 11 illustrates an exemplary communications systems implementing the present invention.
[0036]FIG. 12 illustrates an exemplary base station implemented in accordance with the present invention.
[0037]FIG. 13 illustrates an exemplary wireless terminal implemented in accordance with the present invention.
[0038]FIG. 14 illustrates the steps of transmitting pilot tones in multiple sectors of a cell in a synchronized manner in accordance with the present invention.
[0040]FIG. 18 illustrates a chart showing the transmission of signals on ten different tones during a single symbol transmission period in accordance with the present invention.
[0041]FIG. 19 is a flowchart illustrating the operation of an exemplary wireless terminal implementing the methods of the present invention.
[0042]FIG. 20 is a flowchart illustrating the operation of an exemplary base station implementing the methods of the present invention.
The methods and apparatus of the present invention are well suited for use in a wireless communications system which uses one or more multi-sector cells. FIG. 11 illustrates an exemplary system 1100 with a single cell 1104 shown but it is to be understood that the system may, and often does, include many of such cells 1104. Each cell 1104 is divided into a plurality of N sectors wherein N is a positive integer greater than 1. System 1100 illustrates the case where each cell 1104 is subdivided into 3 sectors: a first sector S0 1106, a second sector S1 1108, and a third sector S2 1110. Cell 1104 includes a S0/S1 sector boundary 1150, a S1/S2 sector boundary 1152, and a S2/S0 sector boundary 1154. Sector boundaries are boundaries where the signals from multiple sectors, e.g., adjoining sectors, may be received at almost the same level making it difficult for a receiver to distinguish between transmissions from the sector in which it is located and the adjoining sector. In the cell 1104, multiple end nodes (ENs), e.g., wireless terminals (WTs), such as mobile nodes, communicate with a base station (BS) 1102. Cells with two sectors (N=2) and more than 3 sectors (N>3) are also possible. In sector S0 1106, a plurality of end nodes EN(1) 1116, EN (X) 1118 are coupled to base station 1 1102 via wireless links 1117, 1119, respectively. In sector S1 1108, a plurality of end nodes EN(1′) 1120, EN (X′) 1122 are coupled to base station 1 1102 via wireless links 1121, 1123, respectively. In sector S2 1110, a plurality of end nodes EN(1″) 1124, EN (X″) 1126 are coupled to base station 1 1102 via wireless links 1125, 1127, respectively. In accordance with the invention, the base station 1102 transmits pilot signals at multiple power levels to the ENs 1116, 1118, 1120, 1122, 1124, 1126, and there is synchronization of the transmission of pilot signals of various predetermined and known levels between the three sectors. In accordance with the invention, the end nodes, e.g., EN(1) 1116 report feedback information, e.g., channel quality indicator values to the base station 1102, allowing the base station 1102 to determine the wireless terminals received SNR as a function of base station transmitted signal power. Base station 1102 is coupled to a network node 1112 via network link 1114. The network node 1112 is coupled to other network nodes, e.g., intermediate nodes, other base station, AAA nodes, home agent nodes, etc., and the internet via network link 1129. Network node 1112 provides an interface outside cell 1104, so that ENs operating within the cell may communicate with peer nodes outside the cell 1104. The ENs within cell 1104 may move within the sectors 1106, 1108, 1110 of the cell 1104 or may move to another cell corresponding to another base station. Network links 1114 and 1129, maybe, e.g., fiber optic cables.
[0044]FIG. 12 illustrates an exemplary base station (BS) 1200, implemented in accordance with the invention. Base station 1200 is a more detailed representation of base station 1102 shown in the exemplary communication system 1100 of FIG. 11. The base station 1200 includes sectorized antennas 1203, 1205 coupled to receiver 1202 and transmitter 1204, respectively. The receiver 1202 includes a decoder 1212 while the transmitter 1204 includes an encoder 1214. Base station 1200 also includes an I/O interface 1208, a processor, e.g., CPU, 1206 and memory 1210. The transmitter 1204 is used to transmit pilot signals into multiple sectors in a synchronized manner via sectorized transmit antenna 1205. The receiver 1202, the transmitter 1204, the processor 1206, the I/O interface 1208, and the memory 1210 are couple together via bus 1209 over which the various elements can interchange data and information. The I/O interface 1208 couples the base station 1200 to the Internet and to other network nodes.
The memory 1210 includes routines 1218 and data/information 1220. Routines 1218, which when executed by the processor 1206, cause the base station 1200 to operate in accordance with the invention. Routines 1218 include communications routine 1222, a received signal processing routine 1260, and base station control routines 1224. The received signal processing routine 1260 includes a channel quality indicator value extraction module 1262 which extracts channel quality indicator values from received signals, e.g., WT report messages, and a position information extraction module 1264 for extracting WT position information from received messages. The position information, in some embodiments, indicates a WT's position relative to a sector boundary. Extracted channel quality indicator values, e.g., SNR or power values, are provide to the transmission power calculation routine 1226 for use in calculating transmission power for signals transmitted to a WT. The base station control routines 1224 include a scheduler module 1225, a transmission power calculation routine 1226, and signaling routines 1228 including a pilot signal generation and transmission control routine.
The sector ID 1248 identifies which of the three sectors, S0, S1, S2, WT 1 is operating in. The channel quality indicator values 1250 include information conveyed by WT 1 to the base station in channel quality report messages, that the base station may use to calculate the expected received WT1 SNR level as a function of base station transmission signal power. The channel quality indicator values 1250 are derived by WT1 from measurements performed by WT1 on the various strength pilot signals transmitted by the base station, in accordance with the present invention. The sector boundary position information 1252 includes: information identifying whether WT1 has detected that it is near a sector boundary, experiencing high levels of interference and information identifying which sector boundary WT1 is located near. This information is obtained or derived from position feedback information transmitted by the WT1 and received by the BS. The channel quality indicator values 1250 and the sector boundary position information 1252 represent channel quality feedback information from the WT1 to the base station 1200, providing information about one or more downlink channels between the base station 1200 and WT1.
Communications routines 1222 is used for controlling the base station 1200 to perform various communications operations and implement various communications protocols. Base station control routines 1224 used to control the base station 1200 to perform basic base station functionality, e.g., signal generation and reception, scheduling, and to implement the steps of the method of the present invention including generation of pilot signals at different transmission strength levels, reception and processing and use of wireless terminal reported information. The signaling routine 1228 controls the transmitter 1204 and the receiver 1204 which generate and detect signals to and from the wireless terminals, e.g. OFDM signals following data tone hopping sequences. Pilot signal generation and transmission control routine uses the data/ information 1220 including the pilot hopping sequence info 1234 to generate a specific pilot tone hopping sequences for each sector. The power levels of the pilot tones, included in power level info 1236 and the specific tones selected to receive specific pilot tones for each pilot in each sector at specific times are coordinated and controlled under the direction of the pilot signal generation and transmission control routine 1230. This routine 1230 controls the transmission of pilot tones, e.g., as illustrated in FIGS. 15-17. Individual processing instructions, e.g., software commands, responsible for the transmission of different pilot tones are individual components or modules which may be interpreted as separate means which operate together to control the base station to transmit the pilot tone sequences described and shown in FIGS. 15-17. Coordinating and/or synchronizing the transmission of various types of pilot signals between the sectors of a cell, e.g., in terms of transmission frequency, and/or symbol transmission time while controlling transmission power, enables a wireless terminal receiving the various levels of transmitted pilot tones, e.g., known predetermined fixed level pilot tones, sector null pilot tones, and cell null pilot tones, to obtain, e.g., compute from measured signal values, channel quality indicator values 1250. In accordance with the invention, regular (non-null) pilot tones, sector null pilot tones, and cell null pilot tones may punch through or replace data tones that would normally be transmitted. Scheduling module 1225 is used to control transmission scheduling and/or communication resource allocation. The scheduler 1225, in accordance with the invention, may be supplied with information indicating each wireless terminal's received SNR as a function of the base station transmitted signal power. Such information, derived from the channel quality indicator values 1250, may be used by the scheduler to allocate channel segments to WTs. This allows the BS 1200 to allocate segments on channels having sufficient transmission power to meet received SNR requirements for a particular data rate, coding scheme, and/or modulation selected to be provide to a WT.
[0049]FIG. 13 illustrates an exemplary wireless terminal 1300 implemented in accordance with the present invention. The wireless terminal 1300 may be used as a wireless end node, e.g., a mobile node. Wireless terminal 1300 is a more detailed representation of the ENs 1114, 1116, 1118, 1120, 1122, 1124 shown in the exemplary communications system 1100 of FIG. 11. Wireless terminal 1300 includes a receiver 1302, a transmitter 1304, a processor, e.g., CPU, 1306, and memory 1308 coupled together via a bus 1310 over which the elements may interchange data and information. The wireless terminal 1300 includes receiver and transmitter antennas 1303, 1305 which are coupled to receiver and transmitter 1302, 1304 respectively. The receiver 1302 includes a decoder 1312 while the transmitter 1304 includes an encoder 1314. Processor 1306, under control of one or more routines 1320 stored in memory 1308 causes the wireless terminal 1300 to operate in accordance with the methods of the present invention as described herein. Memory 1320 includes routines 1320 and data/information 1322. Routines 1320 includes communications routine 1324 and wireless terminal control routines 1326. The wireless terminal control routines 1326 includes signaling routine 1328 including a pilot signal measuring module 1330, a channel quality indicator value generating module 1332, a sector boundary position determining module 1331, and a channel quality indicator value transmission control module 1333. Data/information 1322 includes user data 1334, e.g. information to be transmitted from the wireless terminal 1300 to a peer node, user info 1336, and pilot signaling info 1350. User info 1336 includes measured signal values info 1337, quality indicator value information 1338, sector boundary position information 1340, terminal ID information 1342, base station ID information, and channel report information 1346. Pilot signaling info 1350 includes hopping sequence info 1352, power level info 1354, and tone info 1356. The measured signal value info 1337 includes measured signal values obtained from measurements, performed under the control of pilot signal measuring module 1330, of a at least one of an amplitude and phase of a received pilot signal. The quality indicator value information 1338 includes output from the channel quality indicator value generating module 1332. The channel quality indicator value information 1338, when transmitted to a base station may allow the base station to determine the WTs received SNR as a function of transmitted signal power. Sector boundary position information 1340 includes information identifying that the wireless terminal is in a sector boundary region, e.g., the wireless terminal is experiencing high inter-sector interference levels, and information identifying which of the two adjacent sectors is the boundary region sector. The base station may use the sector boundary information to identify channels in adjacent sectors where the transmission power should be turned off to reduce inter-sector interference. Channel report information 1346 includes the quality channel indicator values 1338 obtained or portions of the channel quality indicator values 1338 and may also include sector boundary position information 1340. The channel report information 1346 may be structured with individual messages for each quality indicator value or with groups of quality indicator values included in a single message. The messages may be sent out at periodically at predetermined times on dedicated channels. The terminal IUD information 1342 represents a base station assigned identification applied to the wireless terminal 1300 while operating within the cellular coverage area of the base station. The base station ID info 1344 includes information specific to the base station, e.g., a slope value in a hopping sequence, and may also include sector identification information.
[0053]FIG. 1 is a simplified diagram showing a transmitter 101 and a receiver 103 which will be used for explaining the invention. Transmitter 101 may be, e.g., the transmitter 1204 of base station 1200, while receiver 103 may be, e.g., the receiver 1302 of wireless terminal 1300. In a communications system, such as the system 1100, the transmitter 101 often needs to make choices about the appropriate method for transmitting data to the receiver 103. The choices may include the code rate of the error-correcting code, the modulation constellation, and the transmit power level. In general, in order to make sensible choices, it is desirable for the transmitter 101 to have knowledge about the communication channel from the transmitter 101 to the receiver 103. FIG. 1 shows an exemplary system 100, in which a transmitter 101 sends data traffic 102 to a receiver 103 on a forward link 105. On a reverse link 107 from the receiver 103 to the transmitter 101, the receiver 103 reports the forward link's channel condition 106 to the transmitter 101. The transmitter 101 then uses the reported channel condition information 106 to set its parameters properly for transmission.
[0054]FIG. 2 shows an exemplary wireless cellular system 200 where a transmitter is included in a base station (BS) 201 with antenna 205 and a receiver is included in a wireless terminal (WT), 203, e.g., a mobile terminal or a fixed terminal, with antenna 207, enabling the base station 201 to communicate information on the downlink channel(s) 208 to the wireless terminal 203. The BS 201 often transmits pilot signals 209, which are typically transmitted on a small fraction of the transmission resource and are generally comprised of known (pre-determined) symbols transmitted at a constant power. The WT 203 measures the downlink channel condition 213 based on the received pilot signals 209, and reports the channel conditions 213 to the BS 201 on an uplink channel 215. Note that since the channel conditions 213 often change over time due to fading and Doppler effects, it is desirable that the BS 201 transmit the pilots 209 frequently or even continuously so that the WT 203 can track and report channel conditions 213 as they vary with time. The WT 203 can evaluate the downlink channel conditions 213 based on the received signal strength and the noise and interference on the pilot signals 209. The combination of noise and interference will be referred to subsequently as ‘noise/interference’ or sometimes just ‘noise’. In the prior art techniques, this type of information is normally reported in the form of a single scalar ratio such as signal-to-noise ratio (SNR) or an equivalent metric. In the case where noise/interference is not dependent on the transmitted signal, such a single scalar metric is usually all that is required at the BS 201 to predict how the received SNR will change with signal transmit power. In such a case, the BS 201 can determine the correct (minimum) transmit power for the coding and modulation it selects to transmit from the single received value. Unfortunately, in the multi-sector case, noise resulting from transmitted signals can be a significant signal component making a single scalar value insufficient for accurate SNR predictions for different transmission power levels.
In many communication situations, especially in cellular wireless systems, such as the multi-sector system 1100 of the invention, the noise is not independent of the signal transmit power but depends on it. There is generally a component of noise called ‘self-noise’, which is proportional or roughly proportional to the power of the signal. FIG. 3, shows an example where noise is dependent on signal transmit power. In FIG. 3, graph 300 shows received power of the signal of interest on the vertical axis 317 vs total noise on the horizontal axis 303. Total noise, represented by line 305 which is the sum of a signal dependent portion 309 and a signal independent portion 307, is plotted against the received signal power 317. There may be many reasons for the self-noise. An example of self-noise is the unequalized signal energy that interferes with the received signal. This noise is proportional to the signal strength. The unequalized signal energy could result from error in channel estimation or error in the equalizer coefficients or from many other reasons. In situations where the self-noise is comparable to or larger than the signal-independent noise, a single scalar downlink SNR value (which may be measured on a pilot) is no longer adequate for the BS 1200 to accurately predict the received SNR at the WT 1300 as a function of the signal transmit power.
This invention provides a methods and apparatus which enable each WT 1300 to predict its downlink receive SNR as a function of the signal transmit power in the presence of signal dependent noise 309 and communicate this information to the BS 1200. This enables the BS 1200 to transmit to different WTs at different (minimum) signal powers depending upon the respective SNRs required at each of the WTs. The total power transmitted by the BS 1200 is typically known or fixed but the proportion allocated to different WTs 1300 may be different and may vary over time. At a WT receiver 1302, the dependence of total noise 303 as a function of the received signal power 317 can be modeled by a straight line 305, referred to as the ‘noise characteristic line’ in this application, as shown in FIG. 3. Since the noise characteristic line 305 does not in general go through the origin, a single scalar parameter is not enough to characterize this line 305. At least two parameters, e.g., two channel quality indicator values, are required to determine this line 305. A simple method of determining this line is to identify the location of two distinct points, e.g., points 311 and 315, on it, since any two distinct points uniquely determine a straight line. Note that as a practical matter, the points can be determined with a limited accuracy, so that the accuracy with which the line is determined is better if the points are chosen farther apart than if the points are closer together.
Signal noise and various signaling issues will now be discussed further. Graph 400 of FIG. 4 plots received power of a signal of interest on the vertical axis 401 vs total noise on the horizontal axis 403. FIG. 4 gives an illustration of an exemplary noise characteristic line 405. To characterize the line 405, in accordance with the invention, the BS 1200 transmits signals that enable the WT 1300 to make measurements of at least two distinct points on the line, e.g. points 407 and 409, information, characterizing the line 405, obtained from those measurements is then transmitted to the BS 1200. For example, the BS 1200 can transmit two different signal powers P1 and P2 that will be received as powers Y1 and Y2 as shown in FIG. 4. The WT 1300 measures the corresponding received signal powers, denoted as Y1 415 and Y2 419, and the corresponding total noise, denoted as X1 413 and X2 417, respectively. From X1 413, X2 417, Y1 415, and Y2 419, the slope and the intercept of the line 405 can be uniquely determined. In one embodiment, P1 and P2 are known and fixed. In another embodiment, P2 can be the pilot power, corresponding to a pilot signal, while P1 can be zero, representing a null signal, which occupies some transmission resource but with zero transmission power. In general, however, P1 does not necessarily have to be zero. For example, P1 can and in some embodiments is some positive number smaller than P2.
Once the noise characteristic line 405 has been determined by the BS 1200 from received feedback information, the BS 1200 can calculate the SNR at the WT receiver 1302 for any given transmission power Q. For example, FIG. 4 shows the procedure of determining the SNR corresponding to a given transmission power Q. First, the BS 1200 finds the corresponding received signal power Y 421 of transmission power Q, by linearly interpolating between the points (Y2, P2) and (Y1, P1): Y = Y1 + Y2 - Y1 P2 - P1 · ( Q - P1 ) .
The corresponding noise power corresponding to a transmission power Q is given by linearly interpolating between the points (X2, P2) and (X1, P1): X = X1 + X2 - X1 P2 - P1 · ( Q - P1 )
Then SNR(Q), the SNR as seen by the WT 1300 for a BS transmit power Q, is given by: SNR  ( Q ) = Y X = Y1  ( P2 - P1 ) + ( Y2 - Y1 )  ( Q - P1 ) X1  ( P2 - P1 ) + ( X2 - X1 )  ( Q - P1 )
[0064]FIG. 5 shows a graph 500 plotting power on the vertical axis 501 vs frequency on the horizontal axis 503. FIG. 5 corresponds to one exemplary embodiment of this invention, in which the wireless cellular network uses Orthogonal Frequency Division Modulation (OFDM). In this exemplary case, the frequency 505 is divided into 31 orthogonal tones, such that transmissions on different tones do not interfere with each other at the receiver, even in the presence of multipath fading in the channel. The minimum unit of signal transmission is a single tone in an OFDM symbol, which corresponds to a combination of time and frequency resources.
[0065]FIG. 5 shows the power profile of the tones at a given OFDM symbol. In this embodiment, a pilot 515 is a known symbol sent at a fixed pilot power 507 on a tone, and the null pilot 513 is a tone with zero transmission power. These pilot tones 515 and null pilot tones 513 may hop over time, meaning that from one OFDM symbol to the next, the position that they occupy may vary. Over extended periods of time, the pilot signal transmissions are periodic due to the repetition of the hopping sequences. Four pilot tones 515 and one null pilot tone 513 are shown in FIG. 5. The tone locations of the pilots 515 and the null pilots 513 are known to both the BS 1200 and the WT 1300. Twenty-six data tones 511 are also shown in FIG. 5 with corresponding transmission power level 509. FIG. 5 illustrates that the pilot tone transmission power level 515 is significantly higher than the data tone transmission power level 509, allowing the wireless terminals to easily recognize pilot tones. In general, the data tone transmission power 509 may not necessarily be the same across all the data tones as shown in FIG. 5, but level 509 may vary from data tone to data tone.
The knowledge of the received SNR is important since it determines the combination of coding rates and modulation constellations that can be supported. For a specified target block error rate (e. g., the probability that the transmission of a single codeword is incorrect) and for each coding rate and modulation constellation, it is possible to define a minimum SNR that the received SNR must exceed in order for the probability of unsuccessful transmission to be less than the specified target rate (e.g., 1% block error rate). From this point of view, it is desirable for the BS 1200 be able to accurately estimate SNR(Q) in order to solve for the transmit power Q that will produce an SNR that exceeds the minimum SNR for the desired code rate and modulation constellation.
Let α denote the channel gain, so that when the BS transmits at power P, the received power by wireless terminal is αP . Let N denote the signal-independent noise, and γP represent the signal-dependent noise, where γ is the proportionality factor to the transmit power P. Then when measuring the SNR on pilot tones, the WT 1300 measures an SNR of SNR1  ( P ) = α   P N + γ   P ,
By using the null pilot, it is possible for the WT 1300 to separately measure the signal-independent noise N, since there is no power transmitted by the BS 1200 on this null tone. By comparing this signal-independent noise N with the received power αP of the BS pilot, it is possible to estimate an SNR that is free of signal-dependent noise. Let us represent this ratio by SNR0  ( P ) = α   P N ,
where the name ‘SNR0’ indicates that it considers no signal-dependent noise. Then the relationship between SNR1(P) and SNR0(P) is given by: 1 SNR1  ( P ) = 1 SNR0  ( P ) + γ α .
For notational simplicity, let us define SRR1 = γ α .
Comparing with the noise characteristic line shown in FIGS. 3 and 4, one can see that SNR0(P) corresponds to the x-axis intercept of the line, while SRR1 is equivalent to the slope of the line. Then as a function of SNR0(P) and SRR1, we can write: SNR1  ( P ) = SNR0  ( P ) SRR1 · SNR0  ( P ) + 1 .
Graph 600 of FIG. 6 illustrates the relationship between SNR1(P) on the vertical axis 601 and SNR0(P) on the horizontal axis 603, where the SNRs are plotted in dB. Three curves illustrates by lines 605, 607, and 609 representing SRR1=0, SRR1=0.5 and SRR1=1, respectively. The case of SRR1=0 (line 605) corresponds to the situation where noise is independent of the signal, so that SNR1(P)=SNR0(P). The case of SRR1=1 (line 609) corresponds to the case where the signal-dependent noise is equal to the signal so that it is never possible for SNR1(P) to exceed 0 dB.
From the information received from the WT 1300, the BS 1200 can then compute the received SNR as a function of the transmit power Q for the data traffic. The received SNR by the WT 1300 will include signal-dependent noise, and takes the form SNR1  ( Q ) = α   Q N + γ   Q .
Inverting and performing substitutions gives: 1 SNR1  ( Q ) = N α   Q + γ α = 1 SNR0  ( P )  P Q + SRR1 SNR1  ( Q ) = SNR0  ( P ) SNR0  ( P ) · SRR1 + P Q
Hence as a function of the values SNR0(P) and SRR1 reported by the WT 1300, it is possible to predict the SNR as seen by the WT 1300 for any transmit power Q. These derivations illustrate that using the null pilot, the WT 1300 can determine and transmit statistics to the BS 1200 which enable the BS 1200 to predict SNR as a function of transmit power in the presence of signal-dependent noise that is proportional to the transmit power.
The methods and apparatus of the present invention are particularly useful in a multi-sector cell. In wireless cellular systems, base stations 1200 are often deployed in a configuration where each cell is divided into multiple sectors as shown in FIG. 11. For a sectorized environment, the interference between sectors 1106, 1108, 1110 has a significant impact on the received SNR. In addition to the signal-independent portion, the total noise also includes signal-dependent portions, each of which is proportional to the signal power from other sectors of the same cell 1104. The noise characteristics in this case are more complex than what is shown in FIG. 3, because in this sectorized situation, the total noise includes two or more signal-dependent components instead of one. However, the total noise can still be characterized by a straight line, which is now defined in a higher dimensional space. This noise characteristic line can be described, for example, by an intercept and slopes. The intercept is a function of the signal-independent noise portion and each slope corresponds to the proportionality of the signal-dependent noise portion with respect to a particular signal power.
In certain scenarios, however, the description of the noise characteristic line can be simplified. For example, in an exemplary method of sectorization, where the each of the sectors of a cell may use the entire or nearly the entire transmission resource, e.g., frequency band, to transmit in each of the sectors. The total power transmitted from each sector is typically fixed or known but different WTs 1300 may receive a different fraction of it. Since the isolation between the sectors is not perfect, signal transmitted on one sector becomes noise (interference) to other sectors. Furthermore, if each of the sectors 1106, 1108, 1110 is constrained to transmit identical, proportional or nearly proportional signal power on a given degree of freedom, the interference from other sectors to a WT 1300 in a given sector 1106, 1108, 1110 appears like signal dependent noise or self-noise. This is the case because the interference from other sectors scales with signal power, so that the noise characteristic line is similar to what is shown in FIG. 3.
In accordance with the invention, the BS 1200 transmits signals such as the ‘cell null pilot’ that enable the WT 1300 to evaluate the intercept of the noise characteristic line with all of the signal-independent noise. In addition, as an example, the scheduling amongst the sectors 1106, 1108, 1110 may be coordinated so that WTs 1300 at the boundary 1150, 1152, 1154 of sectors do not receive any interference (or receive reduced interference) from other sectors. In accordance with the invention, the BS 1200 transmits signals such as the 'sector null pilot’ that enable the WT 1300 to evaluate the slope of the noise characteristic line taking into account only the signal-dependent noise from a subset of sectors. In accordance with the invention, the WT 1300 then reports the signal-independent SNR and these different slopes, or some equivalent set of information, back to the BS 1200 on a reverse link.
[0084]FIG. 7 shows in diagram 700 the signaling for an embodiment of the invention in the case of a sectorized cellular wireless system using Orthogonal Frequency Division Modulation (OFDM). Consider a BS 1200 with three sectors 701, 703, 705, in which the same carrier frequency is reused in all sectors 701, 703, 705. The pilot power level corresponding to sectors 701, 703, 705 are indicated by reference numbers 709, 713 and 717, respectively. Data signal power levels are indicated by reference numbers 711, 715, 719 for each of the first through third sectors, respectively. The situation of other numbers of sectors will be discussed below. Let the three sectors 1106, 1108, 1110 of the base station 1200 be represented by S0 701, S1 703, and S2 705 as shown in FIG. 7. FIG. 7 shows a tone allocation for the downlink transmission at a given OFDM symbol 707, including an example of the placement of data tones, e.g. exemplary data tone 728, pilot tones, e.g. exemplary pilot tone 728, and null pilot tones, e.g. exemplary null pilot tone 721, across the three sectors. Since it is assumed that each of the sectors share the same frequency band, the corresponding tones between sectors will interfere with each other. Note that the position and order of the tones are shown for illustrative purposes only and may vary in different implementations.
In accordance with the invention, the downlink signal includes one or more cell null pilots, which are null tones that are shared by each of the sectors 701, 703, 705. In a cell null pilot 729, there is zero transmission power in each of the sectors 701,703, 705. In addition, the downlink signal includes one or more sector nulls 721, 723, 725 where the transmission power is zero only in a subset of the sectors 701, 703, 705. In the same tone as the sector null pilot, it is desirable to have a pilot tone or a data tone whose transmission power is fixed and known to the WT 1300 in the other sectors. For example, sector S1 703 sector null pilot 723, has corresponding sector S0 701 pilot tone 731 and corresponding sector S2 705 pilot tone 737.
Suppose that the WT 1300 has a link established with sector SO of the base station 1200, and that the channel gain from S0 to WT 1300 is given by α. Similarly, suppose that the channel gain from S1 to WT 1300 is given by β, and from S2 to WT 1300 is given by γ. Finally for completeness, suppose that the signal-dependent noise in the link from S0 to WT 1300 includes self-noise that is proportional to the transmit power with a channel gain of δ.
Suppose that the transmit power for the data tones on the three sectors is given by Q0, Q1, and Q2, respectively. Then the received SNR for the link from S0 to WT 1300 is given by SNR S0  ( Q0 , Q1 , Q2 ) = α   Q0 δ   Q0 + β   Q1 + γ   Q2 + N .
The WT 1300 should provide a set of parameters to the base station so that it has enough information to predict the received SNR for the downlink data transmission from S0 to WT 1300. To obtain that information, it may use the null pilot tones. Using a cell null pilot, in which the transmission in each of the sectors is 0, it is possible to measure the signal-independent noise. Comparing that with the received strength of the pilot from S0 gives the following SNR: SNR0  ( P ) = α   P N
Next, the sector null pilot tones can be, and in various embodiments are, used to measure the SNR in the situation when one of the neighboring sectors is not transmitting. In particular, for sector S0, consider the pilot tone that corresponds to a sector null pilot tone in S2. Then measuring the SNR based on this pilot in sector S0 will give the value SNR1 β  ( P ) = α   P β   P + N ,
where the interfering sector is S1 (with path gain β). Similarly, by measuring the SNR on the pilot tone that is a sector null tone in S1, the interfering sector is sector S2 (with path gain γ), and the resulting SNR is given by SNR1 γ  ( P ) = α   P γ   P + N .
The slopes of the noise characteristic line in these two cases are β α   and   γ α ,
Next, if the SNR is directly measured using pilot tones that do not correspond to sector null pilots in the other sectors, then this SNR measurement takes into account the interference from the other two sectors. This measurement is called SNR2, since it includes interference from two sectors. SNR2  ( P ) = α   P β   P + γ   P + N
The slope of the noise characteristic line in this case is β + γ α .
By defining the following SRR as proper slope values of the noise characteristic lines, it is possible to relate SNR1 β(P), SNR1 γ(P), and SNR2(P) to SNR0(P): SRR2 = β + γ α SRR1 β = β α SRR1 γ = γ α
The SRRs themselves can be computed in terms of the SNRs as follows: SRR2 = 1 SNR2  ( P ) - 1 SNR0  ( P ) SRR1 β = 1 SNR1 β  ( P ) - 1 SNR0  ( P ) SRR1 γ = 1 SNR1 γ  ( P ) - 1 SNR0  ( P )
Note that SRR2 can be found as the sum of SRR1 β and SRR1 γ.
Then the SNRs can be written in terms of SNR0(P) and the SRRs: SNR2  ( P ) = SNR0  ( P ) 1 + SRR2 · SNR0  ( P ) SNR1 γ  ( P ) = SNR0  ( P ) 1 + SRR1 γ · SNR0  ( P ) SNR1 β  ( P ) = SNR0  ( P ) 1 + SRR1 β · SNR0  ( P )
If the WT 1300 reports a sufficient set of these statistics (e.g., SNR0(P), SRR1 β, SRR1 γ, SRR2) to the base station 1200, the base station 1200 can predict the received SNR by the WT 1300 based on the transmit powers Q0, Q1, and Q2. In general, the SNR as seen by the WT 1300 for a data transmission with power Q0, with interference from sectors S1 and S2 with powers Q1 and Q2, is given in terms of the measurements made on the pilot tone with transmit power P as: SNR S0  ( Q0 , Q1 , Q2 ) =  α   Q0 β   Q1 + γ   Q2 + N =  SNR0  ( P ) ( Q1 Q0  SRR1 β + Q2 Q0  SRR1 γ ) · SNR0  ( P ) + P Q0
In an embodiment of the invention, for each of these three situations, the WT sends a subset of the measured statistics to the BS 1200, in order to reduce the amount of information conveyed on the reverse link, e.g., the uplink.
In the situation shown in FIG. 9 with respect to cell 901, suppose that the WT 909 in sector S0 903 receives significant interference from sector S1 905. Then a coordinated scheduler 1225 for the base station can turn off the data transmissions in sector S1 905 that interfere with the transmissions from sector S0 903 to the WT 909. Meanwhile, the transmission in sector S2 907 is coordinated so that it has the same or nearly the same transmit power Q as in sector S0. Then the SNR seen by the WT 909 will be given by SNR S0  ( Q , 0 , Q ) =  α   Q γ   Q + N =  SNR0  ( P ) SRR1 γ · SNR0  ( P ) + P Q
in which case it is sufficient to report SNR0(P) and SRR1 γ.
Next, for the situation shown in FIG. 9 with respect to cell 921, in which the WT 929 is not near a sector boundary, it is possible to transmit on most or all sectors without causing too much interference to the WT 929. In this case, suppose the base station scheduler 1225 makes the simplifying assumption that each of the three sectors should transmit data with the same power Q. Then the SNR seen by the WT 929 for a transmission from sector S0 923 is given by SNR S0  ( Q , Q , Q ) =  α   Q β   Q + γ   Q + N =  SNR0  ( P ) SRR2 · SNR0  ( P ) + P Q
Next, for the situation shown in FIG. 9 with respect to cell 941, the WT 949 is located near the sector boundary with sector S2 947. Since the WT 949 receives significant interference from sector S2 947, a coordinated scheduler 1225 for the base station 1200 can turn off the corresponding data transmissions in sector S2 947. Meanwhile, suppose the transmission for sector S1 945 is scheduled with the same transmit power Q as in sector S0 943. Then the SNR seen by the WT 949 will be given by SNR S0  ( Q , Q , 0 ) =  α   Q  β   Q + N =  SNR0  ( P ) SRR1 β · SNR0  ( P ) + P Q
in which case it is sufficient to report SNR0(P) and SRR1 β.
Indicator SNR Other sectors WT reports
00 SNRS0(Q, Q, Q) Transmit on all sectors SNR0(P), SRR2
10 SNRS0(Q, Q, Q) Turn off sector S2 SNR0(P), SRR1γ
11 SNRS0(Q, Q, 0) Turn off sector S1 SNR0(P), SRR1β
A multi-sector cell with an arbitrary number of sectors will now be discussed. In another embodiment of this invention, for the situation where there are an arbitrary number of sectors, the sectors are divided into three sector types, which we will label S0, S1 and S2. This classification into sector types is done in such a way that two adjacent sectors will not have the same type. It is assumed that for two non-adjacent sectors, the effect of interference is considered small enough as to not be significant, so that the main cause of interference is from adjacent sectors of different types. Hence it is possible to treat this situation in an analogous fashion to the case of the 3-sector cell, since the primary source of interference in each sector comes from its two neighboring sectors.
[0116]FIG. 10 includes a diagram 1000 that shows the sector types for exemplary cells 1001, 1021, and 1041 with 3, 4 and 5 sectors, respectively. Cell 1001 includes a first sector S0 type sector 1003, a first sector S1 type sector 1005, and a first sector S2 type sector 1007. Cell 1021 includes a first sector S0 type sector 1023, a first sector S1 type sector 1025, a first sector S2 type sector 1027, and a second S2 type sector 1029. Cell 1041 includes a first sector S0 type sector 1043, a first sector S1 type sector 1045, a first sector S2 type sector 1047, a second S0 type sector 1049, and a second S1 type sector 1051. Table 2 set forth below gives an example of a plan for different numbers of sectors, where the order of the list of sector types corresponds to the order proceeding (e.g., clockwise) around the sector.
Number of sectors Sector types
2 S0, S1
3 S0, S1, S2
4 S0, S1, S2, S1
5 S0, S1, S2, S0, S1
6 S0, S1, S2, S0, S1, S2
7 S0, S1, S2, S0, S1, S2, S1
8 S0, S1, S2, S0, S1, S2, S0, S1
9 S0, S1, S2, S0, S1, S2, S0, S1, S2
[0121]FIG. 14 illustrates the steps of an exemplary method 1400 of transmitting pilot tones in multiple sectors of a cell in a synchronized manner in accordance with the present invention. The method starts in start node 1402 and proceeds to step 1404 wherein a current symbol time counter is initialized, e.g., to 1. Symbols are transmitted in the exemplary system on a per symbol basis with a symbol time being the time used to transmit one symbol along with a cyclic prefix which is normally a copy of a portion of the transmitted symbol that is added for redundancy to protect against multipath interference and minor symbol transmission timing errors.
[0129]FIG. 15 is a chart 1500 showing a two-sector pilot tone transmission sequence implemented in one exemplary embodiment of the present invention. As will be discussed below, the sequence shown in FIG. 15 can be extend to systems with N sectors, where N is an arbitrary number greater than 1. The sequence shown in FIG. 15 is implemented for a cell which includes two sectors, sector A and sector B. The symbol times in each sector may be slightly offset but substantially overlap and therefore will be described as the same symbol time although actually being two slightly different symbol times in many cases. The first column 1502 titled time refers to the symbol time in which a tone is transmitted assuming perfect synchronization between sectors. In one embodiment, where the same tone is used in each symbol time for pilot signal purposes, each symbol time 1 through 4, corresponds to a different current symbol time. The second column 1504 titled TONE lists the tone, e.g., frequency, on which the pilot signals are transmitted. Each row corresponds to one tone. Different rows may correspond to the same or different tones depending on the particular implementation. For example, in cases where the first through fourth symbol times are the same current symbol time, then the first through fourth tones listed in column 1504 will be different since each pilot signal requires one tone. However, in cases where the first through 4th symbol times in column 1502 correspond to different current symbol times, the tones listed in column 1504 may be the same or different.
Row 1512 shows that at symbol time 1, using tone 1, a 1 pilot signal is transmitted in sector A while a NULL pilot signal is transmitted in sector B. This makes it possible to measure the contribution of inter-sector interference in sector B caused by sector A transmission on the same tone. It also allows sector A to make accurate measurements of the attenuation in sector A without the presence of interference due to sector B transmission. Row 1514 corresponds to symbol time 2 wherein tone 2 is used to transmit a NULL tone in sector A and a 1 pilot signal in sector B. This allows sector A to determine the amount of signal interference due to sector B transmission on the same tone. Row 1516 corresponds to symbol time 3 wherein tone 3 is used to transmit a NULL pilot signal in both sectors A and B making general background noise measurements possible on tone 3. Row 1518 corresponds to symbol time 4 wherein tone 4 is used in both sectors A and B to transmit 1 pilot signals. In such a case each sector can measure the effect of having a signal transmitted with the same non-zero power level in each of sectors A and B at the same time. Normally pilot signals are transmitted in accordance with both the first and second rows 1512, 1514 of FIG. 15 and at least one of rows 1516 and 1518 in order to provide a wireless terminal to make sufficient signal measurements which required as input to the two different functions used to generate the first and second channel quality indicator values that are feedback to the base station 1200 in accordance with one feature of the invention.
[0133]FIG. 16 illustrates an exemplary pilot tone transmission sequence for a three sector system. As in the FIG. 15 example, the first column 1602 corresponds to symbol transmission time, the second column 1604 corresponds to tone while columns 1606, 1608 and 1610 indicate pilot signal transmissions in each of three sectors A, B and C of a cell, respectively. Thus, as in the FIG. 15 example, each rectangle of column 1606, 1608 and 1610 which corresponds to one of the first through fifth rows, 1612, 1614, 1616, 1618, 1620 represents the step of transmitting a pilot signal on the indicated tone in the indicated sector. While the tones used in each row are the same in each sector, as discussed above, when each of the symbol times corresponds to the same current symbol time, the each of the first through fifth tones will be different. However, when each of the first through fifth symbol times are different the first through fifth tones may be the same or different.
[0135]FIG. 17 is a chart 1700 showing a three sector implementation similar to FIG. 16 with the pilots transmitted in each sector being described in a more general manner in terms of power levels. The transmission of 15 pilots P1 through P15 are shown in the FIG. 17 embodiment with each pilot being transmitted at a different symbol time in the case where each row corresponds to a different transmission symbol period. In the case where each of the listed signals are to be transmitted in the same symbol time, three different symbol times are shown, with the transmission time of each sector being slightly different but corresponding to substantially the same symbol time as used in the other sectors.
As in the FIG. 15 and 16 examples the pilots of each row 1712, 1714, 1716, 1718, 1720 are transmitted using the same tone but different rows may correspond to different tones. While being shown as being transmitted at 5 different symbol times as listed in the fist column 1702, when variations in sector transmission times is taken into consideration each rectangle listed on the heading Sector may actually correspond to a different symbol time with the symbol times of each row substantially overlapping and being identical in the case of precise synchronization. The power level of each of the first through 15th pilots P1 through P15 are represented in parenthesis, e.g., the transmission power for P1 is p1. While in some cases such as in the FIG. 16 example two different power levels are supported, multiple known power levels may be supported. The last row 1720 of FIG. 17 represents the transmission of a NULL pilot signal using tone 5 in each of sectors A, B and C according the power level of these pilot signals is 0 in each case.
[0137]FIG. 18 illustrates a chart 1750 showing the transmission of signals on 10 different tones during a single symbol transmission time period. In the FIG. 17 implementation the 0 is used to represent a NULL pilot signal, while a 1 is used to represent a pilot at a single known non-zero transmission power level which is normally higher than the power level at which data is transmitted. D is used in the chart 1700 to illustrate the transmission of data in one of the sectors A, B and C. The data signal D is usually transmitted on the tone at a power level lower than the pilot signal level 1 and therefore may not cause significant interference with the pilot in the neighboring sector. Data is normally transmitted in each of the sectors on additional tones not shown in FIG. 17 during the illustrated symbol time. In the OFDM embodiment of the present invention, in a given sector such additional data tones do not interfere with the pilot tones since they are orthogonal to the tones used to transmit pilot signals. FIG. 19 illustrates a method 1800 of operating a wireless terminal to process pilots signals received from a base station 1200, which were transmitted in accordance with the present invention. The received pilot signals may be pilot signals that were transmitted with known different transmission power levels allowing the receiving device to make various signal measurements and computations useful for determining various noise contributions, e.g., background noise as well as inter-sector interference.
[0145]FIG. 20 shows a flowchart 1900 illustrating a method of operating base station (BS) 1200 in accordance with the present invention, e.g., to transmit pilot tones and to receive and process feedback information to determine the power level at which to transmit data signals. The method starts with step 1902 where the base station 1200 is powered on and operational. In step 1904, base station's transmitter 1204, coupled to a multi-sector antenna 1205, is transmits pilot signals into each sector, e.g. S0 1106, S1 1108, S2 1110 of a multi-sector cell, e.g., 1104 at the same time in a synchronized manner using predetermined power levels and tones such that the transmission of the pilot tones into each of the sectors 1106, 1108, 1110 of the cell 1104 use the same set of tones and are transmitted at substantially the same time in each of the sectors 1106, 1108, 1110. The transmission of pilot tones in step 1904 is performed under the direction of the pilot signal generation and transmission control routine 1230 using pilot tone power level info 1236 and tone info 1238. Operation proceeds to step 1906 where BS 1200 receives messages from at least one wireless terminal (WT) 1300 including, e.g., a set of channel quality indicator values, e.g., first and second channel quality indicator values, and sector boundary position information. The messages are received under the direction of the received signal processing routine 1260 included in base station 1200. In step 1908, the base station, under the direction of channel quality indicator value extraction module 1262 extracts at least two different channel quality indicator values 1250, e.g., from a single message or from multiple messages received from a wireless terminal 1300. In some embodiments each channel quality indicator value is in a separate message. In other embodiments multiple channel quality indicator values are include in a single message from a WT 1300. Next, in step 1910, the base station 1200, under control of position information extraction module 1264, extracts location information from received messages, e.g., boundary position indicator value, indicating the position of a wireless terminal 1300 relative to a boundary in a multi-sector cell. This location information may have been transmitted by WT 1300 in a separate message or may have been included in a message including channel quality indicator values. This location information may identify whether the WT 1300 is near a sector boundary, and identify which the sector boundary, e.g., identify the adjacent sector from which a higher level of transmission power dependent interference is being received. Sector boundary information extracted from received messages is stored in sector boundary position information 1252 in BS 1200.
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Classificatie in de VS 455/522, 455/69, 455/456.1
Internationale classificatie H04B17/00, H04B7/005, H04B7/04, H04W16/24, H04W52/32, H04W52/24
Coöperatieve classificatie H04B7/0491, H04B17/309, H04B17/24, H04W16/24, H04W52/24, H04W52/325
Europese classificatie H04B7/04S, H04W52/32C
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:LAROIA, RAJIV;FAN, JOHN L.;LI, JUNYI;REEL/FRAME:015029/0047