Adaptive cyclic delay diversity for mobile devices

Embodiments of the present invention provide for adaptive cyclic delay diversity to be used in wireless transmissions to mobile devices. The cyclic delay diversity may be adapted through cyclic delay parameters determined based at least in part on a determined speed of the mobile device. Other embodiments may be described and claimed.

FIELD

Embodiments of the present invention relate to the field of wireless networks, and more particularly, to adaptive cyclic delay diversity for mobile devices used in said wireless networks.

BACKGROUND

Cyclic delay diversity (CDD) is provided in wireless communications as a method of spatially diversifying transmissions in orthogonal frequency division multiplexing (OFDM) systems. When spatial diversity is desired, e.g., when throughput conditions are compromised due to poor channel conditions, CDD may be employed. CDD provides that a signal be repeated throughout a plurality of transmit branches of a transmitting device (e.g., a base station), with each transmit branch providing a cyclic delay to the transmitted information. The cyclic delay may be provided by copying a certain number of samples from the end of the transmission and placing those samples at the beginning of the transmission. The number of samples of the cyclic delay is predetermined given the characteristics of a given platform, e.g., number of antennas. The spatial diversity provided through such a CDD transmission may increase the throughput over channels with substantial signal to noise ratios (SNRs).

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

For the purposes of the present invention, the phrase “A and/or B” means “(A), (B), or (A and B).” For the purposes of the present invention, the phrase “A, B, and/or C” means “(A), (B), (C), (A and B), (A and C), (B and C), or (A, B and C).”

FIG. 1illustrates a wireless communication system100in accordance with an embodiment of this invention. In this embodiment, the communication system100is shown with two wireless communication devices, e.g., base station104and mobile device108, communicatively coupled to one another via one or more wireless communication channels (“channels”)112.

The wireless communication devices104and108may have respective antenna structures116and120to facilitate the communicative coupling. Each of the antenna structures116and120may have one or more antennas. An antenna may be directional or omnidirectional antenna, including, e.g., a dipole antenna, a monopole antenna, a patch antenna, a loop antenna, a microstrip antenna or any other type of antenna suitable for transmission/reception of radio frequency (RF) signals.

The base station104may include a controller124coupled to a transmitter128and a receiver132. If the conditions of the channels112deteriorate, e.g., the SNR decreases, the controller124may control the transmitter128to employ CDD to obtain spatial diversity in the transmissions to the mobile device108in an effort to increase throughput of the transmissions. As briefly introduced above, and discussed in further detail below, CDD may be employed by the transmitter128by repeating a signal through a plurality of transmit branches, within the transmitter128, with a branch-specific cyclic delay added through each transmit branch.

The mobile device108may also include a controller136coupled to a transmitter140and a receiver144. The transmitter140and receiver144may be collectively referred to as a transceiver. The mobile device may be traveling in a direction148at a particular speed while communicating with the base station104. The controller136may determine this speed, which may thereafter serve as at least a partial basis for determining a cyclic delay parameter to be used in wireless transmissions between the base station104and the mobile device108. In particular, the cyclic delay parameter may be used to adapt CDD utilized in said wireless transmissions. In one embodiment, the controller136may access a lookup table152that correlates various speeds to cyclic delay parameters, e.g., by providing throughput values for a number of different cyclic delay parameter values at different speeds of the mobile device108.

The speed of the mobile device108may be determined through receipt of a speed-determinative signal. For example, the receiver144may include a global positioning system (GPS) receiver, controlled by the controller136, to receive a speed-determinative signal as a series of GPS transmissions and to determine the speed of the mobile device108. In other embodiments, the speed-determinative signal may be communications from the base station104and the controller136may determine the speed by referencing pilot symbols in said communications. Other methods for determining the speed of the mobile device may be additionally/alternatively employed in various embodiments.

In some embodiments, both the transmitting device and the receiving devices may be mobile devices. In these embodiments, it may be that speeds of both wireless communication devices are determined and factored into the determination of the cyclic delay.

In some embodiments, a direction that the mobile device is traveling may additionally/alternatively be considered when adapting CDD.

In various embodiments, the communication system100may be compatible with any wireless communication standards including, e.g., cellular system standards, wireless computer network standards, etc.

In various embodiments, the mobile device108may be a mobile computer, a personal digital assistant, a mobile phone, etc. The base station104may be a fixed device or a mobile device that may provide the mobile device108network access. The base station104may be a mobile computer, a personal digital assistant, a mobile phone, an access point, a base transceiver station, a radio base station, a node B, etc.

FIG. 2illustrates a transmitter200that may be used in place of transmitter128in accordance with various embodiments. The transmitter200may be an N—transmit branch OFDM transmitter with CDD. An input signal may be modulated by an OFDM module204into an OFDM transmission sequence (e.g., an OFDM symbol), which is then replicated to the N transmit branches. In each transmit branch, the OFDM symbol may be cyclically shifted through shift modules208by a branch specific cyclic delay δn, n=0, . . . , N−1 (inFIG. 2, δ0may be assumed to be zero). A cyclic delay may be a number of inverse fast fourier transform (IFFT) samples copied from the end of the symbol and placed at the beginning.

This cyclical shifting utilizing the cyclic delay in a time domain is one example of implementing CDD. Other examples of implementing CDD may include, e.g., a phase rotation ramp in a frequency domain. Accordingly, in various embodiments, CDD may be implemented by feeding back a cyclic delay parameter that may be either a cyclic delay or a phase rotation ramp. Additionally, one type of cyclic delay parameter may be used to derive the other. For example, a phase rotation ramp may be fed back and used by the transmitter to determine the cyclic delay to use in CDD. Either types of the cyclic delay parameter may be fed back as a single number to limit consumption of bandwidth. Discussion of one type of cyclic delay parameter may apply equally well to the other of cyclic delay parameter with appropriate modifications.

Following the cyclic shifting, the symbols may have extensions added through cyclic extension modules212, to mitigate for intersymbol interference, and upconverted through upconverting modules216. The symbols may then be transmitted via antennas220.

FIG. 3illustrates an adaptation of CDD in accordance with various embodiments. The controller136may determine a speed at which the mobile device108is traveling at block304. As discussed above, the controller136may determine the travel speed through any of a number of mechanisms. Having determined the speed, the controller136may determine a cyclic delay parameter, e.g., cyclic delay δt, to be used in time period t at block308. This cyclic delay parameter may then be transmitted to a transmitting device, e.g., the base station104. The base station104may then use the cyclic delay parameter for CDD transmissions to the mobile device108.

The mobile device108may then detect for a CDD adaptation trigger at block316and, when detected, loop back to block304for a repeated determination of the speed of the mobile device108. In some embodiments, a CDD adaptation trigger may be time-based, e.g., expiration of the time period t. In other embodiments, a CDD adaptation trigger may be event-based, e.g., detecting a change of speed of a predetermined magnitude.

In embodiments utilizing the speed of the mobile device108as an event-based CDD adaptation trigger, the detection of the CDD adaptation trigger at block316may include a repeated determination of the speed so that the current speed of the mobile device108may be considered. The process may then loop back from block316to block308.

In another embodiment, an event-based CDD adaptation trigger may be the determination of a different cyclic delay parameter, e.g., cyclic delay δt. That is, the speed and the resulting cyclic delay parameter may be repeatedly determined and, when the speed changes in such a degree there is also a change in the corresponding cyclic delay parameter, the updated cyclic delay parameter (or its difference with the previously determined value) may be transmitted. In this embodiment, the detection of the CDD adaptation trigger at block316may include the repeated determination of the speed and cyclic delay parameter. The process may then loop back from block316to block312.

Thus, in various embodiments a CDD adaptation trigger may instigate, or be the result of, repeated determinations of speed and/or cyclic delay parameter. Repeated determinations of speed and associated cyclic delay parameter (and subsequent transmission of the cyclic delay parameter) may allow the base station104and the mobile device108to adapt CDD in wireless transmissions to real-time conditions. This adaptation may be performed without computational expensive determinations of instantaneous channel conditions.

While the embodiments discussed above contemplate the mobile device108determining the cyclic delay parameter and transmitting the determined cyclic delay parameter to the base station104; in other embodiments, the base station104may perform the cyclic delay parameter determination based on a speed determined by the mobile device108and fed back to the base station104.

Furthermore, in some embodiments, the mobile device108may determine the cyclic delay parameter to be used in transmissions to the base station104by manners similar those described herein.

FIG. 4illustrates another transmitter400that may be used in place of transmitter128in accordance with various embodiments. The transmitter400may be a group selection per antenna rate control (GS-PARC) scheme configured to enhance multiple-input, multiple-output (MIMO) OFDM performance. The transmitter400may be compatible with 3rdGeneration Partnership Project (3GPP) Long Term Evolution (LTE) project, projected to result in release 8 of the Universal Mobile Telecommunications System (UMTS) technologies.

The transmitter400may have a demultiplexor404to distribute information bits between groups408and412within the transmitter400. Each group may have a coder/symbol mapper416, an OFDM module420, and a number of transmit branches424(two in this embodiment), similar to the transmitter200. For subsequent discussions, cyclic delay δ1of group408may be assumed to be equal to cyclic delay δ2of group412.

In other embodiments, transmitters having other configurations may be employed. For example, in one embodiment, a variation of transmitter400may include a coder and interleaver before a demultiplexer, and the symbol mappers may be only on the outputs of the demultiplexor. Other variations may also be employed.

Table500, shown inFIG. 5, provides various operating parameters for transmitter400that may be used in subsequent performance comparison descriptions. The operating parameters of table500may include an FFT size of 1024 samples; a cyclic prefix of 256 samples; a subframe duration of 0.5 milliseconds (ms); a subcarrier spacing of 15 kilohertz (kHz); a channel model that is a Global System for Mobile Communication (GSM) typical urban (TU) propagation model with a 6-tap delay line with MIMO extensions, a base station correlation of 0.25, and a mobile device correlation of 0; an ideal channel estimation method; mobile device speeds of 3 kilometer/hour (km/h), 30 km/h, and 120 km/h; and a transmit packet size of 960 information bits.

FIG. 6is a chart600illustrating throughputs of cyclic delays for given speeds of the mobile device108in accordance with various embodiments. The chart600includes a first cyclic delay604of 8 IFFT samples; a second cyclic delay608of 16 IFFT samples; a third cyclic delay612of 22 IFFT samples; a fourth cyclic delay616of 38 IFFT samples; and a fifth cyclic delay620of 68 IFFT samples. The throughput comparisons of chart600may be through a channel having an SNR of 12.3 decibels (dB) and a code rate selection being fed back with a feedback delay of 2 subframes.

As can be seen by chart600, the cyclic delay value resulting in the greatest throughput may be dependent on the speed of the mobile device108. For example, for relatively low speeds, e.g., 3 km/h, the first cyclic delay604may have the greatest throughput. However, the fifth cyclic delay620may have higher throughputs for relatively high speeds, e.g., 120 km/h. This information correlating the speed, throughput, and cyclic delays may be modeled for a given platform and stored in the lookup table152. The controller136may then be able to determine a cyclic delay resulting in desired throughput by accessing the information stored in the lookup table152.

FIG. 7is a chart700illustrating a performance comparison in accordance with various embodiments. In particular, the chart700illustrates a performance comparison between an adaptive CDD transmitter704, utilizing various teachings presented herein, and a PARC transmitter708of the prior art. The spectral efficiency, which may be similar to the throughput measurement ofFIG. 6, given in terms of bits/subframe/subcarrier over a range of mobile device108speeds is shown. As can be seen, the adaptive CDD transmitter704is shown to be more spectrally efficient than the fixed CDD transmitter708over the range of speeds of the mobile device108. This increased spectral efficiency may result in an increase in the overall performance of a wireless communication system, e.g., system100.

FIG. 8illustrates a computing device800capable of implementing a wireless network device in accordance with various embodiments. As illustrated, for the embodiments, computing device800includes processor804, memory808, and bus812, coupled to each other as shown. Additionally, computing device800includes storage816, and communication interfaces820, e.g., a wireless network interface card (WNIC), coupled to each other, and the earlier described elements as shown.

Memory808and storage816may include in particular, temporal and persistent copies of CDD adaptation logic824, respectively. The CDD adaptation logic824may include instructions that when accessed by the processor804result in the computing device800performing CDD adaptation operations described in conjunction with various wireless network devices in accordance with embodiments of this invention. These CDD adaptation operations include, but are not limited to, determining speed of a mobile device, determining cyclic delay parameter to be used in wireless transmissions, and/or transmitting determined speed and/or cyclic delay parameter to a wireless communication device.

In various embodiments, storage816may include integrated and/or peripheral storage devices, such as, but not limited to, disks and associated drives (e.g., magnetic, optical), universal serial bus (USB) storage devices and associated ports, flash memory, read-only memory (ROM), non-volatile semiconductor devices, etc.

In various embodiments, storage816may be a storage resource physically part of the computing device800or it may be accessible by, but not necessarily a part of, the computing device800. For example, the storage816may be accessed by the computing device800over a network.

In various embodiments, computing device800may have more or less components, and/or different architectures. In various embodiments, computing device800may be a mobile device and/or a base station.