WO2009004570A2

WO2009004570A2 - Cyclic prefix in the downlink and zero prefix in the uplink or unit chooses cyclic or zero prefix choice according to the power available to it

Info

Publication number: WO2009004570A2
Application number: PCT/IB2008/052631
Authority: WO
Inventors: Vasanth Gaddam; Dagnachew Birru
Original assignee: Koninklijke Philips Electronics, N.V.
Priority date: 2007-06-29
Filing date: 2008-06-30
Publication date: 2009-01-08
Also published as: WO2009004570A3

Abstract

The base station transmits using a cyclic prefix and the mobile station transmits using a zero prefix. Devices with more power available to them use a cyclic prefix or a zero prefix. Devices with less power available to them use a zero prefix.

Description

AN ADAPTIVE GUARD INTERVAL FOR BLOCK TRANSMISSION PROTOCOLS

This application claims the benefit of U.S. Provisional Application No. 60/946,992 filed on June 29, 2007, the contents of which are herein incorporated by reference. The invention generally relates to block transmission schemes.

The operating modes of wireless communication networks can be classified as either an infrastructure or ad-hoc (i.e., peer-to-peer) depending upon how the network devices are setup and communicate among each other. Fig. 1 shows a schematic illustration of a network 100 configured to operate in an infrastructure mode. In this mode, a central coordinator 110 (e.g., an access point or a base station) manages and controls the communication among the various devices 120-1 through 120-N, within its operating region. Hereinafter, the central coordinator 110 will be referred to as a base station (BS) 110, and each device 120-X (where X is an integer greater than or equal to 1) will be referred to as a subscriber station (SS) 120- X.

BS 110 manages the communication link with SS 120-X. Typically, BS 110 is a device having more computation and power resources compared to SS 120-X. In the network 100 two different transmission directions are defined: a downlink transmission from BS 110 to SS 120-X, and an uplink transmission from SS 120-X to BS 110. Infrastructure based networks include, for example, a wireless metropolitan area network (WMAN), a wireless regional area network (WRAN), and the like.

Typically, block transmission protocols are utilized to facilitate the communication between BS 110 and SS 120-X. Examples of such protocols include an orthogonal frequency division multiplexing (OFDM), an orthogonal frequency division multiple access (OFDMA), a single carrier block transmission (SCBT), and the like. One of the properties of these protocols is that a predefined pattern is transmitted between two consecutive transmission blocks. For example, in an OFDM based transmission, a guard interval is inserted between two successive symbols in order to avoid inter-symbol interference introduced by the multi- path channels. The duration of the guard interval is determined by the delay spread of the channel. Many of the existing systems provide dynamic adaptation of the guard interval duration in order to efficiently utilize the channel bandwidth. The location of the guard interval can be either at the beginning of the symbol (prefix) or at the end of the symbol (suffix).

The pattern of the guard interval can be either a cyclic prefix/suffix (hereinafter "CP") or a zero prefix/suffix (hereinafter "ZP"). Fig. 2A is an example of an OFDM symbol 200 that includes a CP type guard interval 210. The CP is a copy of a data portion 205 inserted at the beginning of the symbol 200. Fig. 2B is an example of an OFDM symbol 220 that includes a ZP type guard interval 230. The ZP data portion includes zeros.

The decision whether to use a guard interval based on a CP or ZP is a design tradeoff. If CP guard intervals are transmitted, it is easier for a receiver to recover symbols using any standard channel estimation and equalization techniques, thereby simplifying the functionality of the receiver. On the other hand, transmission of CP results in a significant overhead in terms of power and symbol rate. When ZP guard intervals are utilized, no energy is transmitted during the guard interval period, whereby the transmission power is significantly reduced. The ZP also guarantees symbol recovery independent of channel characteristics. However, the receiver is required to support advanced channel estimation and equalization functions to recover symbols and realize full benefits of ZP transmissions. Advanced channel estimation and equalization techniques require complex computational algorithms in comparison to those utilized to decode CP based symbols (i.e., standard estimation and equalization techniques).

Generally, transmission standards specify one common scheme for communication in both directions. That is, the type of the guard interval being transmitted in infrastructure based networks can be either CP or ZP. This is a limiting factor as the guard interval is not determined based on the resources of BS 110 and SS 120-1 through 120-N.

Certain embodiments of the invention include a base station operative in an infrastructure based network. The base station comprises a transmitter for transmitting a cyclic prefix (CP) guard interval, and a receiver for receiving a zero prefix (ZP) guard interval.

Certain embodiments of the invention include a subscriber station operative in an infrastructure based network. The subscriber station comprises a transmitter for transmitting a zero prefix (ZP) guard interval, and a receiver for receiving a cyclic prefix (CP) guard interval. Certain embodiments of the invention include a communication protocol for adaptive Iy generating guard intervals based on available resources of devices in an infrastructure based network. According to the protocol, a first device in the network transmits zero prefix guard intervals based upon the first device's power resource, and a second device in the network transmits cyclic prefix guard intervals based upon power resource of the second device being greater than that of the first device.

The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings.

Figure 1 is a diagram of an infrastructure based network;

Figure 2 illustrates the different types of a guard interval;

Figure 3 is a block diagram of a BS and a SS constructed in accordance with an embodiment of the present invention;

Figure 4 is a block diagram of a CP receiver constructed in accordance with an embodiment of the invention;

Figure 5 is a block diagram of a ZP receiver constructed in accordance with an embodiment of the invention; and

Figure 6 is a block diagram of a ZP receiver constructed in accordance with an embodiment of the invention.

It is important to note that the embodiments disclosed by the invention are only examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in plural and vice versa with no loss of generality . In the drawings, like numerals refer to like parts through several views.

In accordance with principles of the invention, the type of the guard interval is selected based on the resources of the transmitting and receiving devices. Specifically, the CP is used for downlink transmissions and ZP for uplink transmissions. This maximizes the utilization of network resources, thereby improving the performance gains of the network. Fig. 3 shows a block diagram of BS 110 and SS 120-X constructed in accordance with an embodiment of the invention. BS 110 includes CP transmitter 310 and ZP receiver 320, while SS 120-X includes ZP transmitter 330 and CP receiver 340. In one embodiment, CP transmitter 310 is adapted to transmit OFDM or OFDMA symbols that include CP based guard interval and CP receiver 340 is adapted to receive and decode such symbols. Likewise, ZP transmitter 330 is adapted to transmit OFDM or OFDMA symbols that include ZP based guard interval and ZP receiver 320 is adapted to receive and decode such symbols.

In accordance with an embodiment of the invention, symbols including ZP guard intervals are transmitted in the uplink direction. BS 110 has prior information about the transmissions from various SS 120-X. Thus, BS 110 does not have to detect the presence of the signal. Therefore, transmission of ZP guard intervals does not affect the signal acquisition. In a preferred embodiment of the invention, ZP receiver 320 can implement either standard or advanced channel estimation and equalization techniques to decode the signal depending on performance requirements and/or the resources of BS 110. Different embodiments to implement ZP receiver 320 are illustrated in Figs. 5 and 6.

In the downlink direction, symbols including CP type guard interval are transmitted. This enables CP receiver 340 to quickly detect the presence of transmitted signals by performing auto-correlation. In addition, CP receiver 340 can also decode the received signal using standard channel estimation and equalization techniques.

Fig. 4 shows an exemplary block diagram of CP receiver 340 constructed in accordance with an embodiment of the invention. CP receiver 340 is adapted to receive and decode CP- OFDM symbols, i.e., OFDM symbols that include CP type guard interval. CP receiver 340 includes fast Fourier transform (FFT) module 410 adapted to transform time domain OFDM symbols to frequency domain OFDM symbols, channel estimation module 420, demapper 430 adapted to de-map OFDM symbols to bits or bit metrics, and decoder 440 for decoding the bits. Channel estimation module 420 estimates the properties of the channel using standard channel estimation and equalization techniques. Specifically, channel estimation module 420 derives an estimate of the transmission channel, and an equalization process is performed to correct the received symbols for channel impairments using the derived channel estimations. Specifically, for CP based symbols, the equalization is performed by scaling each received symbol sub-carrier by its corresponding channel estimate. Fig. 5 shows an exemplary block diagram of ZP receiver 320 constructed in accordance with an embodiment of the invention. ZP receiver 320 is adapted to receive and decode ZP- OFDMs symbols, i.e., OFDM symbols that include a ZP type guard interval. ZP receiver 320 includes FFT module 510 adapted to transform time domain OFDM symbols to frequency domain OFDM symbols, channel estimation module 520, demapper 530 adapted to de-map OFDM symbols to bits or bit metrics, and decoder 540 for decoding the bits. ZP receiver 320 uses overlap and add (OLA) techniques to convert the received symbols equivalent to CP based symbols. In order to compensate for the performance loss due to OLA processing, ZP transmitter 330 increases the transmission power of the symbols accordingly. Furthermore, channel estimation module 520 can estimate the properties of the channel using standard channel estimation and equalization techniques. Specifically, after processing the ZP-OFDM symbols using an OLA, the symbols can be equalized and estimated using processes performed for CP based symbols, whereby standard equalization techniques can be utilized.

In another embodiment of the invention, ZP receiver 320 can estimate and equalize the channel effects without implementing OLA techniques, but rather performing advanced channel estimation and equalization techniques. Advanced channel equalization includes, for example, calculating matrix inversion which is dependent on the channel itself. Typically, this requires additional computational resources, thereby such process can be implemented by BS 110. This guarantees symbol recovery even in the presence of channel zeros, and therefore improves the performance of the system. An exemplary block diagram of ZP receiver 320 implemented in accordance with this embodiment is shown in Fig. 6.

The ZP receiver 320 shown in Fig. 6 includes FFT module 610 adapted to transform time domain OFDM symbols to frequency domain OFDM symbols, channel estimation module 620 for performing the advanced channel equalization, demapper 630 adapted to de-map OFDM symbols to bits or bit metrics, and decoder 640 for decoding the bits.

It should be noted that the embodiments described herein can be adapted to implement an efficient communication protocol to be utilized in infrastructure based networks. The protocol determines the type of a guard interval to be transmitted based on the resources of the network. Specifically, as described in detail above, for a BS that has no power resources limitation, CP based guard intervals are transmitted. Moreover, for a SS having limited power recourse, ZP based guard intervals are transmitted. It is appreciated that by transmitting ZP signals the performance gain of the SS can be improved, as the overhead power in transmitting non-zero guard intervals is eliminated. Instead the power resources can be utilized for data transmission. Typically, saving in transmission power increases at least a battery life time for portable devices and the network capacity (as each SS causes less interference to other SS). A BS receiving symbols with ZP can decode the symbols using either standard or advanced channel estimation and equalization schemes.

The foregoing detailed description has set forth a few of the many forms that the invention can take. It is intended that the foregoing detailed description be understood as an illustration of selected forms that the invention can take and not as a limitation to the definition of the invention. It is only the claims, including all equivalents that are intended to define the scope of this invention.

Preferably, the principles of the invention can be utilized in any form of block transmission technique using a guard interval between two consecutive data blocks (or symbols). These techniques include, but are not limited to, an OFDM, an OFDMA, a SCBT, and the like. Most preferably, the principles of the invention are implemented as a combination of hardware, firmware and software. Moreover, the software is preferably implemented as an application program tangibly embodied on a program storage unit or computer readable medium. The application program may be uploaded to, and executed by, a machine comprising any suitable architecture. Preferably, the machine is implemented on a computer platform having hardware such as one or more central processing units ("CPU"), a memory, and input/output interfaces. The computer platform may also include an operating system and microinstruction code. The various processes and functions described herein may be either part of the microinstruction code or part of the application program, or any combination thereof, which may be executed by a CPU, whether or not such computer or processor is explicitly shown. In addition, various other peripheral units may be connected to the computer platform such as an additional data storage unit and a printing unit.

Claims

1. A base station (110) operative in an infrastructure based network (100), comprising: a transmitter (310) for transmitting a cyclic prefix (CP) guard interval; and a receiver (320) for receiving a zero prefix (ZP) guard interval.

2. The base station of claim 1 , wherein the transmitter transmits a cyclic suffix guard interval, and wherein the receiver receives a zero suffix guard interval.

3. The base station of claim 1, wherein symbols including guard intervals are transmitted using a block transmission protocols, and wherein the block transmission protocols include at least one of: an orthogonal frequency division multiplexing (OFDM), an orthogonal frequency division multiple access (OFDMA), a single carrier block transmission (SCBT).

4. The base station of claim 1 , wherein the receiver equalizes channel effects using overlap and add (OLA) techniques.

5. The base station of claim 1, wherein the receiver equalizes the channel effects using advanced equalization techniques, and wherein the advanced equalization techniques include at least calculating matrix inversion depending on the channel.

6. A subscriber station (120-X) operative in an infrastructure based network (110) adapted to transmit an adaptive guard interval for block transmission protocols, comprising: a transmitter(330) for transmitting a zero prefix (ZP) guard interval; and a receiver (340) for receiving a cyclic prefix (CP) guard interval.

7. The subscriber station of claim 6, wherein the transmitter transmits a zero suffix guard interval, and wherein the receiver receives a cyclic suffix guard interval.

8. The subscriber station of claim 6, wherein symbols including guards interval are transmitted using a block transmission protocols, and wherein the block transmission protocols include at least one of: an orthogonal frequency division multiplexing (OFDM), an orthogonal frequency division multiple access (OFDMA), a single carrier block transmission (SCBT).

9. The subscriber station of claim 7, wherein the receiver equalizes channel effects using channel estimations computed for the channel.

10. A communication protocol for adaptive Iy generating guard intervals based on available resources of devices in an infrastructure based network, comprising: causing a first device in the network to transmit zero prefix guard intervals based upon a power resource of the first device; and causing a second device in the network to transmit cyclic prefix guard intervals based upon a power resource of the second device being greater than the power resource of the first device.

11. The communication protocol of claim 10, further comprising decoding, by the second device, symbols including zero prefix guard intervals using advance channel estimation and equalization techniques.

12. The communication protocol of claim 10, further comprising decoding, by the second device, symbols including zero prefix guard intervals using using overlap and add (OLA) techniques.

13. The communication protocol of claim 10, further comprising decoding, by the first device, symbols including cyclic prefix guard intervals using standard channel estimation and equalization techniques.

14. The communication protocol of claim 10, wherein symbols including guard interval are transmitted using a block transmission protocols, and wherein the block transmission protocol includes at least one of: an orthogonal frequency division multiplexing (OFDM), an orthogonal frequency division multiple access (OFDMA), a single carrier block transmission (SCBT).