Todor Cooklev

An Implementation of IEEE 1588 Over IEEE 802.11b for Synchronization of Wireless Local Area Network Nodes

IEEE 1588 is a new standard to synchronize independent clocks running on separate nodes of a distributed measurement and control system. It is intended for high-accuracy implementations on compact systems such as a single subnet. This paper examines potential accuracy limitations introduced by the physical layer of the IEEE 802.11b wireless local area network. Experimental results are presented that show that these limitations do not preclude clock-synchronization accuracy of several hundred nanoseconds.

An efficient architecture for orthogonal wavelet transforms

A novel architecture is advanced for the efficient high-speed computation of orthogonal one-dimensional two-channel discrete wavelet transforms. The structure is derived when the multirate operations (downsampling and upsampling) are staggered. Compared with the polyphase and lattice architectures the developed architecture requires the minimum number of multiplications. It preserves orthogonality even when the coefficients are quantized. For multilevel discrete wavelet transform, the proposed architecture allows balanced pipeline implementations.

Introducing Finer Prioritization in EDCA using Random AIFSN

The recently adopted WLAN standard IEEE 802.11e provides quality of service (QoS) differentiation by grouping the traffic into different access categories (ACs) with different priorities. It defines different arbitration inter-frame space number (AIFSN) on a per-category basis to ensure that each category has different probability of accessing the channel. Within an AC, random back-off mechanism is used to spread the traffic. However, when there are a large number of QoS stations (QSTAs) within an AC, the probability of two or more stations choosing the identical back-off values is increased leading to increased packet collisions. In this paper, we propose the use of randomized AIFSN. The random AIFSN method can be exploited to introduce finer priority values within an AC. Each AC is assigned an AIFSN interval and a probability distribution function (PDF) defined over that interval. The simulation results indicate that the proposed method leads to reduced probability for collisions and higher throughput. We also present some testbed results to verify the improvement using random AIFSN method.

Improved performance of CSMA/CA WLAN using a random inter-frame spacing algorithm

The recently adopted WLAN standard IEEE 802.11e provides quality of service (QoS) differentiation by grouping the traffic into different access categories (ACs) with different priorities. It defines different arbitration inter-frame space number (AIFSN) on a per-category basis to ensure that each category has different probability of accessing the channel. Within an AC, random back-off mechanism is used to spread the traffic. However, when there are a large number of QoS stations (QSTAs) within an AC, the probability of two or more stations choosing the identical back-off values is increased leading to increased packet collisions. In this paper, we propose the use of randomized AIFSN. The proposed technology leads to reduced probability for collisions and higher throughput. It also allows the introduction of many more priority values within an AC. Each AC is assigned an AIFSN interval and a probability distribution function (PDF) defined over that interval. The simulation results indicate that the proposed method also improves the throughput and latency of traffic at all priority levels.

Tutorial: Software-defined radio technology

Software defined radio (SDR) is one of the most important emerging disruptive technologies that shaped wireless communication and mobile computing industries. The ideal software radio consists of a wideband antenna, wideband ADC and DAC, and a programmable processor. This paper discusses the development of software radios along with their applications in different fields of telecommunication.

Vector transform-based OFDM

In this paper we propose a new type of a system for multicarrier modulation. The modulation and demodulation in the proposed system are performed by an inverse and forward vector transforms. The properties of the vector transform allow fast vector transform algorithms to be derived in a manner similar to the FFT Real-valued vector transforms are also possible. Appropriate vector transforms are designed. Simulation results are presented, which show that in multipath channels compared with conventional OFDM systems the proposed vector-transform-based system achieves lower bit error rate.

An improved method for lossless data compression

Summary form only given. This paper describes an improved lossless data compression scheme. The proposed scheme contains three innovations: first, an efficient algorithm is introduced to decide when and how to switch from transparent mode to compressed mode, second, a temporary buffer is introduced at the encoder, and third, an approach to decide when to discard the entire dictionary is advanced. According to the developed method the changes are only at the transmitter and any V.42bis-compatible receiver can be used as a decoder. Therefore devices using V.42bis can use the proposed method after a firmware upgrade. To make a decision when to switch modes, we introduce two look-ahead buffers, B/sub C/ and B/sub T/, for each mode of operation of the encoder. Regardless of which mode the encoder is in, the output of both modes of operation is written to the corresponding look-ahead buffer. The simulation results demonstrate that the proposed method achieves higher compression ratios in most cases. Another goal of the work presented in this paper is to analyze what is the improvement obtained after the dictionary is reset and when is a good time to discard the dictionary. Our results for different file types show that consistently the compression ratio before dictionary reset is 0.87 and after dictionary reset is 1.07, an increase of 22.6 %. It is noted that V.44 is a newer compression standard, based on a different compression algorithm. While our results do not apply directly to V.44, they may be used after appropriate modifications.

Using Finer AIFSN Granularity to Accurately Tune the Flow Ratios in IEEE 802.11e

The current wireless LAN (WLAN) standard, IEEE 802.11e, provides Quality of Service (QoS) differentiation by grouping the traffic into access classes (AC) with different priorities. The traffic prioritization is accomplished by using the enhanced distributed channel access (EDCA) parameters - arbitration inter-frame space (AIFS) interval, contention window (CW), and transmission opportunity (TXOP) - defined on a per-class basis, thus ensuring that each class has different probabilities of accessing the channel. The EDCA parameters are broadcasted by the access point (AP) during the beacon frames. Although, the WLAN standard IEEE 802.11e provides the means to achieve traffic differentiation, it does not specify the method to tune various EDCA parameters to achieve the desired throughput ratios across different ACs. This paper proposes a method to tune the different EDCA parameters in order to get the desired throughput ratios across different ACs that utilizes the available network bandwidth optimally.

Analytic Constructions of Periodic and Non-periodic Complementary Sequences

An analytic approach for the generation of non-periodic and periodic complementary sequences is advanced for lengths that are powers of two. The periodic complementary sequences can be obtained using symmetric or anti-symmetric extensions. The properties of their autocorrelation functions are studied. The non-periodic complementary sequences are the intersection between anti-symmetric and symmetric periodic sequences. These non-periodic and periodic complementary sequences are identified to be special cases of non-periodic and periodic (or cyclic) orthogonal wavelet transforms. This relationship leads to the novel approach.

A wavelet transform approach to the design of complementary sequences for communications

In this paper periodic complementary sequences with symmetric and anti-symmetric extension are designed using a periodic wavelet transform approach. Explicit formulae for these sequences are derived. The non-periodic complementary sequences are members of both of these sets. A systematic approach for the generation of all Golay sequences of a given length is also described