Patent Application
20110013568
This invention relates generally to mobile wireless networks, and more particularly to frequency allocation in femtocells.
A femtocell is a relatively small wireless network serviced by a femtocell base station or alternatively called a home node base station (HNB) in residential or small business environments. User equipment (UE) communicates either with macro base station (BS) or femto BS. When the UE communicates with the femtocell BS, it is also called a home UE (HUE). Typically, the service provider is assigned a frequency spectrum. The frequency spectrum can be partitioned into bands to allocate to the macrocells and femtocells as needed. Potentially, this can cause interference.
The femtocell extends coverage of the macrocell indoors, especially where access would otherwise be limited or unavailable, see Chandrasekhar et al., “Femtocell networks: a survey,” Communications Magazine, vol. 46, no. 9, pp. 59-67, September 2008, and Yeh et al., “WiMAX Femtocells: A perspective on network architecture, capacity, and coverage,” IEEE Communications Magazine, vol. 46, no. 10, pp. 58-65, October 2008.
Clearly there is a trade-off between the coverage area of the femtocell and the interference outage area for other cells, whether these cells are cochannel macrocells, adjacent channel macrocells or co-channel adjacent femtocells.
It is desired to minimize the interference by HNBs to macrocells, when the HNBs are allocated channels from the same set of frequency bands as the macrocells in which they operate.
Interference Reduction
Interference can be reduced by allocating different frequency bands to femtocells than macrocells. However, unallocated frequency bands may not always be available or economically feasible, particularly when the number of femtocells is large. Because femtocells are relatively small, when compared with macrocells, transmission power can be reduced to minimize interference with the macrocells operating in the same and adjacent frequency bands. Typically, the transmit power in femtocells is less than one hundredth the power of macrocells. However, reducing power also reduces throughput to 300 kbs or less. Other means, such as interference rejection and beamforming, are too complex for large scale deployment.
Embodiments of the invention provide a method for allocating frequencies used for guard bands in macrocells to femtocells. This significantly reduces the interference between femtocells and macrocells.
One function performed by the processor 160 is resource allocation. An important limited resource is frequency. Typically, the service provider for the network is assigned a bandwidth of frequencies 170. The service provider can partition the bandwidth into frequency bands to assign to the macrocells, and the base stations can further partition the bands into subbands to allocate to user equipment. One important consideration during the frequency allocation is to minimize inter- and intra-cell interference.
Therefore, macrocells use frequency guard bands to separate channels, e.g., multicarrier OFDM or OFDMA networks such as Worldwide Interoperability for Microwave Access (WiMAX) and LTE, and TDMA or TDMA/FDMA networks such GSM and IS136+. Even CDMA Evolution-Data Optimized (EVDO) networks have guard band between groups of channels. The guard bands minimize interference when adjacent cells are assigned adjacent frequency bands.
Because it is hard to implement such a transmit filter, some OFDM subcarriers 410 in the guard bands are not used at all, or null, i.e., no data are carried. Other active subcarriers 420 carry data information. Even though the transmit mask is non-zero in some of the null bands, no data are transmitted in the null band by any of the macrocell. For a 20 MHz network with 2048 OFDM subcarriers, 184 leftmost subcarriers and 183 rightmost subcarriers are unused, i.e., of the 2048 subcarriers, 368 subcarriers are null, which correspond to 1.787 MHz on the rightmost part of each carrier, and 1.797 MHz on the leftmost part of each carrier. Therefore, as defined herein, a guard band includes frequencies that are not allocated to any macrocells.
In prior art system, as shown in
As shown in
To minimize interference, even though the transmit mask of the femtocell extends beyond the guard band, the femtocell only transmits data on subcarriers that are within the guard band. Alternatively, the femtocell optionally selects to transmit data also on subcarriers that are located within the transmit mask and outside of the guard band of the macrocells. This way, the femtocell achieves higher data rate, and the macrocell UE has more interference.
Adjacent femtocells can operate in different guard-bands as in
To further minimize interference from adjacent macrocells and femtocells, the macrocell can assign the subcarriers used for transmitting control information to be near the center frequency of the channel.
The frequency allocation as described herein does not impact the operation of conventional UE for macrocells, or and of the applicable standards. UE for the femtocells needs to be enabled to operate in the guard bands. Specifically, in addition to the center frequencies of the macrocells, the UE also searches over the center frequencies of the femtocells.
Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications may be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.
