In 3GPP (3rd Generation Partnership Project), the packet-switched communication systems HSPA (High Speed Packet Access) and LTE (Long Term Evolution) have been specified for wireless transmission of data packets between user terminals and base stations in a cellular/mobile network. In this description, the term “base station” is used to generally represent any system entity capable of wireless communication with a user terminal.
LTE systems generally use OFDM (Orthogonal Frequency Division Multiplexing) involving multiple narrowband subcarriers which are further divided into time slots to form a so-called “time-frequency grid” where each frequency/timeslot combination is referred to as a “Resource Element RE”. In LTE, multiple antennas can also be employed in both user terminals and base stations for obtaining parallel and spatially multiplexed data streams, e.g. according to MIMO (Multiple Input Multiple Output), which is well-known in the art. Other wireless communication systems relevant for the following description include but is not limited to WCDMA (Wideband Code Division Multiple Access), WiMAX, UMB (Ultra Mobile Broadband), GPRS (General Packet Radio Service) and GSM (Global System for Mobile communications).
A base station of a cell in a wireless network may transmit data and control information in a physical downlink channel to a user terminal or “UE” (User Equipment), and a user terminal may likewise transmit data and control information in a physical uplink channel in the opposite direction to the base station. In this description, a physical downlink or uplink channel is generally referred to as a wireless link between a sending entity and a receiving entity. Further, the terms “sending entity” and “receiving entity” are used here merely to imply the direction of the wireless link considered, although these entities can of course both receive and send data and messages in an ongoing communication. Further, the term “Resource Element RE” is used in this description to generally represent a signal bearer element that can carry a signal over a wireless link, without limitation to any transmission technology such as LTE. For example, an RE can incorporate a specific code and timeslot in a system using CDMA (Code Division Multiple Access), or a specific frequency and timeslot in a system using TDMA (Time Division Multiple Access), and so forth.
When two entities in a cell communicate over a wireless link that is configured according to various link parameters, one or more such link parameters can be adapted to the current state of the link on a dynamic basis, often referred to as link adaptation. Such link parameters may include transmission power, transmit antenna weights, modulation schemes, encoding schemes, and the number of parallel data streams when multiple antennas are used, the latter link parameter being called “transmission rank”. Link adaptation is used to generally optimise transmission in order to increase capacity and data throughput in the network. Further, link adaptation can be employed for the uplink and the downlink independently, if applicable, since the current state of the uplink and downlink can be very different, e.g. due to different interference and when frequency and/or time are widely separated for uplink and downlink transmissions between the two entities.
To support link adaptation during an ongoing communication between a sending entity and a receiving entity, either on the uplink or downlink, the receiving entity is often required to measure certain link parameters and report recommended link parameters to the sending entity, such as a recommended transmission rank and/or a recommended precoder matrix and channel quality indicators (CQI). The quality of the received signal is often measured, typically in terms of a Signal to Interference and Noise Ratio SINR, e.g. separately for different parallel data streams. Based on the measured SINR value(s), the receiving entity estimates so-called “Channel Quality Indicators” CQIs, e.g. one CQI for each coded data block (codeword). The CQIs, which may be expressed in terms of a recommended modulation and coding scheme, are used together with the other link parameters to indicate the current state of the link. In this description, the recommended link parameters, including CQI and/or transmission rank and/or a precoder matrix, will be called a “link state report” for short. The sending entity can then adapt one or more link parameters depending on the received link state report. The reported CQIs may also be used for packet scheduling decisions.
Typically, specific known reference symbols RS, or equivalently pilot symbols, are regularly transmitted over a wireless link according to a predetermined scheme to support the above link quality estimation. In an OFDM-based LTE system, these RSs are transmitted from base stations in predetermined REs in the time-frequency grid as known by the receiving terminal.
In general, a received signal “r” in an RE is basically comprised of transmitted symbols “s” as well as noise and interference “n”. Thus:r=Hs+n  (1)
Generally, r, s and n are vectors and H is a matrix, where “H” represents the channel response which can be derived from a channel estimator in the receiver. However, the noise and interference of a signal in an RE may display different characteristics because the interference mix hitting the REs may typically have different transmission power and spatial characteristics, e.g. due to time and/or frequency synchronization in neighbouring cells. The interference is a mix of interference from REs containing payload data, REs containing control signalling and REs containing RSs, where each component can vary in size from non-existent to being the only component in the mix. The interference/noise “I” in these different signal types may be characterised in terms of second order statistics that can be obtained by frequently measuring the signals over time, although “I” can be characterised in other ways as well.
If an RE contains an RS signal received by a user terminal, the terminal is able to estimate the interference/noise n=I(RS) of the RS signal since s are known symbols in this case and H is given by the channel estimator. If the RE contains data scheduled for the terminal, the interference and noise n=I(data) can also be estimated once the data symbols have been detected (i.e. decoded) by the terminal, s thereby being known at that point. In order to obtain proper link quality estimation and to determine an accurate CQI and/or link parameter recommendation for a link, the receiving entity needs sufficient statistics from measuring signals transmitted on the link. Furthermore, the characteristics of intercell interference may be significantly different depending on what signal type is causing the interference from neighbouring cells, i.e. RS signals or data signals. If payload data is transmitted over the link to be estimated, the receiving entity should preferably measure the interference I(data) that hits the data signals. However, the measurements would then be limited to REs that contain data scheduled for the user terminal involved, which may be too scarce such that the statistic basis for determining the CQI is insufficient. Moreover, the data symbols must be detected and decoded, and possibly also re-encoded, before the interference I(data) can be properly estimated, which may impose substantial costs and/or unacceptable delays due to the data processing.
Alternatively or additionally, the receiving entity can measure the interference I(RS) for REs containing an RS which may occur more frequently than the REs containing scheduled data. Measuring I(RS) is also generally more reliable since the RS is always known to the receiving entity. However, the interference that hits RS signals may be significantly different from that hitting the data signals, e.g. with respect to statistics. Therefore, a CQI and/or link parameter recommendation determined from I(RS) measurements may not be representative for a link with payload data transmission. As a result, the link adaptation at the sending entity may not be optimal for data due to either too optimistic or too pessimistic CQI and/or link parameter recommendation from the receiving entity. Hence, if the measured I(RS) is significantly greater than the actual I(data), the CQI and/or link parameter recommendation will be based on an overestimated interference (or underestimated SINR) and therefore unduly pessimistic, and vice versa.
For example, when MIMO is employed in an LTE system and enabled by cell-specific RSs, each antenna must have its own RSs and the REs holding an RS of one antenna at the sending entity must be empty for all other antennas in the same cell, which substantially limits the number of REs available for RS transmissions. As a result, the interference that hits REs containing an RS will largely come from RS transmissions in other cells due to reuse of the RS transmission pattern. As mentioned above, RSs are always transmitted from base stations according to a predetermined scheme and at a relatively high fixed power in order to be received by any terminal in the cell, whereas payload data is only transmitted when scheduled for a specific terminal. Thus, in a situation with low data traffic and/or low transmission power for data signals, I(data) is generally lower than I(RS).
Hence, it is often difficult to obtain accurate estimates of the intercell interference that hits data transmissions, in particular if the interference measurements are performed on RS transmissions, as explained above. Inaccurate estimates of the SINR may thus result in misleading CQIs and non-optimal link parameter recommendations such as transmission rank. A consequence for MIMO systems is that an underestimated SINR may result in a too pessimistic transmission rank when the used link can actually support a transmission rank greater than the recommended one. Both of these issues may well result in reduced throughput. On the other hand, if the SINR is overestimated, the link may not be able to support any recommended CQIs (including a recommended Modulation and Coding Scheme MCS) and transmission rank, resulting in excessive decoding errors and thereby reduced throughput also in this case.
However, the base station may monitor so-called “ACK/NACK signalling” from the terminal for received data blocks, and detect if a Block Error Rate BLER or the like is below or above a predetermined target value. From this information, the base station can decide to use a more offensive or defensive MCS than recommended by the terminal. However, if the base station selects a transmission rank different from the recommended one, the reported CQI will be largely irrelevant since, in most cases, it relates directly to the transmission rank. Consequently, the base station would not have a proper basis for selecting the MCS and other link parameters for the different data streams.
With reference to FIG. 1, an example of determining an estimate of the channel quality with so called outer-loop adjustment is described.
In a first step 1:1, data packets are transmitted by a base station, formatted according to a set of link parameters recommended by a communication terminal, e.g. in a link state report. The communication terminal determines whether the data packets are successfully received or not, in a subsequent step 1:2. In case of successful reception of the data packets, the communication terminal sends one or more ACKs in a following step 1:3, else it continues with step 1:7 below. In the following step 1:4 the base station adjusts at least one of the transmission parameters, and sends further data packets formatted according with the adjusted transmission parameters in the subsequent step 1:5. The steps 1:2 to 1:5 are repeated until the data packets are not successfully received. When the communication terminal determines that the data packets are not successfully received in the step 1:6, it sends one or more NACKs to the base station in the following step 1:7. In step 1:8, the base station changes then the transmission parameters to the latest ones being acknowledged, and proceeds transmit data packets in the following step 1:9. The sequence of steps 1:2-1:9 may then be repeated, in order to perform transmission of data packets formatted according to the set of transmission parameters representing the highest channel quality which achieves reliable transmission of data packets.
It is thus generally a problem that, in a communication with dynamic link adaptation, a signal sending entity may receive inaccurate link quality estimations and/or link parameter recommendations from a signal receiving entity, such that the used link parameters are not optimal or appropriate for the actual link used in the communication.