Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J11/00—Orthogonal multiplex systems, e.g. using WALSH codes
  • H04J11/0023—Interference mitigation or co-ordination
  • H04J11/005—Interference mitigation or co-ordination of intercell interference
  • H04J11/0053—Interference mitigation or co-ordination of intercell interference using co-ordinated multipoint transmission/reception
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B15/00—Suppression or limitation of noise or interference
  • H04B15/02—Reducing interference from electric apparatus by means located at or near the interfering apparatus

Definitions

• the present invention relates generally to wireless communication networks, and in particular to a network-centric system and method of downlink link adaptation for Coordinated Multi-Point (CoMP) cells.
• CoMP Coordinated Multi-Point
• Wireless cellular communication networks are well known and widely deployed, and provide mobile voice and data communications to millions of subscribers.
• a fixed transceiver base station, Node B, etc.
• a sempiternal design goal of cellular communication networks is to efficiently and consistently deliver communication services to mobile subscribers at high data rates.
• Link adaptation also known in the art as adaptive modulation and coding - is a technique to maximize data rates by dynamically altering the modulation (e.g., QPSK, 16-QAM, 64-QAM), the level or degree of redundancy in Forward Error Correction (FEC) coding, and other signal and protocol parameters, to deliver the maximum rate to a UE given the radio link conditions.
• modulation e.g., QPSK, 16-QAM, 64-QAM
• FEC Forward Error Correction
• the network transceiver selects from among a defined set of modulation techniques, coding schemes, and the like, based on an estimate of the instantaneous quality of the downlink channel to each UE.
• the Channel Quality Information is typically reported by the UE, and may comprise the Signal to Interference and Noise Ratio (SINR) measured or estimated by the UE.
• SINR Signal to Interference and Noise Ratio
• OFDM Orthogonal Frequency Division Multiplexing
• SINR(t) [SINR(kl ;t) SINR(k2;t) .... SINR(K; t)],
• the SINR(k;t) experienced by a UE depends on the desired signal transmitted to the UE, interference from transmissions to other UEs in the same sub- cell, interference from transmissions to other UEs in other sub-cells, and thermal noise.
• Conventional link adaptation can be described as UE-centric, in that each UE periodically measures SINR(k;t), and these measurements are reported to the network - with a delay of several Transmission Time Intervals (TTI) - on the uplink, e.g., in Channel Quality Information (CQI) reports.
• TTI Transmission Time Intervals
• CQI Channel Quality Information
• a significant shortcoming of such UE- centric link adaptation is that in packet-oriented cellular system, the own-cell and other- cell interference typically change from one TTI to the next, depending on scheduling at the network transceivers. Accordingly, the UE-reported SINR(k;t) is a very poor predictor of SINR(k; t+d), where "d" is a positive delay. This poor predication leads to underutilization of precious radio resources, and can significantly reduce the overall spectral efficiency of the system. Furthermore, attempts to improve the predictive value of UE-reported SINR(k; t+d) by increasing the CQI reporting frequency, to shorten "d,” increase uplink congestion and interference, and reduce the uplink data .
• a network-centric link adaptation process is performed by each CoMP cell controller.
• the CoMP cell controller receives at least infrequent channel estimates from a UE in the CoMP cell, from which it estimates downlink channel quality and thermal noise at the UE.
• the CoMP cell controller is aware of the desired signal to be received at the UE, and the intra-CoMP cell interference to the UE caused by transmissions to other UEs in the CoMP cell.
• the CoMP cell receives from the UE reports of inter-CoMP cell interference caused by transmissions by other CoMP cells.
• the CoMP cell controller Based on the downlink channel quality, the desired signal, the intra-CoMP cell interference, the inter-CoMP cell interference, and the thermal noise, the CoMP cell controller performs link adaptation by selecting modulation and coding schemes, and other transmission parameters, for an upcoming transmission duration (such as a TTI).
• the CoMP cell controller may facilitate the estimation of the inter-CoMP cell interference by not transmitting from the network transmitters serving the UE during certain intervals known to the UE.
• Figure 1 is a functional block diagram of a Coordinated Multi-Point (CoMP) cell in a wireless communication network.
• CoMP Coordinated Multi-Point
• Figure 2 is a functional block diagram of a plurality of CoMP cells in a wireless communication network.
• Figure 3 is a flow diagram of a method of link adaptation by a CoMP cell controller.
• Figure 4 depicts two graphs of simulation results.
• TDMA scheduling each cell schedules each resource block (RB) independently; hence, in one TTI, a cell might decide to transmit on a particular RB, and this same cell might decide not to transmit on this RB in the next TTI.
• RB resource block
• the matrix-valued transmit power spectral density of the signal transmitted from each network transmitter (where each network transmitter may consists of one or more transmit antennas) on each RB might also change from one TTI to the next, depending on which UE is scheduled on each RB.
• LTE typically only one user is scheduled on each RB in each cell; hence, own-cell interference is typically zero in LTE. This implies that in LTE, the dominant source of errors in predicting SINR is the fast varying other-cell interference.
• Coordinated multipoint is a technology to minimize inter-cell interference.
• a plurality of geographically contiguous cells - referred to as sub-cells - are grouped together to form a CoMP cell.
• Each CoMP cell has a central controller that coordinates transmission within its constituent sub-cells so as to maintain inter-cell interference within the CoMP cell (referred to herein as intra-CoMP cell interference) below a predetermined amount.
• the CoMP cell controller coordinates scheduling of transmissions to and from user equipment (UE) within the cells, and/or actively suppresses interference using signal processing techniques.
• UE user equipment
• CoMP cell and “sub-cell” is used herein, more current CoMP terminology refers to a CoMP cell as a “CoMP cooperating set” or a “CoMP cluster", while a constituent cell is simply referred to as a "cell” rather than a “sub-cell”. This change in terminology does not impact the CoMP functionality described herein.
• CoMP technology as described in the previous paragraph includes a plurality of sub-cells.
• a CoMP cell may comprise a plurality of geographically contiguous cells with base stations located at different sites, often referred to as "inter-site” CoMP technology.
• a CoMP cell may include a plurality of geographically contiguous cells (or sectors) with base stations located at a common site, often referred to as "intra-site" CoMP technology.
• a site in a cellular system may be divided into three sectors, each covering 120 degrees, with three logically separated base stations.
• a CoMP cell consists of a single cell with a single base station. Accordingly, the following description, and Figures 1 -4, shall be construed to encompass any CoMP technology that includes a plurality of sub-cells or a CoMP cell that consists of a single sub-cell with a single base station.
• FIG. 1 depicts a Coordinated Multi-point (CoMP) cell 12 comprising, in this example, seven conventional cells, referred to herein as sub-cells 14.
• Each sub-cell 14 includes a network transceiver 16 (also known as a base station, NodeB, Access Point, or the like) providing wireless communications to subscribers within the sub-cell 14, including mobile UEs 18.
• a CoMP cell controller 20 (also known as Evolved NodeB or eNodeB) coordinates transmissions to UEs 18 within the CoMP cell to maximize data rates to selected UEs, while maintaining intra-CoMP cell interference below a predetermined level.
• the CoMP cell controller 20 may accomplish this through scheduling, and/or by combining weighted transmissions from two or more network transceivers 16 to any UE 18.
• Figure 2 depicts a wireless communication network 10 comprising a plurality of CoMP cells 12, 22, 24, each of which comprises a plurality of sub-cells 14. While the CoMP cell controller 20 is effective in mitigating intra-CoMP cell interference within a single CoMP cell 12, it generally has no knowledge of transmissions scheduled in neighboring CoMP cells 22, 24. Accordingly, the CoMP cell controller 20 lacks information from which to estimate interference from other CoMP cells, or inter-Comp cell interference. The same deficiency described above regarding TDMA scheduling, and variations between own-cell interference and other-cell interference from one TTI to the next, also apply to intra-CoMP interference and intra-CoMP interference, respectively, as transmissions between different CoMP cells are not coordinated.
• Figures 1 and 2 also encompass CoMP cells 12, one or more of which consists of a single sub-cell 14.
• the controller 20 in each CoMP cell 12 already has enough information to accurately predict most of the signals that contribute to SINR(k;t+d) during a given TTI. From the downlink channel state information to the UEs 18 served by a CoMP cell 12, the CoMP cell controller 20 can easily predict the desired signal to be observed by each UE 18 and the intra-CoMP cell interference to be observed by each UE 18. Furthermore, an estimate of the thermal noise and average inter-CoMP cell interference observed by each UE 18 can be reported back by the UE to the CoMP cell controller 20. This enables the CoMP cell controller 20 to perform accurate network- centric link adaptation. Such network-centric link adaptation not only improves downlink performance over conventional UE-centric link adaptation, it additionally reduces channel reporting by the UEs 18 on the uplink.
• CSI Channel State Information
• r 0 (k; t) H 0 (k; t)x 0 (k; t) + ⁇ H 0 (k; t)x, (k; t) + I otk (k; t) + W 0 (k; t)
• H 0 (k; t) is the channel between the transmit antennas of the network transceivers 16 in CoMP cell zero and the antenna(s) of UEo;
• (k; t) is the signal transmitted from the transmit antennas of the network transceivers
• S 0 (k; t) is the set of UEs that are served simultaneously with UEo by cell zero;
• I oth (k; t) is inter-CoMP cell interference (that is, interference from CoMP cells other than CoMP cell zero) observed by UEo, with variance a 0 2 th (k; t) ;
• W 0 (k; t) is thermal noise received, with variance N 0 (k; t) .
• SINR(k;t) observed by UEo at sub-carrier "k: and time "t” can then be expressed as
• the CoMP controller 20 is aware of all downlink channels to all the UEs 18 served by the CoMP cell 12.
• the CoMP cell controller 20 can thus estimate various quantities in equation (1) with greater precision than relying on measurements and reports from the UEs 18, with their concomitant delays.
• the CoMP cell controller 20 is aware of (or at least estimates) the downlink channel quality to the UEs 18 that it serves, thus the quantity H 0 (k; t) is known.
• the CoMP cell controller 20 is also aware of the other UEs 18 in its own CoMP cell, thus the quantity S 0 (k; t) is known, as is ⁇ (k; t) .
• the variance of the thermal noise at each UE 18 is constant over time and frequency; thus, it can be safely assumed that the CoMP cell controller 20 can easily acquire or estimate N 0 (k; t) .
• each UE 18 computes the average of the power of inter- CoMP cell interference over all sub-carriers, and reports to its serving CoMP cell controller 20 just one frequency-independent average value for the power of inter- CoMP cell interference.
• a mechanism for UEs 18 to report to the network their observed average power (averaged over sub-carriers and time) of the inter-CoMP cell interference may be defined by extensions to the relevant network protocol. The network protocol extensions may also define how often such reports should be sent by each UE 18 to its serving CoMP cell controller 20. Since this reported quantity is frequency-independent, the amount of feedback required to implement the network- centric link adaptation is significantly less than the amount of feedback needed to implement conventional, UE-centric link adaptation. In some embodiments, a practical implementation may direct the UEs 18 to report the sum of intra-CoMP cell interference and thermal noise.
• Figure 3 depicts a method 100 of performing network-centric link adaptation for a first UE 18, performed by a controller 20 of a first CoMP cell 12 comprising a plurality of network transceivers 16, each serving UEs 18 in respective sub-cells.
• the method 100 repeats at predetermined durations over which link adaptation is performed, for example, once per TTI.
• the CoMP cell controller 20 determines the downlink channel between one or more network transmitters 16 in the first CoMP cell 20 scheduled to transmit to the first UE 18, and receive antenna(s) of the first UE 18 (block 102). This may result from Channel State Information (CSI) or similar reports by the UE 18, based on reference, or pilot, symbols transmitted by the relevant network transmitters 16.
• CSI Channel State Information
• the CoMP cell controller 20 determines the desired signal to be received at the first UE 18 (block 104), such as for example an appropriately modulated and coded data packet received by the network 12.
• the CoMP cell controller 20 also determines the interference caused to the first UE 18 by transmissions to other UEs 18 in the first CoMP cell 12 (block 106).
• the CoMP cell controller 20 utilizes sophisticated signal processing algorithms to weight transmissions from different network transmitters 16 so as to maximize the data rate to selected UEs 18, while simultaneously minimizing the interference presented to other UEs 18. Accordingly, the CoMP cell controller 20 is uniquely aware of the interference presented to any given UE 18 resulting from intra-CoMP cell interference.
• the CoMP cell controller 20 further determines the thermal noise observed at the first UE 18 (block 108). Since the variance of the thermal noise at each UE 18 is constant over time and frequency, the thermal noise may be accurately estimated based on relatively infrequent reports from the UEs 18. Furthermore, the UEs 18 may average thermal noise measurements over frequency, reducing the number of reports required, and hence conserving uplink bandwidth.
• the CoMP cell controller 20 receives from the first UE 18 a measure of interference from one or more other CoMP cells 22, 24 (block 1 10).
• the UE 18 measurement of total inter-CoMP cell interference is facilitated by the CoMP cell controller 20 transmitting no symbols from any of its network transceivers 16 during a certain known interval. During such an interval, all signals received by the UE 18 are from other CoMP cells 22, 24.
• the UE 18 averages the inter-CoMP cell interference over sub-carriers, and hence its uplink reporting is significantly reduced compared to conventional, UE-centric methods of link adaptation.
• the CoMP cell controller 20 Based on the downlink channel quality, the desired signal, the intra-CoMP cell interference, the inter-CoMP cell interference, and the thermal noise, the CoMP cell controller 20 performs link adaptation for the first UE 18 by determining the modulation and coding, and other transmission parameters, to be applied to CoMP cell 12 transmissions to the first UE 18 during the next predetermined transmission duration, e.g., TTI (block 1 12). The method 100 then repeats for the next predetermined transmission duration (although not all steps, e.g., block 108, will necessarily be performed anew at each iteration).
• Figure 4 graphs the results of system-level simulations performed to compare the performance of conventional, UE-centric link adaptation to the performance of the inventive, network-centric link adaptation disclosed herein.
• the simulation environment comprised downlink transmissions in a CoMP system with seven sub- cells, each comprising three sectors - that is, 21 separately controllable network transceivers 16 per CoMP cell 12.
• the distance between sites of network transceivers 16 in the simulations was 500 meters.
• Each network transceiver 16 has four transmit antennas, and each UE 18 has two receive antennas.
• the simulations computed the overall spectral efficiency and cell-edge bit rate for two different link adaptation approaches - UE-centric and network-centric.
• the network-centric link adaptation results in approximately 50% higher spectral efficiency (throughput, measured in bits per second per Hz per cell) than the UE-centric link adaptation.
• the network-centric link adaptation results in 90% higher achievable cell-edge bit rate than the UE-centric link adaptation (most inter-CoMP cell interference occur in sub-cells at the CoMP cell edges).

Landscapes

• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Mobile Radio Communication Systems (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=48906462

Family Applications (1)

Country Status (9)

Families Citing this family (2)

Family Cites Families (5)

• 2013
  • 2013-05-17 TW TW102117608A patent/TW201351940A/zh unknown
  • 2013-06-06 WO PCT/IB2013/054673 patent/WO2013183029A1/en not_active Ceased
  • 2013-06-06 AU AU2013273112A patent/AU2013273112B2/en not_active Ceased
  • 2013-06-06 KR KR20157000249A patent/KR20150029681A/ko not_active Ceased
  • 2013-06-06 EP EP13742749.8A patent/EP2859672A1/de not_active Ceased
  • 2013-06-06 BR BR112014029648A patent/BR112014029648A2/pt not_active Application Discontinuation
  • 2013-06-06 IN IN10419DEN2014 patent/IN2014DN10419A/en unknown
  • 2013-06-06 JP JP2015515639A patent/JP6242387B2/ja not_active Expired - Fee Related
  • 2013-06-06 CN CN201380030056.5A patent/CN104350697A/zh active Pending

Non-Patent Citations (2)

Also Published As

Similar Documents

Legal Events