Fokke W. Hoeksema

Transcoding of MPEG Bitstreams

This paper discusses the problem of transcoding as it may occur in, for instance, the following situation. Suppose a satellite transmits an MPEG-compressed video signal at say 9 Mbit/s. This signal must be relayed at a cable head end. However, since the cable capacity is only limited, the cable head end will want to relay this incoming signal at a lower bit-rate of, say, 5 Mbit/s. The problem is how to convert a compressed video signal of a given bit-rate into a compressed video signal of a lower bit-rate. The specific transcoding problem discussed in this paper is referred to as bit-rate conversion. Basically, a transcoder used for such a purpose will consist of a cascaded decoder and encoder. It is shown in the paper that the complexity of this combination can be significantly reduced. The paper also investigates the loss of picture quality that may be expected when a transcoder is in the transmission chain. The loss of quality as compared to that resulting in the case of transmission without a transcoder is studied by means of computations using simplified models of the transmission chains and by means of using computer simulations of the complete transmission chain. It will be shown that the presence of two quantizers, i.e. cascaded quantization, in the transmission chain is the main cause of extra losses, and it will be shown that the losses in terms of SNR will be some 0.5 ? 1.0 dB greater than in the case of a transmission chain without a transcoder.

A real-time GPP software-defined radio testbed for the physical layer of wireless standards

We present our contribution to the general-purpose-processor-(GPP)-based radio. We describe a baseband software-defined radio testbed for the physical layer of wireless LAN standards. All physical layer functions have been successfully mapped on a Pentium 4 processor that performs these functions in real time. The testbed consists of a transmitter PC with a DAC board and a receiver PC with an ADC board. In our project, we have implemented two different types of standards on this testbed, a continuous-phase-modulation-based standard, Bluetooth, and an OFDM-based standard, HiperLAN/2. However, our testbed can easily be extended to other standards, because the only limitation in our testbed is the maximal channel bandwidth of 20 MHz and of course the processing capabilities of the used PC. The transmitter functions require at most 714 M cycles per second and the receiver functions need 1225 M cycles per second on a Pentium 4 processor. In addition, baseband experiments have been carried out successfully.

Implementation alternatives for a flexible wireless LAN transceiver

In our software-defined radio project we have implemented two different types of standards, a continuous-phase-modulation (CPM) based standard, Bluetooth, and an OFDM based standard, HiperLAN/2, on a general-purpose processor. First we describe our baseband software-defined radio testbed for the physical layer of wireless LAN standards. All physical layer functions have been successfully mapped on a Pentium 4 processor that performs these functions in real-time. The testbed consists of a transmitter PC with a DAC board and a receiver PC with an ADC board. Channel selection functionality is performed at the DAC and ADC board, whereas all modulation and demodulation functions are mapped on software running on the CPU. Then, three implementation alternatives for the digital part of the transceiver are introduced. These include: -the testbed: a PCI card equipped with analog frontend functionality. All demodulation functions are performed in software running on the CPU of the notebook. - integration of analog front-end functionality in the chipset of the motherboard and demodulation functions are performed by the processor. - a low power DSP plus analog front-end functionality mounted on a PCI card. The alternatives are evaluated with respect to computational-power requirements, power consumption and expected manufacturing costs.

Some remarks on the determination of source traffic descriptors for complex sources

The traffic descriptors defined by the ITU and the ATM Forum include parameters for the GCRA (leaky bucket) algorithms. While the peak cell rate (PCR) is relatively easy to specify for a particular source stream, the cell delay variation tolerance (CDVT) is not. Further, both the parameters of the average rate leaky bucket, the sustainable cell rate (SCR) and the burst tolerance (BT), are rather difficult to determine.