Wednesday, January 27, 2010

Single Carrier FDMA - for 4G wireleess

Over the last decade, the bit rates achieved in wireless communications systems have increased steadily.
TDMA and CDMA has been the major technologies in multiple access. The highest bit rates in commercially deployed wireless systems are achieved by means of Orthogonal Frequency Division Multiplexing (OFDM). The next advance in cellular systems, under investigation by the Third Generation Partnership Project (3GPP), also anticipates the adoption of OFDMA to achieve higher bit rates. Single carrier frequency division multiple access (SC-FDMA), a modified form of Orthogonal FDMA (OFDMA), is a promising technique for high data rate up-link communications in future cellular systems.

An SC system transmits a single carrier, modulated, for example, with QAM, at a high symbol rate. The transmitters use different orthogonal subcarriers to transmit information symbols. The transmission is sequential, which reduces the variations in the transmitted signal envelope. This results in a lower peak-to-average-power ratio. Frequency domain equalization os carried out to counter the severe delay spreads the signal might encounter. The advantages may be listed as:
  • Small variations in the instantaneous power of the transmitted signal
  • Possibility for low-complexity high-quality equalization in the frequency domain.
  • Possibility for FDMA with flexible bandwidth assignment.
  • SC-FDMA can be seen as normal OFDM with a DFT-based precoding

SC-FDMA transmitter and receiver
The block diagram of the SC-FDMA receiver and transmitter is given the figure. The figure is self-explanatory. Similar to OFDM modulation, DFTS-OFDM relies on block-based signal generation.
By adjusting the transmitter DFT size and the size of the block of modulation symbols the nominal bandwidth of the DFTS-OFDM signal can be dynamically adjusted.

Information throughput is another indication of the system performance. Here the throughput depends on the manner in which information is applied to the subcarriers. The two main methods are localized and distributed. The benefit of distributed system, compared to localized, is the possibility for additional frequency diversity as even a low-rate distributed signal can be spread over a potentially very large overall transmission bandwidth. It has been shown that the SC-FDMA can be tuned to achieve data rates in excess of 40Mbps.

Within a specific SC-FDMA system configuration, there are many design and operational choices that affect performance in a complex manner . The impact of channel estimation error on the throughput performance of SC-FDMA is still not understood clearly. Still, SC-FDMA is a promising technique for high data rate
uplink communication in future cellular systems.

Tuesday, January 5, 2010

Operator ID for WMAN

IEEE Std 802.16 defines a 24-bit Operator ID to identify the operator of an IEEE 802.16 base station. The 24-bit Operator ID shall be assigned as an IEEE 802.16 Operator ID by the IEEE Registration Authority. The IEEE Registration Authority is the sole authorized number space administrator for this function. This Operator ID (OpID) is combined with an additional 24-bit programmable field to define the 48-bit Base Station ID. Provided that the operator assigns unique numbers to the least significant 24 bits of the Base Station Identifier, this results in a globally unique Base Station Identifier, as long as the OpID is globally unique.

IEEE 802.16 Operator ID & Base station ID
The IEEE 802.16 Operator ID is a sequence of 24 bits. It is administered by the IEEE Registration Authority. A Base Station ID is defined as a sequence of 48 bits. The first 24 bits take the values of the 24 bits of the Operator ID.

Operator ID Usage
The Operator ID referenced in the assignee's IEEE Registration Authority Assignment is described as a 24-bit globally assigned Operator ID and as an integral part of a 48-bit globally assigned Base Station ID. An Operator ID assignment allows the operator to generate approximately 16 million Base Station IDs, by varying the last three octets.

The method that an operator uses to ensure that no two of its Base Stations carry the same ID will, of course, depend on the assignment process and the operator's philosophy. However, the network selection algorithms may expect Base Stations to have unique IDs. The ultimate responsibility for assuring that expectations and requirements are met, therefore, lies with the operator of the Base Station.