Improving Voice Capacity in 3G Systems
Higher data rates are only part of the story of next-generation wireless architecture.By Stash Czaja and Hong Kui Yang, LSI Logic
Listening to descriptions of 3G wireless standards, it would be easy to assume that the only important attribute this new technology will bring is its enhanced data rate. Much of the discussion about the next-generation wireless architecture has centered on the many exciting multimedia applications 3G data rates will enable, ranging from web browsing to video conferencing.
In fact, the technology's higher data rate is only part of the story. Just as important to service providers is the technology's improved voice capacity. While voice traffic has assumed a secondary status in the shadow of packet-based services, service providers must still build next-generation wireless networks capable of carrying both circuit-switched calls and packet-switched data. Moreover, given the many years it may take to build the market for those new data services, traditional circuit-switched traffic will not only have to coexist with, but in many cases subsidize, each service provider's investment in new data services.
Standards development efforts vividly illustrate the crucial role and increasing demand for voice capacity. First generation 3G systems (cdma2000 1x) offer double the voice capacity of existing 2G systems (IS-95A/B). Follow-on 3G technologies (1x EV-DV) promise to double voice capacity again.
The key question facing developers is which technology offers the best solution for extending voice capacity in next-generation systems. Standards developers used new modulation and coding schemes, forward power control, and coherent reverse link to double voice capacity from IS-95 to cdma2000 1x generations. But for the coming jump to cmda2000 1xEV-DV systems, they are considering a number of alternative approaches. One of the most attractive techniques under consideration is Cell Selection Handoff (CSHO).
CSHO lowers the interference inherent in soft handoffs by selecting only the cell sites necessary to achieve the desired quality of service (QoS). In effect, the number of base stations transmitting at any time to the same mobile station is limited to the absolute minimum, and under many channel conditions will be limited to a single one.
One of the unique features of the CDMA system is its ability to provide extensive path and space diversity. One method the technology uses to exploit space and path diversity is the Soft Handoff (SHO), in which the mobile station Rake receiver combines multiple paths from multiple base stations or cell sites. Typical Rake receivers, such as LSI Logic's CBP3.0 targeted for the IS-95-A/B market, or the CBP4.0 targeted for the IS-2000-A market, employ four demodulating elements or fingers. This feature set allows the receiver to track and demodulate signals from up to four base stations. Each finger of the Rake receiver can demodulate voice, data, and control channels simultaneously using transmit diversity. In addition to the Rake receiver, the baseband processor employs a Double Dwell highly parallel search engine (equivalent to 96 parallel correlators), which continuously measures the power of each base station pilot power. Searcher measurement results are reported to the base stations and assist in the maintenance (allocation/de-allocation) of the link (Active Set) between the base stations and the mobile station.
In general, the performance of any mobile fading environment improves as the number of base stations in the Active Set grows. However, since each base station in the Active Set increases the interference for all other users, the capacity of the total system quickly reaches a limit. Additionally, the number of base stations in the Active Set can be as large as six, and the typical multipath profile (signal components arriving at the receiver at a distinct time) for urban and suburban environments have at least two usable rays. From these factors, one may conclude that on average there will be 12 usable signal components arriving at the mobile station at any given time.
As previously mentioned, a typical cdma2000 baseband processor will feature only four demodulating fingers in its Rake receiver. That limitation prohibits the mobile station receiver from using a significant amount of transmitted signal. Reducing the size of the Active Set without sacrificing voice QoS is not an option due to the fading characteristics of the mobile environment. Instead, designers have developed a new technique called CSHO for disabling selected base station transmitters when a desired QoS is satisfied by the remaining base stations.
Cell Selection Handoff
In principal, CSHO is similar to the fast cell selection method typically employed for data transfer. Unlike traditional methods, which select the single strongest base station for transmission, CSHO maintains the SHO characteristics by disabling the transmission of selected base stations for the period during which the signal conditions at the mobile station guarantee the desired QoS. Furthermore, although the fundamental channel frame size remains unchanged (20 ms), the gating-off duration is defined in terms of the power control groups (PCG) or the pocket data channel slots. This allows the selection of the transmitting base stations to occur at a 1.25 ms rate that allows the cell selection to follow slow-to-moderate fading, which maximizes the diversity gain.
By gating-off the transmission of selected cells from the Active Set, CSHO limits the number of actively transmitting cells. This provides a significant increase in voice capacity while maintaining compatibility with the existing voice frame structure and SHO. The mobile station selects the transmitting base stations to avoid the latency associated with a network-centric solution.
It is important to note one major difference between the current SHO and the CSHO. While all the base stations in the Active Set are transmitting during the SHO, the CSHO limits the number of actively transmitting base stations by switching between base stations with a PCG resolution to combat slow fading. SHO cannot perform this operation.
There are many differences between CSHO and hard handoff (HHO). CSHO allows more than one base station to transmit simultaneously with the mobile station by soft combining all the base stations transmissions. During a CSHO, the reverse link is in soft handoff, since all base stations from the Active Set demodulate the signal transmitted by the mobile station. In addition, CSHO allows the frequent selection of cell sites. HHO cannot perform these operations.
The operations during the CSHO can be summarized as follows:
1. The mobile station continuously estimates the channel conditions of all base stations in its Active Set, averaging the measurement over any uncertainty period (i.e. fading). The measured pilot strength is the sum of all usable multipath signals. The measurement can be done over each PCG to combat fading.
2. The results are sorted and the strongest pilot(s) compared to the T_QOS_dB threshold. T_QOS_dB is sent by the base station and is defined as, for example, T_QOS_dB = FPC_FCH_SETPT + •, where • is a parameter dependent on the processing gain of data channel, coding rate, FPC accumulated gain, etc.
3. If the strongest pilot from the Active Set is larger than T_QOS_dB, the mobile station selects this base station for transmission during PCGN+2 and gates-off (GATE_OFF = 1) all remaining base stations.
4. If the strongest pilot from the Active Set is smaller than T_QOS_dB, the mobile station selects additional pilot(s) until the condition SUM_PILOTS > T_QOS_dB is met.
5. The selection of the base station(s) is fed back to all base stations in the Active Set via a low latency feedback channel. This feedback channel is TDM between all base stations in the Active Set.
6. The mobile station assigns the position in the feedback channel to each base station in its Active Set and communicates this through the Handoff Completion Message.
7. Forward Power Control is identical to that specified by IS-2000-A. All base stations will respond to the Forward Power Control commands, but only those selected will transmit.
8. The mobile station responds only to the Reverse Power Control commands from the base station(s) selected for transmission. If more than one base station is selected, the mobile station performs a conventional OR of the received power control bits.
The operation of CSHO can be visualized in Figure 1.
When all the base stations in the Active Set are selected, CSHO operates identically to an SHO. In certain instances, however, the CSHO will overcome some problems associated with the current SHO. For example, in the current SHO all members of the Active Set are continuously transmitting to the mobile station. In most instances that condition will result in a much higher total transmitted power than what is required to satisfy the desired QoS. Since the mobile station possesses limited resources (i.e. the number of elements in its Rake receiver), most of the transmitted signal energy is not used. However, it still contributes added interference to other users.
CSHO employs a fast feedback channel, which allows the system to adapt to the changing conditions of a slow fading channel, thus combating slow fading. The slow fading channel has the most negative impact on the performance of the mobile station receiver. In contrast, the SHO exhibits a latency of several frames, so it provides no immunity to the slow-fading channel. Given these factors, it is very likely that the CSHO will exceed the performance of SHO in most cases, while enabling increased system voice capacity.
Extensive simulations of CSHO were performed by LSI Logic using the mobile station Active Set statistics obtained during field tests in the existing commercial system. Those simulations indicate an improvement of between 30% and 60% in the voice channel capacity can be achieved as compared with the current SHO while providing the same QoS. The results of those simulations are tabulated in Tables 1 and 2.
While the emerging 3G infrastructure promises dramatic improvements in data services, an additional benefit is increased voice capacity. One of the most promising techniques to maximize use of that new capacity is CSHO. While this innovative approach retains many similarities to CDMA's traditional SHO feature, it allows designers to significantly lower signal interference. At the same time it adds fast feedback and cell-selection capabilities, which promise to mitigate many of the drawbacks of the current SHO. Building on the inherent capabilities already integrated into existing CDMA technology, CSHO promises to significantly increase voice capacity in next-generation 3G systems.
Stash Czaja is a Principal Systems Engineer and Hong Kui Yang a Staff Systems Engineer, for LSI Logic.