Patent Application
20050018758
The invention is applied to radio systems realized by the spread spectrum technique. The invention relates particularly to a rake receiver used in such a radio system.
In radio systems, such as mobile systems, a radio signal between a mobile station and a base station propagates along several paths between a transmitter and a receiver. The signal can propagate directly from the mobile station to the base station if no obstacles occur between them. In urban environments, buildings, vehicles and other obstacles subject a radio signal to reflection and scattering. The multipath-propagated signal components may travel different distances over the radio path, resulting in different arrival times of the components at the receiver. Some radio systems, such as systems realized by the spread spectrum technique and applying code division multiple access (CDMA), are able to utilize the multipath propagation of radio signals. A receiver thus receives several multipath-propagated signal components, which are amplified and combined in order to better identify the signal that was originally transmitted.
In CDMA, user signals are distinguished from one another by means of channelization and scrambling codes allocated individually to users, the codes modulating the baseband and thus spreading the data signal band. A combination of channelization and scrambling codes will be referred to hereinafter as a spreading code. Correlators provided in receivers synchronize with a desired signal, identified by means of the spreading code, and they restore the signal band to the original width. Signals arriving at the receiver and containing some other spreading code do not ideally correlate but retain their wide band and are thus visible as noise in the receiver. The channelization codes used in the system are preferably selected so as to be mutually orthogonal, i.e. they do not correlate with one another. One user may utilize one or more channelization codes, depending on the amount of transmission capacity needed.
A receiver generally used in a CDMA system is a rake receiver consisting of one or more rake fingers, i.e. correlators. The rake fingers are independent receiver units, the function of each unit being to compose and demodulate one multipath-propagated received signal component. The signals received by different rake fingers are combined in a receiver to provide a more reliable signal. In addition to rake fingers intended for receiving signals, a receiver typically comprises at least one separate searcher, the function of which is to form an impulse response for a user signal, representing different user signal components by means of a delay and a amplitude. A searcher typically employs a correlator, i.e. a matched filter (MF). A matched filter calculates one or more points of a user channel impulse response at a time. An actual measuring window, which covers the duration of forming an impulse response, is typically for example 64 chips in length. A matched filter requires 64 separate measurements for calculating an impulse response if the filter correlates only one code phase at a time, whereas a matched filter of e.g. 16 chips requires four consecutive measurements. Significant signal components located from the impulse response are allocated to the fingers of the rake receiver for monitoring the delay component in time.
A measuring window used in a known manner in the reception of a spread spectrum signal is oversized in several occasions, which has resulted in unnecessarily vast use of expensive equipment resources in the receivers, such as matched filters and software designed to control the filters.
An objective of the invention is thus to provide an improved arrangement for allocating equipment resources in receivers realized by the spread spectrum technique. This is achieved by a method according to the invention of receiving a spread spectrum signal in a radio system receiver, the method comprising the steps of receiving a spread spectrum signal comprising one or more user signals in a receiver, forming an impulse response representing delay components of a user signal on the time domain. The method comprises measuring, from the formed impulse response, a delay spread of the delay components that are significant for the reception of the user signal, and determining, in the receiver, lengths of measuring windows for impulse responses related to one or more user signals by means of the measured delay spread.
The invention also relates to a method of preliminary synchronization of a user signal contained in a spread spectrum signal in a radio system receiver, the method comprising the steps of receiving a spread spectrum signal comprising one or more user signals in the receiver, searching the user signal on the delay domain for a first delay component. The method comprises searching for delay components of the user signal from both sides of the first located delay component on the delay domain, so that the user signal is searched from both sides of the located signal component for a period of time equal to a maximum delay spread of the impulse response in the operating range of the receiver.
The invention also relates to a rake receiver comprising means for receiving a spread spectrum signal in a radio system, means for forming an impulse response representing, on the time domain, delay components of a user signal contained in the spread spectrum signal. The rake receiver comprises means for measuring, from the formed impulse response, a delay spread of delay components that are significant for the reception of the user signal, means for determining the length of the measuring window for the impulse response by means of the measured delay spread of the user signal.
The invention further relates to a rake receiver comprising means for receiving a spread spectrum signal containing one or more user signals, means for locating the first delay component from the user signal. The locating means are configured to search for other delay components of the user signal from both sides of the first located delay components on the delay domain by searching the user signal from both sides for a period of time equal to a maximum delay spread of the impulse response of the user signal in the operating range of the receiver.
The invention thus relates to an arrangement for optimum allocation of equipment resources in radio systems realized by the spread spectrum technique. The present invention relates to receivers implemented in such a radio system. However, the invention is not restricted to CDMA methods only, but the multiple access method can also be time division multiple access (TDMA) or frequency division multiple access (FDMA) combined with the CDMA. An example of a radio system according to the invention is thus the 3G universal mobile telecommunications system (UMTS), which is realized by means of wideband CDMA (WCDMA).
The invention is preferably implemented in a rake-type receiver comprising one or more searchers, or delay estimators, and one or more fingers. The invention relates particularly to the operation of a searcher, which is intended to form an impulse response for a user signal by searching for delays and amplitudes of multipath-propagated signal components. The located signal components are allocated to the fingers of the rake receiver, each finger monitoring a phase of a spreading code allocated thereto. One of the functions of the searcher in trying to locate multipath-propagated components is to search for the correct code phase by means of a matched filter. A received signal is input into the matched filter, whereafter the signal is sampled. The formed samples are correlated with predetermined data, which consists for example of pilot symbols multiplied by the user's spreading code. The radio system channel on which the length of the measuring window for the impulse response should be adapted to the existing conditions is not significant for the invention, but the adaptation can be carried out either collectively on all the radio channels or specifically on each channel. For example in a UMTS, the invention is typically utilized on dedicated physical channels (DPCH).
A basic idea of the arrangement according to the invention is that in a radio system receiver, such as a base station, the length of a measuring window to be used in a searcher is adapted to the existing conditions of signal reception. An impulse response formed from a user signal is used to compose a delay spread, which refers to the difference in time between two significant delay components that are remotest from one another. A delay spread can exhibit great variations depending on the surroundings. The invention utilizes information on variation in the delay spread in determining the length of the measuring window of the searcher. For example in urban conditions, where signal components are reflected from objects located close to one another, delay spreads are typically in the range of 2 ms. In such a case a sufficient measuring window for an impulse response has duration of for example 4 ms. In mountain conditions, a delay spread can be as long as 30 ms, which may require a measuring window of up to 40 ms.
In a preferred embodiment of the invention, a delay spread is measured from a user signal during connection set-up or as soon as possible after the connection has been set up. The delay spread is used to suitably adapt the length of the measuring window used to receive the user signal in question. A delay spread can be measured more than once during a connection, and correspondingly, the length of the measuring window can be allocated several times during a connection. In an embodiment, if the user signal is subjected to fading, a very long measuring window is temporarily allocated during the connection.
Another preferred embodiment comprises measuring an average delay spread of terminal equipments located in the coverage area of the radio system, and allocating a measuring window that is equal to or in practice slightly longer than the average delay spread to the terminal equipments arriving at the coverage area. It is clear that the measurement of an average can be replaced with some other statistical value to indicate the extent of the delay spreads. A radio system coverage area refers herein preferably to a base station cell, but the allocation can also be carried out at a more specific level, such as specifically for each sector.
In an embodiment of the invention, information measured from the delay spread or an estimate of the delay spread is used for receiver synchronization. This is implemented by searching the received spread spectrum signal for a first delay component, followed by searching for other delay components from both sides of the first located delay component for a period of time equal to a maximum delay spread. This ensures that all the signal energy is utilized in the reception of a user signal. The search is preferably a z search, where the signal components found according to the invention are placed centrally in the measuring window. In an embodiment, the outermost signal components located in the search and the edges of the measuring window are separated by a safety margin, which is intended to ensure that new components that may be located later on will fall within the measuring window.
An advantage of the invention is that in easy reception circumstances, when user signals have short delay spreads, the receiver requires fewer resources to implement a measuring window for an impulse response.
In the following, the invention will be described in more detail in connection with preferred embodiments and with reference to the accompanying drawings, in which
The arrangements according to the preferred embodiments are implemented in a spread spectrum radio system. An example of such systems is the 3G UMTS, which will be described below. The structure of the UMTS is mainly divided into system parts, i.e. the terminal equipment and the infrastructure. A terminal equipment refers herein for example to a mobile phone, a laptop computer or a domestic appliance arranged to be used via a telecommunications network. A terminal equipment can be further divided into two subdivisions, i.e. a mobile equipment (ME) and a user services identity module (USIM), the interface between them being called a Cu interface. A mobile equipment implements radio interface functions and also comprises a number of other functions, such as connecting the mobile equipment to a laptop computer. The USIM comprises data and functions for identifying a user in the radio system. The USIM also enables the user to change the terminal equipment he/she is using, similarly as a SIM card is changed in the GSM system. The infrastructure is in turn divided into an access network domain and a core network domain, the interface between the domains being referred to as an Iu interface. The access network domain, which is also referred to as a UMTS terrestrial radio access network (UTRAN), comprises physical equipment and mechanisms enabling the user to use the network, whereas the core network domain controls the network at a higher level, i.e. for example it manages user location information, data transmission and signalling. The core network domain comprises three subdivisions: a serving network, a home network and a transit network. The serving network is responsible for call routing and user data transmission between a data source and a target. The serving network is also connected to the home network and the transit network. The home network manages network functions based on permanent location. The transit network manages connections outside the UMTS network in cases where one party to a connection is located outside the UTMS network.
A Uu radio interface between the terminal equipment UE and the UTRAN is a three-level protocol stack consisting of a physical layer L1, a data link layer L2 and a network layer L3. The L2 is further divided into two sublayers, i.e. an LAC (Link Access Control) layer and an MAC (Medium Access Control) layer. The L3 and the LAC are further divided into control (C) and user (U) planes. The physical layer L1 provides the MAC and higher layers with information transfer services to transport channels. The L2/MAC in turn transmits information between the physical transport channels and logical channels that are located higher on the protocol stack. There are different types of logical channels in the UTMS and in other digital systems, e.g. control channels and traffic channels. Some of the radio channels are uplink channels, i.e. from the terminal equipment to the cellular system, whereas some are downlink channels, i.e. from the mobile phone system to the terminal equipment. A control channel is not used to allocate radio resources for a terminal equipment, but a control channel manages tasks related to the use of the system, such as paging of terminal equipments on a paging channel (PCH) that is shared by all the terminal equipments. An example of uplink control channels is a random access channel (RACH), which is used by a terminal equipment to transmit call set-up requests to the network. The radio resources allocated to a terminal equipment for actual traffic channels depend on the transmission need. An example of logical traffic channels is a dedicated channel (DCH), which carries information both in the downlink and the uplink direction. The UMTS also comprises several other channels, but they are not significant for the invention and will thus not be described herein.
Frame structures used on physical channels differ depending on the channel on which transmission occurs. A frame refers herein to a combination of several time slots, where the function of bits in each time slot is specified. An example of a frame is a frame of a DPCH physical channel in the FDD (Frequency Division Duplex) mode of UMTS. The frame length is 10 ms and the frame is divided into 15 time slots, the length of each slot being 0.667 ms. A time slot is used to transfer not only data bits and other information but also known pilot bits utilized in the rake receiver.
When a receiver, such as a terminal equipment or a radio network base station, receives frames on a channel, it forms a channel estimate and a channel impulse response by means of burse pilot bits. Formation of a channel estimate means that the receiver tries to estimate how the radio path has distorted the data contents of the burst. By means of the obtained information, the receiver may use known methods to try to restore the burst data contents according to the channel estimate. The rake receiver allocates the fingers by means of the pilot bits and the impulse response formed from the pilot bits. Channel quality can be estimated by means of the pilot bits and previously known methods, such as the signal-to-interference ratio (SIR) and the bit error rate (BER).
Information to be transmitted over a radio channel is multiplied by a spreading code in order to spread rather narrowband information to a wide frequency band. Each connection Uu has a specific channelization code(s) used by the receiver to identify transmissions intended for it. The maximum number of mutually orthogonal channelization codes that can be used simultaneously is typically 256 different codes. For example over the downlink transmission path in the UMTS, with a 5 MHz carrier of 3.84 Mch/s, spreading factor 256 corresponds to a transfer rate of 32 kbit/s, and the corresponding highest transfer rate in practice is achieved with spreading factor 4, which gives a data transfer rate of 1920 kbit/s. The transfer rate on a channel thus varies in steps of 30, 60, 120, 240, 480, 960 and 1920 kbit/s, and the spreading factor changes correspondingly as follows: 256, 128, 64, 32, 16, 8 and 4. The data transfer rate allocated to a user depends on the channel coding used. For example with 1/3 convolutional coding, the user data transfer rate is usually about one third of the channel data transfer rate. The spreading factor indicates the length of the spreading code. For example the channelization code corresponding to spreading factor 1 is (1). Spreading factor 2 has two mutually orthogonal channelization codes: (1,1) and (1,−1). Further, spreading factor 4 has four mutually orthogonal channelization codes: below a higher-level channelization code (1,1) are channelization codes (1,1,1,1) and (1,1,−1,−1), and below another higher-level channelization code (1,−1) are channelization codes (1,−1,1,−1) and (1,−1,−1,1). Formation of channelization codes proceeds in this manner to the lower levels of the code tree. Channelization codes of a particular level are always mutually orthogonal. Similarly, a channelization code of a particular level is orthogonal with all the channelization codes of subsequent levels derived from another channelization code of the same level. In transmission, one symbol is multiplied by a spreading code, i.e. a combination of a channelization code and a scrambling code, in order to spread the data to the frequency band used. For example in the case of channelization code 256, one symbol is represented by 256 chips. Correspondingly, with channelization code 16, one symbol is represented by 16 chips.
Method step 306 comprises measuring, from the formed impulse response, a delay spread, which refers to the difference in time between the first and the last significant tap of the impulse response. The significance can be determined for example by means of signal energy or a threshold set for the signal-to-interference ratio. In urban environments, delay spreads are typically in the range of 1 to 2 ms, whereas in mountain areas a delay spread can be even longer than 20 ms. Chip duration at a chip rate of 3.84 Mcps is 0.26 ms, which means that a 1 ms delay spread requires a measuring window of about four chips, and a 20 ms delay spread requires a measuring window of almost 80 chips. In a rake receiver, measuring windows of different lengths can be realized in principle in two manners, i.e. either by allocating matched filters of different lengths or by controlling several measuring time slots by software from a time-multiplexed matched filter. For example a measuring window of 32 chips can be provided by using either one measuring time slot from a matched filter of 32 chips, or by using four measuring time slots from a matched filter of eight chips.
The method diagram of
Method steps 320 to 322 describe an embodiment, where a delay spread measured from a user signal is used in a receiver to determine the length of a measuring window applied in a particular coverage area. A coverage area refers herein to a base station cell or sector, for example. Delay spreads measured in a particular base station site are rather close to one another in practice, wherefore a delay spread can be measured for a base station site or sector, to be used for terminal equipments communicating in the area. In step 320, a general delay spread is calculated for example by means of an average of delay spreads. An example is a situation where a 6 ms delay spread has been measured in the area of a base station cell. In such a case, a measuring window of e.g. 8 ms would be used invariably in the base station area. By means of the method described above, base stations can be rapidly adapted to new environments with different delay profiles. Generally, a new base station can be allocated for example a long measuring window, e.g. 30 ms in length, the window being possibly shortened according to information obtained from measurements of actual delay spreads.
In
The z search described above provides advantages over a conventional z search in several possible problem situations. In the conventional z search, when the first component is located in a search beginning for example from the left, the measuring window is placed with the located component on the very left edge of the window. The arrangement according to the preferred embodiment shown in
A maximum delay spread is obtained for example by estimating the terrain surrounding the base station or by measuring the delay spread from one or more user signals. If the measured maximum delay spread is for example 20 ms in length, the measurement can be extended to both sides of the first located tap for a period of 25 ms, which means that the search includes a guard period of 5 ms. In an embodiment, if the first tap has been located in a search that began from the left and the search has already continued to the left for a period of time equal to the maximum delay spread, the search can be directed only to the right of the located tap.
In a preferred embodiment, when the z search has been finished, the located taps are placed in the receiver substantially in the middle of the measuring window, and safety margins are left between the located taps and the edges of the measuring window. The measuring window to be used is preferably for example 20 ms in length. If the distance in time between tap 206 and the last tap of group 208 is for example 10 ms, safety margins of 5 ms are left on both sides of the taps.
The situation shown in
Even though the invention is described above with reference to an example according to the accompanying drawings, it is evident that the invention is not restricted thereto but it can be modified in several manners within the scope of the inventive idea disclosed in the appended claims.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/FI02/00164 | Mar 2002 | US |
| Child | 10917434 | Aug 2004 | US |
